Cnidaria is a phylum containing over 9,000
speciesIn biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of
animalAnimals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s found exclusively in aquatic and mostly
marineAn ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...
environments. Their distinguishing feature is
cnidocyteA cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s, specialized cells that they use mainly for capturing prey. Their bodies consist of
mesogleaMesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
, a non-living jelly-like substance, sandwiched between two layers of
epitheliumEpithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...
that are mostly one
cell thickThe cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
. They have two basic body forms: swimming
medusaIn biology, a medusa is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...
e and
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
polypA polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...
s, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single
orifice-External orifices:In a typical mammalian body such as the human body, the external body orifices are:* The nostrils, for breathing and the associated sense of smell.* The eyes, for the sense of sight and crying....
and body cavity that are used for digestion and
respiration'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
. Many cnidarian species produce
coloniesIn biology, a colony reference to several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defense or the ability to attack bigger prey. Some insects live only in colonies...
that are single organisms composed of medusa-like or polyp-like
zooidA zooid or zoöid is a single animal that is part of a colonial animal. The zooids can either be directly connected by tissue or share a common exoskeleton...
s, or both. Cnidarians' activities are coordinated by a decentralized
nerveA peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
net and simple receptors. Several free-swimming Cubozoa and
ScyphozoaScyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
possess balance-sensing
statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s, and some have simple eyes. Not all cnidarians
reproduce sexuallySexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
. Many have complex lifecycles with
asexualAsexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
polyp stages and sexual medusae, but some omit either the polyp or the medusa stage.
Cnidarians were for a long time grouped with
CtenophoreThe Ctenophora are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size...
s in the phylum
CoelenterataCoelenterata is an obsolete term encompassing two animal phyla, the Ctenophora and the Cnidaria . The name comes from the Greek "koilos" , referring to the hollow body cavity common to these two phyla...
, but increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four main groups: the almost wholly
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
(
sea anemoneSea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s,
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s,
sea penSea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...
s); swimming
ScyphozoaScyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
(jellyfish); Cubozoa (box jellies); and
HydrozoaHydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as
HydraHydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
and colonial swimmers such as the
Portuguese Man o' WarThe Portuguese Man o' War , also known as the Portuguese man-of-war, man-of-war, or bluebottle, is a jelly-like marine invertebrate of the family Physaliidae...
. Staurozoa have recently been recognised as a
classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
in their own right rather than a sub-group of Scyphozoa, and there is debate about whether
MyxozoaThe Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...
and Polypodiozoa are cnidarians or closer to
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
ns (more complex animals).
Most cnidarians prey on organisms ranging in size from
planktonPlankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
to animals several times larger than themselves, but many obtain much of their nutrition from endosymbiotic
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, and a few are
parasitesParasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
. Many are preyed upon by other animals including starfish,
sea slugsA nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...
,
fishFish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
and
turtleTurtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s.
Coral reefCoral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s, whose polyps are rich in endosymbiotic algae, support some of the world's most productive
ecosystemAn ecosystem is a biological environment consisting of all the organisms living in a particular area, as well as all the nonliving , physical components of the environment with which the organisms interact, such as air, soil, water and sunlight....
s, and protect vegetation in tidal zones and on shorelines from strong currents and tides. While corals are almost entirely restricted to warm, shallow marine waters, other cnidarians live in the depths, in
polarEarth's polar regions are the areas of the globe surrounding the poles also known as frigid zones. The North Pole and South Pole being the centers, these regions are dominated by the polar ice caps, resting respectively on the Arctic Ocean and the continent of Antarctica...
seas and in freshwater.
Fossil cnidarians have been found in rocks formed about , and other fossils show that corals may have been present shortly before and diversified a few million years later. Fossils of cnidarians that do not build mineralized structures are very rare. Scientists currently think that cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians. Recent analyses have concluded that cnidarians, although considered more "primitive" than bilaterians, have a wider range of genes.
Jellyfish stings killed several hundred people in the 20th century, and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a
delicacyA delicacy is a food item that is considered highly desirable in certain cultures. Often this is because of unusual flavors or characteristics or because it is rare....
in eastern and southern
AsiaAsia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
. Coral reefs have long been economically important as providers of fishing grounds, protectors of shore buildings against currents and tides, and more recently as centers of tourism. However, they are vulnerable to over-fishing, mining for construction materials,
pollutionPollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
, and damage caused by tourism.
Distinguishing features
Cnidarians form an
animalAnimals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
phylumIn biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
that is more complex than sponges, about as complex as
ctenophoreThe Ctenophora are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size...
s (comb jellies), and less complex than
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
ns, which include almost all other animals. However, both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like
basement membraneThe basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
s;
muscleMuscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s;
nervous systemThe nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
s; and
some have
sensoryIn a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
organs. Cnidarians are distinguished from all other animals by having
cnidocyteA cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s that fire like
harpoonA harpoon is a long spear-like instrument used in fishing to catch fish or large marine mammals such as whales. It accomplishes this task by impaling the target animal, allowing the fishermen to use a rope or chain attached to the butt of the projectile to catch the animal...
s and are used mainly to capture prey but also as anchors in some species.
Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, which is called the
mesogleaMesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
in cnidarians; more complex
animalAnimals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s have three main cell layers and no intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled diploblastic, along with sponges. However, both cnidarians and ctenophores have a type of
muscleMuscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
that, in more complex animals, arises from the
middle cell layerIn all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
. As a result some recent text books classify ctenophores as triploblastic, and it has been suggested that cnidarians evolved from triploblastic ancestors.
| | Sponges | Cnidarians | CtenophoreThe Ctenophora are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size...
s | BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
|
| Cnidocyte A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s |
No |
Yes |
No |
| Colloblast Colloblasts are a cell type found in ctenophores. They are widespread in the tentacles of these animals and are used to capture prey. Colloblasts consist of a coiled spiral filament that is embedded in the epidermis and an axial filament with a granular dome. The apical surface of these cells...
s |
No |
Yes |
No |
| Digestive and circulatory The circulatory system is an organ system that passes nutrients , gases, hormones, blood cells, etc...
organIn biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s |
No |
Yes |
| Number of main cell layers |
Two, with jelly-like layer between them |
Two or Three |
Three |
| Cells in each layer bound together |
No, except that HomoscleromorphaHomoscleromorpha is a subclass of marine demosponges containing a single order, Homosclerophorida and a single family, Plakinidae.-Taxonomy:This class has recently been recognised as the fourth major line of sponges....
have basement membranes. |
Yes: inter-cell connections; basement membranes |
| Sensory In a sensory system, a sensory receptor is a sensory nerve ending that responds to a stimulus in the internal or external environment of an organism...
organs |
No |
Yes |
| Number of cells in middle "jelly" layer |
Many |
Few |
(Not applicable) |
| Cells in outer layers can move inwards and change functions |
Yes |
No |
(Not applicable) |
| Nervous system |
No |
Yes, simple |
Simple to complex |
MuscleMuscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
s |
None |
Mostly epitheliomuscular |
Mostly myoepithelial |
Mostly myocyte A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long, long chains of sarcomeres, the contractile units of the cell....
s |
Main cell layers
Cnidaria are diploblastic animals, in other words they have two main cell layers, while more complex animals are triploblasts having three main layers. The two main cell layers of cnidarians form epithelia that are mostly one cell thick, and are attached to a fibrous
basement membraneThe basement membrane is a thin sheet of fibers that underlies the epithelium, which lines the cavities and surfaces of organs including skin, or the endothelium, which lines the interior surface of blood vessels.- Composition :...
, which they secrete. They also secrete the jelly-like
mesogleaMesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
that separates the layers. The layer that faces outwards, known as the
ectodermThe "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...
("outside skin"), generally contains the following types of cells:
- Epitheliomuscular cells whose bodies form part of the epithelium but whose bases extend to form muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
fibers in parallel rows. The fibers of the outward-facing cell layer generally run at right angles to the fibers of the inward-facing one. In AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
(anemones, corals, etc.) and ScyphozoaScyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
(jellyfish), the mesoglea also contains some muscle cells.
- Cnidocyte
A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s, the harpoon-like "nettleNettles constitute between 24 and 39 species of flowering plants of the genus Urtica in the family Urticaceae, with a cosmopolitan though mainly temperate distribution. They are mostly herbaceous perennial plants, but some are annual and a few are shrubby...
cells" that give the phylumIn biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Cnidaria its name. These appear between or sometimes on top of the muscle cells.
- Nerve
A peripheral nerve, or simply nerve, is an enclosed, cable-like bundle of peripheral axons . A nerve provides a common pathway for the electrochemical nerve impulses that are transmitted along each of the axons. Nerves are found only in the peripheral nervous system...
cells. SensorySensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...
cells appear between or sometimes on top of the muscle cells, and communicate via synapseIn the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
s (gaps across which chemical signals flow) with motor nerve cells, which lie mostly between the bases of the muscle cells.
- Interstitial cells, which are unspecialized and can replace lost or damaged cells by transforming into the appropriate types. These are found between the bases of muscle cells.
In addition to epitheliomuscular, nerve and interstitial cells, the inward-facing
gastrodermThe gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....
("stomach skin") contains
glandA gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...
cells that secrete digestive
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s. In some species it also contains low concentrations of cnidocytes, which are used to subdue prey that is still struggling.
The mesoglea contains small numbers of amoeba-like cells, and muscle cells in some species. However the number of middle-layer cells and types are much lower than in sponges.
Cnidocytes
These "nettle cells" function as
harpoonA harpoon is a long spear-like instrument used in fishing to catch fish or large marine mammals such as whales. It accomplishes this task by impaling the target animal, allowing the fishermen to use a rope or chain attached to the butt of the projectile to catch the animal...
s, since their payloads remain connected to the bodies of the cells by threads. Three types of
cnidocyteA cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s are known:
- Nematocysts inject venom
Venom is the general term referring to any variety of toxins used by certain types of animals that inject it into their victims by the means of a bite or a sting...
into prey, and usually have barbs to keep them embedded in the victims. Most species have nematocysts.
- Spirocysts do not penetrate the victim or inject venom, but entangle it by means of small sticky hairs on the thread.
- Ptychocysts are not used for prey capture — instead the threads of discharged ptychocysts are used for building protective tubes in which their owners live. Ptychocysts are found only in the order
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Cerianthria, tube anemonesTube-dwelling anemones or cerianthids look very similar to sea anemones, but belong to an entirely different subclass of anthozoans. They are solitary, living buried in soft sediments...
.
The main components of a cnidocyte are:
- A cilium
A cilium is an organelle found in eukaryotic cells. Cilia are slender protuberances that project from the much larger cell body....
(fine hair) which projects above the surface and acts as a trigger. Spirocysts do not have cilia.
- A tough capsule, the cnida, which houses the thread, its payload and a mixture of chemicals which may include venom or adhesive
An adhesive, or glue, is a mixture in a liquid or semi-liquid state that adheres or bonds items together. Adhesives may come from either natural or synthetic sources. The types of materials that can be bonded are vast but they are especially useful for bonding thin materials...
s or both. ("cnida" is derived from the Greek word κνίδη, which means "nettle")
- A tube-like extension of the wall of the cnida that points into the cnida, like the finger of a rubber glove pushed inwards. When a cnidocyte fires, the finger pops out. If the cell is a venomous nematocyte, the "finger"'s tip reveals a set of barbs that anchor it in the prey.
- The thread, which is an extension of the "finger" and coils round it until the cnidocyte fires. The thread is usually hollow and delivers chemicals from the cnida to the target.
- An operculum
An operculum is an anatomical feature, a stiff structure resembling a lid or a small door that opens and closes, and thus controls contact between the outside world and an internal part of an animal...
(lid) over the end of the cnida. The lid may be a single hinged flap or three flaps arranged like slices of pie.
- The cell body which produces all the other parts.
It is difficult to study the firing mechanisms of cnidocytes as these structures are small but very complex. At least four hypotheses have been proposed:
- Rapid contraction of fibers round the cnida may increase its internal pressure.
- The thread may be like a coiled spring that extends rapidly when released.
- In the case of Chironex
Chironex is a genus of box jellyfish in the Chirodropidae family. Traditionally, only a single species, C. fleckeri, was recognized in this genus, but in 2009 a second species, C.yamaguchii, was described. Their stings are highly venomous, and both species have caused human fatalities...
(the "sea wasp"), chemical changes in the cnida's contents may cause them to expand rapidly by polymerizationIn polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
.
- Chemical changes in the liquid in the cnida make it a much more concentrated
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...
solution, so that osmotic pressureOsmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
forces water in very rapidly to dilute it. This mechanism has been observed in nematocysts of the class HydrozoaHydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
, sometimes producing pressures as high as 140 atmospheresThe standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
, similar to that of scubaA scuba set is an independent breathing set that provides a scuba diver with the breathing gas necessary to breathe underwater during scuba diving. It is much used for sport diving and some sorts of work diving....
air tanks, and fully extending the thread in as little as 2 milliseconds (0.002 second).
Cnidocytes can only fire once, and about 25% of a hydra's nematocysts are lost from its tentacles when capturing a
brine shrimpArtemia is a genus of aquatic crustaceans known as brine shrimp. Artemia, the only genus in the family Artemiidae, has changed little externally since the Triassic period...
. Used cnidocytes have to be replaced, which takes about 48 hours. To minimise wasteful firing, two types of stimulus are generally required to trigger cnidocytes: their cilia detect contact, and nearby
sensorySensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...
cells "smell" chemicals in the water. This combination prevents them from firing at distant or non-living objects. Groups of cnidocytes are usually connected by nerves and, if one fires, the rest of the group requires a weaker minimum stimulus than the cells that fire first.
Basic body forms
Adult cnidarians appear as either swimming
medusaIn biology, a medusa is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...
e or
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
polypA polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...
s. Both are radially symmetrical, like a wheel and a tube respectively. Since these animals have no heads, their ends are described as "oral" (nearest the mouth) and "aboral" (furthest from the mouth). Most have fringes of tentacles equipped with cnidocytes around their edges, and medusae generally have an inner ring of tentacles around the mouth. The mesoglea of polyps is usually thin and often soft, but that of medusae is usually thick and springy, so that it returns to its original shape after muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a sort of jet propulsion.
Colonial forms
Cnidaria produce a variety of colonial forms, each of which is one organism but consists of polyp-like zooids. The simplest is a connecting tunnel that runs over the substrate (rock or seabed) and from which single zooids sprout. In some cases the tunnels form visible webs, and in others they are enclosed in a fleshy mat. More complex forms are also based on connecting tunnels but produce "tree-like" groups of zooids. The "trees" may be formed either by a central zooid that functions as a "trunk" with later zooids growing to the sides as "branches", or in a zig-zag shape as a succession of zooids, each of which grows to full size and then produces a single bud at an angle to itself. In many cases the connecting tunnels and the "stems" are covered in periderm, a protective layer of
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
. Some colonial forms have other specialized types of zooid, for example, to pump water through their tunnels.
Siphonophores form complex colonies that consist of: an upside-down polyp that forms a central stem with a gas-filled float at the top; one or more sets of medusa-like zooids that provide propulsion; leaf-like bracts that give some protection to other parts; sets of tentacles that bear nematocytes that capture prey; other tentacles that act as sensors; near the base of each set of tentacles, a polyp-like zooid that acts as a stomach for the colony; medusa-like zooids that serve as
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s. Although some of these zooids resemble polyps or medusae in shape, they lack features that are not relevant to their specific functions, for example the swimming "medusae" have no digestive, sensory or reproductive cells. The best-known siphonophore is the
Portuguese Man o' WarThe Portuguese Man o' War , also known as the Portuguese man-of-war, man-of-war, or bluebottle, is a jelly-like marine invertebrate of the family Physaliidae...
(
Physalia physalis).
Skeletons
In medusae the only supporting structure is the
mesogleaMesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
.
HydraHydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
and most
sea anemoneSea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s close their mouths when they are not feeding, and the water in the digestive cavity then acts as a
hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
, rather like a water-filled balloon. Other polyps such as
TubulariaTubularia is a genus of hydroids that appear to be furry pink tufts or balls at the end of long strings, thus causing them to be sometimes be called "pink-mouthed" or "pink-hearted" hydroids. T. larynx is described as:The stems are tubular, with a yellowish coloured tegument and are branched at the...
use columns of water-filled cells for support.
Sea penSea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...
s stiffen the mesoglea with
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
spiculeSpicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
s and tough fibrous
proteinProteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...
s, rather like sponges.
In some colonial polyps a
chitinChitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous periderm gives support and some protection to the connecting sections and to the lower parts of individual polyps. Stony corals secrete massive calcium carbonate
exoskeletonAn exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
s. A few polyps collect materials such as sand grains and shell fragments, which they attach to their outsides. Some colonial sea anemones stiffen the mesoglea with
sedimentSediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
particles.
Locomotion
Medusae swim by a form of jet propulsion: muscles, especially inside the rim of the bell, squeeze water out of the cavity inside the bell, and the springiness of the mesoglea powers the recovery stroke. Since the tissue layers are very thin, they provide too little power to swim against currents and just enough to control movement within currents.
HydraHydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
s and some
sea anemoneSea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s can move slowly over rocks and sea or stream beds by various means: creeping like snails, crawling like
inchwormThe geometer moths or Geometridae are a family of the order Lepidoptera...
s, or by
somersaultIn gymnastics, a cartwheel is a sideways rotary movement performed by bringing the hands to the ground while the body inverts and the legs travel over the body, coming down to a standing position.-Terminology:...
ing. A few can swim clumsily by waggling their bases.
Nervous system and senses
Cnidaria have no
brainThe brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
s or even
central nervous systemThe central nervous system is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of bilaterian animals—that is, all multicellular animals except sponges and radially symmetric animals such as jellyfish...
s. Instead they have decentralized
nerve netA nerve net is a type of simple nervous system that is found in members of the cnidaria, ctenophora, and echinodermata phyla. Nerve nets consist of interconnected neurons lacking a brain or any form of cephalization. This nervous system allows cnidarians to respond to physical contact. They may...
s consisting of :
sensory neuronSensory neurons are typically classified as the neurons responsible for converting external stimuli from the environment into internal stimuli. They are activated by sensory input , and send projections into the central nervous system that convey sensory information to the brain or spinal cord...
s that generate signals in response to various types of stimulus, such as
odorOlfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
s;
motor neuronIn vertebrates, the term motor neuron classically applies to neurons located in the central nervous system that project their axons outside the CNS and directly or indirectly control muscles...
s that tell muscles to contract; all connected by "cobwebs" of intermediate neurons. As well as forming the "signal cables", intermediate neurons also form ganglia that act as local coordination centers. The cilia of the
cnidocyteA cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s detect physical contact. Nerves inform cnidocytes when odors from prey or attackers are detected and when neighbouring cnidocytes fire. Most of the communications between nerve cells are via
chemical synapseChemical synapses are specialized junctions through which neurons signal to each other and to non-neuronal cells such as those in muscles or glands. Chemical synapses allow neurons to form circuits within the central nervous system. They are crucial to the biological computations that underlie...
s, small gaps across which chemicals flow. As this process is too slow to ensure that the muscles round the rim of a medusa's bell contract simultaneously in swimming the neurons which control this communicate by much faster electrical signals across
gap junctionA gap junction or nexus is a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells....
s.
Medusae and complex swimming colonies such as siphonophores and
chondrophoreThe chondrophores or porpitids are a small and very unusual group of hydrozoans today classified as family Porpitidae. Though it derives from an outdated name for this lineage , some still find the term "chondrophore" useful as a synonym to "porpitid" in discussions of the three genera contained...
s sense tilt and acceleration by means of
statocystThe statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...
s, chambers lined with hairs which detect the movements of internal mineral grains called statoliths. If the body tilts in the wrong direction, the animal rights itself by increasing the strength of the swimming movements on the side that is too low. They also have ocelli ("little eyes"), which can detect the direction from which light is coming. Box jellies have camera eyes, although these probably do not form images, and their
lensA lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
es simply produce a clearer indication of the direction from which light is coming.
Feeding and excretion
Cnidarians feed in several ways:
predationIn ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
, absorbing dissolved
organicOrganic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
chemicals, filtering food particles out of the water, and obtaining nutrients from symbiotic
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
within their cells. Most obtain the majority of their food from predation but some, including the
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s
Hetroxenia and
Leptogorgia, depend almost completely on their
endosymbiontAn endosymbiont is any organism that lives within the body or cells of another organism, i.e. forming an endosymbiosis...
s and on absorbing dissolved nutrients. Cnidaria give their symbiotic algae
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, some nutrients and a place in the sun.
Predatory species use their
cnidocyteA cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria . The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on...
s to poison or entangle prey, and those with venomous nematocysts may start digestion by injecting digestive
enzymeEnzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...
s. The "smell" of fluids from wounded prey makes the tentacles fold inwards and wipe the prey off into the mouth. In medusae the tentacles round the edge of the bell are often short and most of the prey capture is done by "oral arms", which are extensions of the edge of the mouth and are often frilled and sometimes branched to increase their surface area. Medusae often trap prey or suspended food particles by swimming upwards, spreading their tentacles and oral arms and then sinking. In species for which suspended food particles are important, the tentacles and oral arms often have rows of cilia whose beating creates currents that flow towards the mouth, and some produce nets of
mucusIn vertebrates, mucus is a slippery secretion produced by, and covering, mucous membranes. Mucous fluid is typically produced from mucous cells found in mucous glands. Mucous cells secrete products that are rich in glycoproteins and water. Mucous fluid may also originate from mixed glands, which...
to trap particles.
Once the food is in the digestive cavity,
glandA gland is an organ in an animal's body that synthesizes a substance for release of substances such as hormones or breast milk, often into the bloodstream or into cavities inside the body or its outer surface .- Types :...
cells in the
gastrodermThe gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....
release enzymes that reduce the prey to slurry, usually within a few hours. This circulates through the digestive cavity and, in colonial cnidarians, through the connecting tunnels, so that gastroderm cells can absorb the nutrients. Absorption may take a few hours, and digestion within the cells may take a few days. The circulation of nutrients is driven by water currents produced by cilia in the gastroderm or by muscular movements or both, so that nutrients reach all parts of the digestive cavity. Nutrients reach the outer cell layer by
diffusionMolecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
or, for animals or zooids such as medusae which have thick
mesogleaMesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
s, are transported by mobile cells in the mesoglea.
Indigestible remains of prey are expelled through the mouth. The main waste product of cells' internal processes is
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, which is removed by the external and internal water currents.
Respiration
There are no respiratory organs, and both cell layers absorb oxygen from and expel
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
into the surrounding water. When the water in the digestive cavity becomes stale it must be replaced, and nutrients that have not been absorbed will be expelled with it. Some
AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
have ciliated grooves on their tentacles, allowing them to pump water out of and into the digestive cavity without opening the mouth. This improves respiration after feeding and allows these animals, which use the cavity as a
hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding circular and longitudinal muscles are used to change an...
, to control the water pressure in the cavity without expelling undigested food.
Cnidaria that carry photosynthetic symbionts may have the opposite problem, an excess of oxygen, which may prove
toxicOxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at elevated partial pressures. It is also known as oxygen toxicity syndrome, oxygen intoxication, and oxygen poisoning...
. The animals produce large quantities of
antioxidantAn antioxidant is a molecule capable of inhibiting the oxidation of other molecules. Oxidation is a chemical reaction that transfers electrons or hydrogen from a substance to an oxidizing agent. Oxidation reactions can produce free radicals. In turn, these radicals can start chain reactions. When...
s to neutralize the excess oxygen.
Regeneration
All cnidarians can
regenerateIn biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...
, allowing them to recover from injury and to reproduce
asexuallyAsexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
. Medusae have limited ability to regenerate, but polyps can do so from small pieces or even collections of separated cells. This enables corals to recover even after apparently being destroyed by predators.
Reproduction
Sexual
In the Cnidaria
sexual reproductionSexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
often involves a complex life cycle with both
polypA polyp in zoology is one of two forms found in the phylum Cnidaria, the other being the medusa. Polyps are approximately cylindrical in shape and elongated at the axis of the body...
and
medusaIn biology, a medusa is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...
stages. For example in
ScyphozoaScyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
(jellyfish) and Cubozoa (box jellies) a
larvaA larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
swims until it finds a good site, and then becomes a polyp. This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile medusae, a process called
strobilationStrobilation or transverse fission is a form of asexual reproduction consisting of the spontaneous transverse segmentation of the body. It is observed in certain cnidarians and helminths...
. The juveniles swim off and slowly grow to maturity, while the polyp re-grows and may continue strobilating periodically. The adults have
gonadThe gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s in the
gastrodermThe gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....
, and these release
ovaAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
and
spermThe term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
into the water in the breeding season.
Shortened forms of this life cycle are common, for example some oceanic scyphozoans omit the polyp stage completely, and cubozoan polyps produce only one medusa.
HydrozoaHydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
have a variety of life cycles. Some have no polyp stages and some (e.g.
hydraHydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
) have no medusae. In some species the medusae remain attached to the polyp and are responsible for sexual reproduction; in extreme cases these reproductive zooids may not look much like medusae.
AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
have no medusa stage at all and the polyps are responsible for sexual reproduction.
Spawning is generally driven by environmental factors such as changes in the water temperature, and their release is triggered by lighting conditions such as sunrise, sunset or the
phase of the moonA lunar phase or phase of the moon is the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases change cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon, and Sun...
. Many species of Cnidaria may spawn simultaneously in the same location, so that there are too many ova and sperm for predators to eat more than a tiny percentage — one famous example is the
Great Barrier ReefThe Great Barrier Reef is the world'slargest reef system composed of over 2,900 individual reefs and 900 islands stretching for over 2,600 kilometres over an area of approximately...
, where at least 110
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s and a few non-cnidarian
invertebrateAn invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
s produce enough to turn the water cloudy. These mass spawnings may produce hybrids, some of which can settle and form polyps, but it is not known how long these can survive. In some species the ova release chemicals that attract sperm of the same species.
The fertilized eggs develop into larvae by dividing until there are enough cells to form a hollow sphere (
blastulaThe blastula is a hollow sphere of cells formed during an early stage of embryonic development in animals . The blastula is created when the zygote undergoes the cell division process known as cleavage. The blastula is preceded by the morula and is followed by the gastrula in the developmental...
) and then a depression forms at one end (
gastrulationGastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...
) and eventually become the digestive cavity. However in cnidarians the depression forms at the end further from the yolk (at the
animal poleIn developmental biology, a blastula embryo is divided into two hemispheres: the animal pole and the vegetal pole.The animal pole consists of small cells that divide rapidly, in contrast with the vegetal pole below it. The animal pole draws its name from its liveliness relative to the...
), while in bilaterians it forms at the other end (vegetal pole). The larvae, called
planulaA planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
e, swim or crawl by means of cilia. They are cigar-shaped but slightly broader at the "front" end, which is the aboral, vegetal-pole end and eventually attaches to a substrate if the species has a polyp stage.
Anthozoan larvae either have large yolks or are capable of feeding on
planktonPlankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...
, and some already have endosymbiotic
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
that help to feed them. Since the parents are immobile, these feeding capabilities extend the larvae's range and avoid overcrowding of sites. Scyphozoan and hydrozoan larvae have little yolk and most lack endosymbiotic algae, and therefore have to settle quickly and
metamorphose"Métamorphose" - a song by French singer Amanda Lear released in 1989 by Carrere Records.- Song information :"Métamorphose" was the first single from Amanda's French-Italian album Tant qu'il y aura des hommes. The album was actually a re-release of Uomini più uomini, and consisted mostly of down-...
into polyps. Instead these species rely on their medusae to extend their ranges.
Asexual
All known cnidaria can reproduce
asexuallyAsexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
by various means, in addition to regenerating after being fragmented.
HydrozoaHydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
n polyps only bud, while the medusae of some hydrozoans can divide down the middle.
ScyphozoaScyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
n polyps can both bud and split down the middle. In addition to both of these methods,
AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
can split horizontally just above the base.
Classification
Cnidarians were for a long time grouped with
CtenophoreThe Ctenophora are a phylum of animals that live in marine waters worldwide. Their most distinctive feature is the "combs", groups of cilia that they use for swimming, and they are the largest animals that swim by means of cilia – adults of various species range from a few millimeters to in size...
s in the phylum
CoelenterataCoelenterata is an obsolete term encompassing two animal phyla, the Ctenophora and the Cnidaria . The name comes from the Greek "koilos" , referring to the hollow body cavity common to these two phyla...
, but increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four main groups: sessile
AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
(
sea anemoneSea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s,
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s,
sea penSea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...
s); swimming
ScyphozoaScyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
(jellyfish); Cubozoa (box jellies); and
HydrozoaHydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as
HydraHydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
and colonial swimmers such as the
Portuguese Man o' WarThe Portuguese Man o' War , also known as the Portuguese man-of-war, man-of-war, or bluebottle, is a jelly-like marine invertebrate of the family Physaliidae...
. Staurozoa have recently been recognised as a
classIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
in their own right rather than a sub-group of Scyphozoa, and there is debate about whether
MyxozoaThe Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...
and Polypodiozoa are cnidarians or closer to
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
ns.
Modern cnidarians are generally classified into four
classesIn biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
:
| | HydrozoaHydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
| Scyphozoa Scyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
| Cubozoa | AnthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
|
| Number of species |
2,700 |
200 |
20 |
6,000 |
| Examples |
HydraHydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
, siphonophores |
JellyfishJellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
|
Box jellies |
Sea anemoneSea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s, coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, sea penSea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...
s |
| Cells found in mesoglea Mesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
|
No |
Yes |
Yes |
Yes |
| Nematocysts in exodermis The exodermis is a layer layer of cells from the outermost layer of the cortex of many angiosperms. Like the endodermis, its cells are very compact and are surrounded by a Casparian strip, two features which are used to restrict flow of water to symplastic rather than apoplastic flow through...
|
No |
Yes |
Yes |
Yes |
| Medusa phase in life cycle |
In some species |
Yes, except for StauromedusaeStauromedusae are the stalked jellyfishes, of the class Staurozoa within the phylum Cnidaria. They are unique in that they do not have an alternation of polyp and medusa life cycle phases, but are interpreted as an attached medusa stage, with a life style more resembling that of polypoid forms...
if they are scyphozoans |
Yes |
No |
| Number of medusae produced per polyp |
Many |
Many |
One |
(not applicable) |
Stauromedusae, small
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
cnidarians with stalks and no medusa stage, have traditionally been classified as members of the Scyphozoa, but recent research suggests they should be regarded as a separate class, Staurozoa.
The
MyxozoaThe Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...
, microscopic parasites, were first classified as
protozoaProtozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
ns, but recently as heavily modified cnidarians, and more closely related to Hydrozoa and Scyphozoa than to Anthozoa. However other recent research suggests that
Polypodium hydriforme, a parasite
within the egg cells of
sturgeonSturgeon is the common name used for some 26 species of fish in the family Acipenseridae, including the genera Acipenser, Huso, Scaphirhynchus and Pseudoscaphirhynchus. The term includes over 20 species commonly referred to as sturgeon and several closely related species that have distinct common...
, is closely related to the Myxozoa and that both
Polypodium and the Myxozoa are intermediate between cnidarians and
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
n animals.
Some researchers classify the extinct
conulariidThe Conulata, also known as conulariids, form a poorly understood clade of extinct scyphozoan cnidarians.-Structure:The conulariids are fossils preserved as shell-like structures made up of rows of calcium phosphate rods, resembling an ice-cream cone with fourfold symmetry, usually four...
s as cnidarians, while others propose that they form a completely separate
phylumIn biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
.
Ecology
Many cnidarians are limited to shallow waters because they depend on endosymbiotic
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
for much of their nutrients. The life cycles of most have polyp stages, which are limited to locations that offer stable substrates. Nevertheless major cnidarian groups contain species that have escaped these limitations. Hydrozoans have a worldwide range: some, such as
HydraHydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
, live in freshwater;
ObeliaObelia is a genus in the class Hydrozoa, which consists of mainly marine and some freshwater animal species and have both the polyp and medusa stages in their life cycle...
appears in the coastal waters of all the oceans; and
Liriope can form large shoals near the surface in mid-ocean. Among
anthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
ns, a few
scleractiniaScleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...
n
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s,
sea penSea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...
s and sea fans live in deep, cold waters, and some sea anemones inhabit polar seabeds while others live near
hydrothermal ventA hydrothermal vent is a fissure in a planet's surface from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart, ocean basins, and hotspots. Hydrothermal vents exist because the earth is both...
s over 10 kilometres (6.2 mi) below sea-level.
ReefIn nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
-building corals are limited to tropical seas between 30°N and 30°S with a maximum depth of 46 metres (150.9 ft), temperatures between 20°C and 28°C, high
salinitySalinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
and low
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
levels.
StauromedusaeStauromedusae are the stalked jellyfishes, of the class Staurozoa within the phylum Cnidaria. They are unique in that they do not have an alternation of polyp and medusa life cycle phases, but are interpreted as an attached medusa stage, with a life style more resembling that of polypoid forms...
, although usually classified as jellyfish, are stalked,
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
animals that live in cool to
ArcticThe Arctic is a region located at the northern-most part of the Earth. The Arctic consists of the Arctic Ocean and parts of Canada, Russia, Greenland, the United States, Norway, Sweden, Finland, and Iceland. The Arctic region consists of a vast, ice-covered ocean, surrounded by treeless permafrost...
waters. Cnidarians range in size from
Hydra, 5–20 mm (0.196850393700787–0.78740157480315 in) long, to the
Lion's mane jellyfishThe lion's mane jellyfish is the largest known species of jellyfish. Its range is confined to cold, boreal waters of the Arctic, northern Atlantic, and northern Pacific Oceans, seldom found farther south than 42°N latitude. Similar jellyfish, which may be the same species, are known to inhabit...
, which may exceed 2 metres (6.6 ft) in diameter and 75 metres (246.1 ft) in length.
Prey of cnidarians ranges from plankton to animals several times larger than themselves. Some cnidarians are parasites, mainly on jellyfish but a few are major pests of fish. Others obtain most of their nourishment from endosymbiotic algae or dissolved nutrients. Predators of cnidarians include:
sea slugsA nudibranch is a member of what is now a taxonomic clade, and what was previously a suborder, of soft-bodied, marine gastropod mollusks which shed their shell after their larval stage. They are noted for their often extraordinary colors and striking forms...
, which can incorporate nematocysts into their own bodies for self-defense; starfish, notably the crown of thorns starfish, which can devastate corals; butterfly fish and parrot fish, which eat corals; and marine
turtleTurtles are reptiles of the order Testudines , characterised by a special bony or cartilaginous shell developed from their ribs that acts as a shield...
s, which eat jellyfish. Some sea anemones and jellyfish have a symbiotic relationship with some fish; for example clown fish live among the tentacles of sea anemones, and each partner protects the other against predators.
Coral reefCoral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s form some of the world's most productive ecosystems. Common coral reef cnidarians include both Anthozoans (hard corals, octocorals, anemones) and Hydrozoans (fire corals, lace corals) The endosymbiotic algae of many cnidarian species are very effective primary producers, in other words converters of inorganic chemicals into
organicAn organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
ones that other organisms can use, and their coral hosts use these organic chemicals very efficiently. In addition reefs provide complex and varied habitats that support a wide range of other organisms.
Fringing reefA fringing reef is one of the three main types of coral reefs recognized by most coral reef scientists. It is distinguished from the other two main types in that it has either an entirely shallow backreef zone or none at all...
s just below low-
tideTides are the rise and fall of sea levels caused by the combined effects of the gravitational forces exerted by the moon and the sun and the rotation of the Earth....
level also have a mutually beneficial relationship with
mangroveMangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...
forests at high-tide level and sea grass meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage them or
erodeErosion is when materials are removed from the surface and changed into something else. It only works by hydraulic actions and transport of solids in the natural environment, and leads to the deposition of these materials elsewhere...
the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large influxes of
siltSilt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...
, fresh water and
pollutantsPollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
. This additional level of variety in the environment is beneficial to many types of coral reef animals, which for example may feed in the sea grass and use the reefs for protection or breeding.
Fossil record
The earliest widely accepted animal fossils are rather modern-looking cnidarians, possibly from around , although fossils from the
Doushantuo FormationThe Doushantuo Formation is a Lagerstätte in Guizhou Province, China that is notable for being one of the oldest fossil beds to contain highly preserved fossils...
can only be dated approximately. The identification of some of these as embryos of animals has been contested, but other fossils from these rocks strongly resemble tubes and other mineralized structures made by
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s. Their presence implies that the cnidarian and
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
n lineages had already diverged. Although the Ediacaran fossil
CharniaCharnia is the genus name given to a frond-like Ediacaran lifeform with segmented ridges branching alternately to the right and left from a zig-zag medial suture. The genus Charnia was named after Charnwood Forest in Leicestershire, England, where the first fossilised specimen was found.- Diversity...
used to be classified as a
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
or
sea penSea pens are colonial marine cnidarians belonging to the order Pennatulacea. There are 14 families within the order; they are thought to have a cosmopolitan distribution in tropical and temperate waters worldwide...
, more recent study of growth patterns in
Charnia and modern cnidarians has cast doubt on this hypothesis, and there are now no bona-fide cnidarian body fossils in the Ediacaran. Few fossils of cnidarians without mineralized
skeletonThe skeleton is the body part that forms the supporting structure of an organism. There are two different skeletal types: the exoskeleton, which is the stable outer shell of an organism, and the endoskeleton, which forms the support structure inside the body.In a figurative sense, skeleton can...
s are known from more recent rocks, except in
lagerstätteA Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
n that preserved soft-bodied animals.
A few mineralized fossils that resemble
coralCorals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s have been found in rocks from the
CambrianThe Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
period, and corals diversified in the Early
OrdovicianThe Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
. These corals, which were wiped out in the Permian-Triassic extinction about , did not dominate reef construction since sponges and algae also played a major part. During the
MesozoicThe Mesozoic era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the age of reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time...
era rudist bivalves were the main reef-builders, but they were wiped out in the Cretaceous-Tertiary extinction , and since then the main reef-builders have been
scleractiniaScleractinia, also called stony corals, are exclusively marine animals; they are very similar to sea anemones but generate a hard skeleton. They first appeared in the Middle Triassic and replaced tabulate and rugose corals that went extinct at the end of the Permian...
n corals.
Family tree
It is difficult to reconstruct the early stages in the
evolutionEvolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary "family tree" of animals using only
morphologyIn biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
(their shapes and structures), because the large differences between Porifera (sponges), Cnidaria plus Ctenophora (comb jellies), Placozoa and
BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
(all the more complex animals) make comparisons difficult. Hence reconstructions now rely largely or entirely on molecular phylogenetics, which groups organisms according to similarities and differences in their
biochemistryBiochemistry, sometimes called biological chemistry, is the study of chemical processes in living organisms, including, but not limited to, living matter. Biochemistry governs all living organisms and living processes...
, usually in their
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
or
RNARibonucleic acid , or RNA, is one of the three major macromolecules that are essential for all known forms of life....
.
It is now generally thought that the Calcarea (sponges with
calcium carbonateCalcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
spiculeSpicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
s) are more closely related to Cnidaria, Ctenophora (comb jellies) and
BilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back...
(all the more complex animals) than they are to the other groups of sponges. In 1866 it was proposed that Cnidaria and Ctenophora were more closely related to each other than to Bilateria and formed a group called
CoelenterataCoelenterata is an obsolete term encompassing two animal phyla, the Ctenophora and the Cnidaria . The name comes from the Greek "koilos" , referring to the hollow body cavity common to these two phyla...
("hollow guts"), because Cnidaria and Ctenophora both rely on the flow of water in and out of a single cavity for feeding, excretion and respiration. In 1881 it was proposed that Ctenophora and Bilateria were more closely related to each other, since they shared features that Cnidaria lack, for example muscles in the middle layer (
mesogleaMesoglea is the translucent, inert, jelly-like substance that makes up most of the bodies of jellyfish, comb jellies and certain primitive sea creatures in the phyla Cnidaria and Ctenophora. It acts as the creatures' structural support in water, as they lack bones or cartilage, endo- or...
in Ctenophora,
mesodermIn all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
in Bilateria). However more recent analyses indicate that these similarities are rather vague, and the current view, based on molecular phylogenetics, is that Cnidaria and Bilateria are more closely related to each other than either is to Ctenophora. This grouping of Cnidaria and Bilateria has been labelled "
PlanulozoaPlanulozoa is a proposed clade including the Cnidaria and the Bilateria . The clade excludes the Ctenophora ....
" because it suggests that the earliest Bilateria were similar to the
planulaA planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
larvae of Cnidaria.
Within the Cnidaria, the Anthozoa (sea anemones and corals) are regarded as the sister-group of the rest, which suggests that the earliest cnidarians were
sessileIn zoology, sessility is a characteristic of animals which are not able to move about. They are usually permanently attached to a solid substrate of some kind, such as a part of a plant or dead tree trunk, a rock, or the hull of a ship in the case of barnacles. Corals lay down their own...
polyps with no medusa stage. However it is unclear how the other groups acquired the medusa stage, since
HydrozoaHydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
form medusae by budding from the side of the polyp while the other Medusozoa do so by splitting them off from the tip of the polyp. The traditional grouping of
ScyphozoaScyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
included the Staurozoa, but morphology and molecular phylogenetics indicate that Staurozoa are more closely related to Cubozoa (box jellies) than to other "Scyphozoa". Similarities in the double body walls of Staurozoa and the extinct
ConulariidaThe Conulata, also known as conulariids, form a poorly understood clade of extinct scyphozoan cnidarians.-Structure:The conulariids are fossils preserved as shell-like structures made up of rows of calcium phosphate rods, resembling an ice-cream cone with fourfold symmetry, usually four...
suggest that they are closely related. The position of Anthozoa nearest the beginning of the cnidarian family tree also implies that Anthozoa are the cnidarians most closely related to Bilateria, and this is supported by the fact that Anthozoa and Bilateria share some genes that determine the main axes of the body.
However in 2005 Katja Seipel and Volker Schmid suggested that cnidarians and ctenophores are simplified descendants of triploblastic animals, since ctenophores and the medusa stage of some cnidarians have
striated muscleStriated muscle tissue is a form of fibers that are combined into parallel fibers. More specifically, it can refer to:* Cardiac muscle .* Skeletal muscle* Branchiomeric muscles...
, which in bilaterians arises from the
mesodermIn all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...
. They did not commit themselves on whether bilaterians evolved from early cnidarians or from the hypothesized triploblastic ancestors of cnidarians.
In molecular phylogenetics analyses from 2005 onwards, important groups of developmental genes show the same variety in cnidarians as in
chordateChordates are animals which are either vertebrates or one of several closely related invertebrates. They are united by having, for at least some period of their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail...
s. In fact cnidarians, and especially
anthozoaAnthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
ns (sea anemones and corals), retain some genes that are present in
bacteriaBacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
,
protistProtists are a diverse group of eukaryotic microorganisms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy...
s,
plantPlants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
s and fungi but not in bilaterians.
The mitochondial genomes in the medusozoan cnidarians unlike that of other animals is linear with fragmented genes. The reason for this difference is unknown.
Interaction with humans
Jellyfish stings killed about 1,500 people in the 20th century, and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a
delicacyA delicacy is a food item that is considered highly desirable in certain cultures. Often this is because of unusual flavors or characteristics or because it is rare....
in eastern and southern
AsiaAsia is the world's largest and most populous continent, located primarily in the eastern and northern hemispheres. It covers 8.7% of the Earth's total surface area and with approximately 3.879 billion people, it hosts 60% of the world's current human population...
. Coral reefs have long been economically important as providers of fishing grounds, protectors of shore buildings against currents and tides, and more recently as centers of tourism. However, they are vulnerable to over-fishing, mining for construction materials,
pollutionPollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
, and damage caused by tourism.
Beaches protected from tides and storms by
coral reefCoral reefs are underwater structures made from calcium carbonate secreted by corals. Coral reefs are colonies of tiny living animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps...
s are often the best places for housing in tropical countries. Reefs are an important food source for low-technology fishing, both on the reefs themselves and in the adjacent seas. However despite their great
productivity400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...
reefs are vulnerable to over-fishing, because much of the organic carbon they produce is exhaled as
carbon dioxideCarbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
by organisms at the middle levels of the
food chainA food web depicts feeding connections in an ecological community. Ecologists can broadly lump all life forms into one of two categories called trophic levels: 1) the autotrophs, and 2) the heterotrophs...
and never reaches the larger species that are of interest to fishermen. Tourism centered on reefs provides much of the income of some tropical islands, attracting photographers, divers and sports fishermen. However human activities damage reefs in several ways: mining for construction materials;
pollutionPollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
, including large influxes of fresh water from
storm drainA storm drain, storm sewer , stormwater drain or drainage well system or simply a drain or drain system is designed to drain excess rain and ground water from paved streets, parking lots, sidewalks, and roofs. Storm drains vary in design from small residential dry wells to large municipal systems...
s; commercial fishing, including the use of
dynamiteDynamite is an explosive material based on nitroglycerin, initially using diatomaceous earth , or another absorbent substance such as powdered shells, clay, sawdust, or wood pulp. Dynamites using organic materials such as sawdust are less stable and such use has been generally discontinued...
to stun fish and the capture of young fish for
aquariumAn aquarium is a vivarium consisting of at least one transparent side in which water-dwelling plants or animals are kept. Fishkeepers use aquaria to keep fish, invertebrates, amphibians, marine mammals, turtles, and aquatic plants...
s; and tourist damage caused by boat anchors and the cumulative effect of walking on the reefs. Coral, mainly from the
Pacific OceanThe Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
has long been used in
jewelleryJewellery or jewelry is a form of personal adornment, such as brooches, rings, necklaces, earrings, and bracelets.With some exceptions, such as medical alert bracelets or military dog tags, jewellery normally differs from other items of personal adornment in that it has no other purpose than to...
, and demand rose sharply in the 1980s.
Some large
jellyfishJellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...
species have been used in
Chinese cuisineChinese cuisine is any of several styles originating in the regions of China, some of which have become highly popular in other parts of the world – from Asia to the Americas, Australia, Western Europe and Southern Africa...
at least since 200 AD, and are now fished in the seas around most of South East Asia.
JapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
is the largest single consumer of edible jellyfish, importing at first only from China but now from all of South East Asia as prices rose in the 1970s. This fishing industry is restricted to daylight hours and calm conditions in two short seasons, from March to May and August to November. The commercial value of jellyfish food products depends on the skill with which they are prepared, and "Jellyfish Masters" guard their
trade secretA trade secret is a formula, practice, process, design, instrument, pattern, or compilation of information which is not generally known or reasonably ascertainable, by which a business can obtain an economic advantage over competitors or customers...
s carefully. Jellyfish is very low in
cholesterolCholesterol is a complex isoprenoid. Specifically, it is a waxy steroid of fat that is produced in the liver or intestines. It is used to produce hormones and cell membranes and is transported in the blood plasma of all mammals. It is an essential structural component of mammalian cell membranes...
and
sugarSugar is a class of edible crystalline carbohydrates, mainly sucrose, lactose, and fructose, characterized by a sweet flavor.Sucrose in its refined form primarily comes from sugar cane and sugar beet...
s, but cheap preparation can introduce undesirable amounts of heavy metals.
The "sea wasp"
Chironex fleckeriChironex fleckeri, commonly known as sea wasp, is a species of Box jellyfish found in coastal waters from northern Australia and New Guinea north to the Philippines and Vietnam. It has been described as "the most lethal jellyfish in the world", with at least 63 known deaths from 1884 to...
has been described as the world's most venomous animal and is held responsible for 67 deaths, although it is difficult to identify the animal as it is almost transparent. Most stingings by
C. fleckeri cause only mild symptoms. Seven other box jellies can cause a set of symptoms called
Irukandji syndromeIrukandji syndrome is a condition that is induced by venomization through the sting of Carukia barnesi, a species of Irukandji jellyfish, and other cubozoans. The condition is rarely fatal, but if immediate medical action is not taken, within only 20 minutes the victim could go into cardiac arrest...
, which takes about 30 minutes to develop, and from a few hours to two weeks to disappear. Hospital treatment is usually required, and there have been a few deaths.
Books
- Arai, M.N. (1997). A Functional Biology of Scyphozoa. London: Chapman & Hall [p. 316]. ISBN 0-412-45110-7.
- Ax, P. (1999). Das System der Metazoa I. Ein Lehrbuch der phylogenetischen Systematik. Gustav Fischer, Stuttgart-Jena: Gustav Fischer. ISBN 3-437-30803-3.
- Barnes, R.S.K., P. Calow, P. J. W. Olive, D. W. Golding & J. I. Spicer (2001). The invertebrates—a synthesis. Oxford: Blackwell. 3rd edition [chapter 3.4.2, p. 54]. ISBN 0-632-04761-5.
- Brusca, R.C., G.J. Brusca (2003). Invertebrates. Sunderland, Mass.: Sinauer Associates. 2nd edition [chapter 8, p. 219]. ISBN 0-87893-097-3.
- Dalby, A.
Andrew Dalby is an English linguist, translator and historian who has written articles and several books on a wide range of topics including food history, language, Classical texts, and Wikipedia.-Education and early career:...
(2003). Food in the Ancient World: from A to Z. London: Routledge.
- Moore, J.(2001). An Introduction to the Invertebrates. Cambridge: Cambridge University Press [chapter 4, p. 30]. ISBN 0-521-77914-6.
- Schäfer, W. (1997). Cnidaria, Nesseltiere. In Rieger, W. (ed.) Spezielle Zoologie. Teil 1. Einzeller und Wirbellose Tiere. Stuttgart-Jena: Gustav Fischer. Spektrum Akademischer Verl., Heidelberg, 2004. ISBN 3-8274-1482-2.
- Werner, B. 4. Stamm Cnidaria. In: V. Gruner (ed.) Lehrbuch der speziellen Zoologie. Begr. von Kaestner. 2 Bde. Stuttgart-Jena: Gustav Fischer, Stuttgart-Jena. 1954, 1980, 1984, Spektrum Akad. Verl., Heidelberg-Berlin, 1993. 5th edition. ISBN 3-334-60474-8.
Journal articles
- D. Bridge, B. Schierwater, C. W. Cunningham, R. DeSalle R, L. W. Buss: Mitochondrial DNA structure and the molecular phylogeny of recent cnidaria classes. in: Proceedings of the Academy of Natural Sciences of Philadelphia. Philadelphia USA 89.1992, p. 8750.
- D. Bridge, C. W. Cunningham, R. DeSalle, L. W. Buss: Class-level relationships in the phylum Cnidaria—Molecular and morphological evidence. in: Molecular biology and evolution. Oxford University Press, Oxford 12.1995, p. 679.
- D. G. Fautin: Reproduction of Cnidaria. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1735. (PDF, online)
- G. O. Mackie: What's new in cnidarian biology? in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1649. (PDF, online)
- P. Schuchert: Phylogenetic analysis of the Cnidaria. in: Zeitschrift für zoologische Systematik und Evolutionsforschung. Paray, Hamburg-Berlin 31.1993, p. 161.
- G. Kass-Simon, A. A. Scappaticci Jr.: The behavioral and developmental physiology of nematocysts. in: Canadian Journal of Zoology. Ottawa Ont. 80.2002, p. 1772. (PDF, online)
External links