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Sea anemone

Sea anemone

Overview

Sea anemones are a group of water-dwelling, predatory
Predation
In 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...

 animals of the order
Order (biology)
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...

 Actiniaria; they are named after the anemone
Anemone
Anemone , is a genus of about 120 species of flowering plants in the buttercup family Ranunculaceae in the north and south temperate zones...

, a terrestrial flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

. Sea anemones are classified in the phylum Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyp
Polyp
A 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 that allow for digestion of larger prey and also lack a medusa
Medusa (biology)
In 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...

 stage. As cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

ns, sea anemones are closely related to coral
Coral
Corals 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, jellyfish
Jellyfish
Jellyfish 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...

, tube-dwelling anemone
Tube-dwelling anemone
Tube-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...

s, and Hydra
Hydra (genus)
Hydra 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...

.

A sea anemone is a polyp
Polyp
A 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...

 attached at the bottom to the surface beneath it by an adhesive foot, called a basal disc, with a column shaped body ending in an oral disc.
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Encyclopedia

Sea anemones are a group of water-dwelling, predatory
Predation
In 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...

 animals of the order
Order (biology)
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...

 Actiniaria; they are named after the anemone
Anemone
Anemone , is a genus of about 120 species of flowering plants in the buttercup family Ranunculaceae in the north and south temperate zones...

, a terrestrial flower
Flower
A flower, sometimes known as a bloom or blossom, is the reproductive structure found in flowering plants . The biological function of a flower is to effect reproduction, usually by providing a mechanism for the union of sperm with eggs...

. Sea anemones are classified in the phylum Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyp
Polyp
A 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 that allow for digestion of larger prey and also lack a medusa
Medusa (biology)
In 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...

 stage. As cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...

ns, sea anemones are closely related to coral
Coral
Corals 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, jellyfish
Jellyfish
Jellyfish 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...

, tube-dwelling anemone
Tube-dwelling anemone
Tube-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...

s, and Hydra
Hydra (genus)
Hydra 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...

.

Anatomy


A sea anemone is a polyp
Polyp
A 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...

 attached at the bottom to the surface beneath it by an adhesive foot, called a basal disc, with a column shaped body ending in an oral disc. Most are from 1.8 to 3 cm (0.708661417322835 to 1.2 in) in diameter, but anemones as small as 4 millimetre (0.15748031496063 in) or as large as nearly 2 metres (6.6 ft) are known. They can have anywhere from a few tens to a few hundred tentacles.

A few species are pelagic, and are not attached to the bottom; instead they have a gas chamber within the pedal disc, allowing them to float upside down in the water.

The mouth is in the middle of the oral disc surrounded by tentacles armed with many cnidocyte
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, which are cells that function as a defense and as a means to capture prey. Cnidocytes contain nematocyst, capsule-like organelle
Organelle
In cell biology, an organelle is a specialized subunit within a cell that has a specific function, and is usually separately enclosed within its own lipid bilayer....

s capable of everting, giving phylum Cnidaria its name. The cnidae that sting are called nematocysts
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...

. Each nematocyst contains a small vesicle filled with toxins (actinoporin
Actinotoxin
Actinotoxin is an uncharacterized toxin derived from extracts of the tentacles of sea anemones. It was used by M. Portier and Charles Richet in 1902 in their pioneering research into anaphlaxis....

s), an inner filament, and an external sensory hair. When the hair is touched it mechanically triggers the cell explosion, a harpoon-like structure which attaches to organisms that trigger it, and injects a dose of poison in the flesh of the aggressor or prey. This gives the anemone its characteristic sticky feeling. The sea anemone eats small fish and shrimp.

The poison is a mix of toxin
Toxin
A toxin is a poisonous substance produced within living cells or organisms; man-made substances created by artificial processes are thus excluded...

s, including neurotoxin
Neurotoxin
A neurotoxin is a toxin that acts specifically on nerve cells , usually by interacting with membrane proteins such as ion channels. Some sources are more general, and define the effect of neurotoxins as occurring at nerve tissue...

s, which paralyzes the prey and allows it to be moved to the mouth for digestion inside the gastrovascular cavity
Gastrovascular cavity
Gastro vascular cavity, as the name indicates, functions in both digestion and the distribution of nutrients and particles to all parts of the body...

. Actinoporins have been reported as highly toxic to fish and crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s, which are the natural prey of sea anemones. In addition to their role in predation, it has been suggested that actinoporins could act, when released in water, as repellents against potential predators. Anemonefish (clownfish), small banded fish in various colors, are not affected by their host anemone's sting and shelter themselves from predators within its tentacles.

The internal anatomy of anemones is quite complex.

Digestive system


There is a gastrovascular cavity (which functions as a stomach) with a single opening to the outside which functions as both a mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

 and an anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...

; waste and undigested matter is excreted through the mouth/anus, which can be described as an incomplete gut. The mouth is typically slit-like in shape, and bears a groove at one or both ends. The groove, termed a siphonophore, is ciliated, and helps to circulate water through the gastrovascular cavity. Some anemones feed on small particles, which are caught with the aid of a mucus secretion and moving currents that are set up by the tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...

s. Most sea anemones are predacious, immobilizing their prey with the aid of their nematocysts.

The mouth opens into a flattened pharynx
Pharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...

. This consists of an in-folding of the body wall, and is therefore lined by the animal's epidermis
Epidermis (zoology)
The Epidermis is an epithelium that covers the body of an eumetazoan . Eumetazoa have a cavity lined with a similar epithelium, the gastrodermis, which forms a boundary with the epidermis at the mouth.Sponges have no epithelium, and therefore no epidermis or gastrodermis...

. The pharynx typically runs for about two-thirds the length of the body before opening into the gastrovascular cavity that fills the remainder of the body.

The gastrovascular cavity itself is divided into a number of chambers by mesenteries
Mesentery
In anatomy, the mesentery is the double layer of peritoneum that suspends the jejunum and ileum from the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity.-Mesentery :The...

 radiating inwards from the body wall. Some of the mesenteries form complete partitions with a free edge at the base of the pharynx, to which they connect, but others reach only partway across. The mesenteries are usually found in multiples of twelve, and are symmetrically arranged around the central pharynx. They have stomach lining on both sides, separated by a thin layer of mesoglea
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...

, and includes filaments of tissue specialised for secreting digestive enzyme
Digestive enzyme
'Digestive enzymes' are enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption by the body. Digestive enzymes are found in the digestive tract of animals where they aid in the digestion of food as well as inside the cells,...

s. In some species these filaments extend below the lower margin of the mesentery, hanging free in the gastovascular cavity as acontial filaments.

Nerve system


A primitive nervous system, without centralization, coordinates the processes involved in maintaining homeostasis as well as biochemical and physical responses to various stimuli. There are no specialized sense organs.

The muscles and nerves are much simpler than those of most other animals, although more specialised than in other cnidarians, such as corals. Cells in the outer layer (epidermis) and the inner layer (gastrodermis
Gastrodermis
The gastrodermis is the inner layer of cells that lines a gastrovascular cavity of Cnidarians....

) have microfilaments that group into contractile fibers. These fibers are not true muscles because they are not freely suspended in the body cavity as they are in more developed animals. Longitudinal fibres are found in the tentacles and oral disc, and also within the mesenteries, where they can contract the whole length of the body. Circular fibers are found in the body wall and, in some species, around the oral disc, allowing the animal to retract its tentacles into a protective sphincter
Sphincter
A sphincter is an anatomical structure, or a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning...

.

Since the anemone lacks a skeleton, the contractile cells pull against the gastrovascular cavity, which acts as a hydrostatic skeleton. The anemone stabilizes itself by shutting its mouth, which keeps the gastrovascular cavity at a constant volume, making it more rigid. Although generally sessile
Sessility (zoology)
In 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...

, sea anemones are capable of slow movements using their pedal disc, or of swimming, using either their tentacles or by flexing their body.

Life cycle



Unlike other cnidarians, anemones (and other anthozoa
Anthozoa
Anthozoa 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) entirely lack the free-swimming medusa
Medusa (biology)
In 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...

 stage of the life cycle; the polyp
Polyp
A 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...

 produces eggs and sperm, and the fertilized egg develops into a planula
Planula
A 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...

 that develops directly into another polyp.

Anemones tend to stay in the same spot until conditions become unsuitable (prolonged dryness, for example), or a predator attacks them. In that case anemones can release themselves from the substrate and use flexing motions to swim to a new location. Most sea anemones attach temporarily to submerged objects; a few thrust themselves into the sand or live in burrows; a few are parasitic on other marine organisms and some have symbiotic
Symbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

 relationships with hermit crab
Hermit crab
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...

s.

The sexes in sea anemones are separate in some species, while other species, like the brooding anemone (Epiactis prolifera
Epiactis prolifera
Epiactis prolifera, the brooding, proliferating or small green anemone, is a species of marine invertebrate in the family Actiniidae...

), are protandric hermaphrodite
Hermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

s. The gonad
Gonad
The 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 are strips of tissue within the mesenteries. Both sexual
Sexual reproduction
Sexual 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...

 and asexual reproduction
Asexual reproduction
Asexual 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...

 can occur. In sexual reproduction males release sperm to stimulate females to release eggs, and fertilization occurs. Anemones eject eggs and sperm through the mouth. The fertilized egg develops into a planula
Planula
A 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...

, which settles and grows into a single polyp
Polyp
A 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...

.

Anemones can also reproduce asexually, by budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...

, binary fission (the polyp separates into two halves), and pedal laceration, in which small pieces of the pedal disc
Pedal disc
The pedal disc is, in anatomy of the sea anemone, the surface opposite to the mouth, and generally serves to attach the anemone to the substrate, or hard surface, upon which it lives. It is composed of a thin tissue plate and is used by the animal to adhere to and move across the surface....

 break off and regenerate into small anemones.

Ecology



The sea anemone has a pedal disc, which the organism uses to attach itself to rocks or which it anchors in the sand. Others also burrow into a stronger object. Some species attach to kelp while others are free-swimming. Although not plant
Plant
Plants 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 therefore incapable of photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

 themselves, many sea anemones form an important facultative symbiotic relationship with certain single-celled
Cell (biology)
The 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....

 green algae
Green algae
The green algae are the large group of algae from which the embryophytes emerged. As such, they form a paraphyletic group, although the group including both green algae and embryophytes is monophyletic...

 species which reside in the animals' gastrodermal cells. These algae may be either zooxanthella
Zooxanthella
Zooxanthellae are flagellate protozoa that are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the scleractinian corals and the tropical sea anemone, Aiptasia....

e, zoochlorella
Zoochlorella
Zoochlorella is a genus of alga comprising one species, Z. parasitica. The term Zoochlorella is sometimes used to refer to any green alga that lives symbiotically within the body of a freshwater or marine invertebrate or protozoan...

e or both. The sea anemone benefits from the products of the algae's photosynthesis, namely oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 and food in the form of glycerol
Glycerol
Glycerol is a simple polyol compound. It is a colorless, odorless, viscous liquid that is widely used in pharmaceutical formulations. Glycerol has three hydroxyl groups that are responsible for its solubility in water and its hygroscopic nature. The glycerol backbone is central to all lipids...

, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 and alanine
Alanine
Alanine is an α-amino acid with the chemical formula CH3CHCOOH. The L-isomer is one of the 20 amino acids encoded by the genetic code. Its codons are GCU, GCC, GCA, and GCG. It is classified as a nonpolar amino acid...

; the algae in turn are assured a reliable exposure to sunlight and protection from micro-feeders, which the sea anemones actively maintain. The algae also benefit by being protected due to the presence of stinging cells called nematocysts, reducing the likelihood of being eaten by herbivores. Most species inhabit tropical reefs, although there are species adapted to relatively cold waters, intertidal reefs, and sand/kelp environments.

Exploitation



The global trade of marine ornamentals has been a rapidly expanding industry involving numerous countries worldwide. In the early 1980s, the estimated value of imported marine fish and invertebrates was US$24–40 million annually. Current estimates place that value at US$200–330 million, with the United States accounting for 80% of the industry imports. Despite advances and the expansion of aquaculture
Aquaculture
Aquaculture, also known as aquafarming, is the farming of aquatic organisms such as fish, crustaceans, molluscs and aquatic plants. Aquaculture involves cultivating freshwater and saltwater populations under controlled conditions, and can be contrasted with commercial fishing, which is the...

, post-larval capture and rearing, the majority of marine ornamentals are collected in the wild as adults or juveniles. Anemones are susceptible to overexploitation due to their long life spans, slower relative growth rates, and lower reproductive rates than their resident fish, which are also affected due to the fact that they settle exclusively and are restricted to specific host sea anemones. The demand for these organisms is reflected in fishermen's catch records, which document the value they are paid per catch, and on average sea anemones were valued at five times the average value of anemonefish, and ten times the value of the most abundant anemonefish, and in fact only made up 4.1% of the total value of the catch. Research has shown that aquarium fishing activities significantly impact the populations of anemones and anemonefish by drastically reducing the densities of each in exploited areas, and could also negatively impact anemone shrimp, and any organisms obligately associated with sea anemones. It should be noted that anemonefish can survive alone in captivity, as has been shown by multiple research efforts.

Fossil record


Most Actiniaria do not form hard parts that can be recognized as fossils but a few fossils do exist; Mackenzia, from the Middle Cambrian Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...

of Canada, is the oldest fossil identified as a sea anemone.

External links