Hydrozoa

	Email		Print
	Bookmark		Link

Hydrozoa

Hydrozoa (hydrozoans) are a taxonomic class

Class (biology)

A class is the taxonomic rank in the biological classification of organisms in biology below phylum and above Order .The orders of taxonomy are life, Domain , kingdom , phylum, class , order , family , genus, and species....
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. Hydrozoans are related to jellyfish

Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. They have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa ....
and corals and belong to the phylum Cnidaria

Cnidaria

Cnidaria Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla....
.

Some examples of hydrozoans are the Freshwater Jelly (Craspedacusta sowerbyi), the freshwater polyps (Hydra
Hydra (genus)

Hydra is a genus of simple fresh-water animals possessing symmetry #Radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa....
), Obelia
Obelia

Obelia 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....
, the Portuguese Man o' War

Portuguese Man o' War

The Portuguese Man o' War , also known as the blue bubble, blue bottle, man-of-war, or the Portuguese man of war, is a jelly-like, marine invertebrate of the family: Physaliidae, order: Siphonophora, class: Hydrozoa, and Phylum: Cnidaria....
(Physalia physalis), the chondrophore

Chondrophore

The 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 Though it derives from an outdated name for this lineage , some still find the term "chondrophore" useful as a synonym to "porpitid" in discussions of the three genera contained herein....
s (Porpitidae), "air fern

Air fern

Air fern or "Neptune plant" is a name given to a product that is in fact composed of a species of marine animal called Sertularia argentea, also known as the "sea fir"....
" (Sertularia argenta) and the pink-hearted hydroids (Tubularia).

ozoan systematics

Systematics

Biological systematics is the study of the diversification of life on the planet Earth, both past and present, and the relationships among living things through time....
is highly complex.

Discussion

Ask a question about 'Hydrozoa'

Start a new discussion about 'Hydrozoa'

Answer questions from other users

Full Discussion Forum

Encyclopedia

Hydrozoa (hydrozoans) are a taxonomic class

Class (biology)

Jellyfish

Cnidaria

Portuguese Man o' War

Chondrophore

Air fern

Systematics

Portuguese Man O War (physalia Physalis)

Hydrozoan systematics

Systematics

Biological systematics is the study of the diversification of life on the planet Earth, both past and present, and the relationships among living things through time....
is highly complex. Several approaches for expressing their interrelationships were proposed and heavily contested since the late 19th century, but in more recent times a consensus seems to be emerging.

For long, the hydrozoans were divided into a number of order

Order (biology)

In Biological classification used in biology, the order is a taxonomic rank between class and family . The superorder is a rank between class and order....
s, according to their mode of growth and reproduction. Most famous among these was probably the assemblage called "Hydroida

Hydroida

Hydroida is an obsolete cnidarian order which united such animals as hydra , hydromedusae, and many marine attached Hydrozoa. However, the group is paraphyletic and not composed from close relatives....
", but this group is apparently paraphyletic, united by plesiomorphic (ancestral) traits. Other such orders were the Anthoathecatae, Actinulidae, Laingiomedusae, Polypodiozoa, Siphonophora

Siphonophora

Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....
and Trachylina.

As far as can be told from the molecular and morphological

Morphology (biology)

The term morphology in biology refers to form, structure and configuration of an organism. This includes aspects of the outward appearance as well as the form and structure of the internal parts like bones and organs....
data at hand, the Siphonophora for example were just highly specialized "hydroids," whereas the Limnomedusae - presumed to be a "hydroid" suborder - were simply very primitive hydrozoans and not closely related to the other "hydroids." Therefore, today the hydrozoans are at least tentatively divided into two subclass

Subclass

Subclass may refer to:* Subclass , a taxonomic rank intermediate between class and superorder* Subclass , a class that is derived from another class or classes...
es, the Leptolinae

Leptolinae

Leptolinae are a cnidarian subclass of the Hydrozoa. They contain the bulk of the paraphyletic "Hydroida" which were one of the main groupings of the Hydrozoa in older classifications and were placed at order rank....
(containing the bulk of the former "Hydroida" and the Siphonophora) and the Trachylinae

Trachylinae

Trachylinae are a cnidarian subclass of the Hydrozoa. They are placed at order rank in many older classifications, and limited to contain the Narcomedusae and Trachymedusae....
, containing the others (including the Limnomedusae). The monophyly

Monophyly

In common cladistic usage, a monophyletic group is a clade, consisting of an ancestor and all its descendants. The term is synonymous with the uncommon term holophyly....
of several of the presumed orders in each subclass is still in need of verification.

In any case, according to this classification, the hydrozoans can be subdivided as follows, with taxon

Taxon

A taxon or taxonomic unit is a name designating an organism or a group of organisms. In biological nomenclature according to Carl Linnaeus, a taxon is assigned a taxonomic rank and can be placed at a particular level in a systematic hierarchy reflecting evolutionary relationships....
names emended to end in "-ae":

CLASS HYDROZOA

Subclass Leptolinae
Leptolinae

Leptolinae are a cnidarian subclass of the Hydrozoa. They contain the bulk of the paraphyletic "Hydroida" which were one of the main groupings of the Hydrozoa in older classifications and were placed at order rank....

Order Anthomedusae
Anthomedusae

Anthomedusae or Anthomedusa, the athecate hydroids, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....
(= Anthoathecata(e), Athecata(e), Stylasterina(e)) - includes Laingoimedusae but monophyly requires verification
Order Leptomedusae
Leptomedusae

Leptomedusae or Leptomedusa, commonly called thecate hydroids, are a cnidarian order in the subclass Leptolinae. They were formerly placed at suborder rank in the paraphyletic "Hydroida"....
(= Leptothecata(e), Thecaphora(e), Thecata(e))
Order Siphonophorae

Subclass Trachylinae
Trachylinae

Trachylinae are a cnidarian subclass of the Hydrozoa. They are placed at order rank in many older classifications, and limited to contain the Narcomedusae and Trachymedusae....

Order Actinulidae
Order Limnomedusae - monophyly requires verification; tentatively placed here
Order Narcomedusae
Order Trachymedusae
Trachymedusae

Trachymedusae are an order in the cnidarian class Hydrozoa. They contain some 50 species, divided among about 30 genera and 5 family , of which the Rhopalonematidae are by far the most diverse....
- monophyly requires verification

ITIS uses the same system but unlike here does not use the oldest available names for many groups.

In addition, there exists a weird cnidarian parasite, Polypodium hydriforme, which lives inside its host's cell

Cell (biology)

The cell is the structural and functional unit of all known Life organisms. It is the smallest unit of an organism that is classified as living, and is often called the building bricks of life....
s. It is sometimes placed in the Hydrozoa, but actually its relationships are better treated as unresolved for the time being - a somewhat controversial 18S rRNA sequence

DNA sequence

A DNA sequence or genetic sequence is a succession of letters representing the primary structure of a real or hypothetical DNA molecule or strand, with the capacity to carry information as described by the central dogma of molecular biology....
analysis found it to be closer to Myxozoa

Myxozoa

The Myxozoa are a group of parasite 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....
. It was traditionally placed in its own class

Class (biology)

A class is the taxonomic rank in the biological classification of organisms in biology below phylum and above Order .The orders of taxonomy are life, Domain , kingdom , phylum, class , order , family , genus, and species....
Polypodiozoa and this view is presently often seen to reflect the uncertainties surrounding this highly distinct animal.

Other classifications

Some of the more widespread classification systems for the Hydrozoa are listed below. Though they are often found in seemingly authoritative Internet sources and databases, they do not agree witnh the currently available data. Especially the presumed phylogenetic distinctness of the Siphonophora

Siphonophora

Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....
is a major flaw that was corrected only recently.

The obsolete classification mentioned above was as follows:

Order Actinulidae
Order Anthoathecatae
Order Hydroida
Hydroida

Hydroida is an obsolete cnidarian order which united such animals as hydra , hydromedusae, and many marine attached Hydrozoa. However, the group is paraphyletic and not composed from close relatives....

Suborder Anthomedusae
Anthomedusae

Anthomedusae or Anthomedusa, the athecate hydroids, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....
Suborder Leptomedusae
Leptomedusae

Leptomedusae or Leptomedusa, commonly called thecate hydroids, are a cnidarian order in the subclass Leptolinae. They were formerly placed at suborder rank in the paraphyletic "Hydroida"....
Suborder Limnomedusae

Order Laingiomedusae
Order Polypodiozoa
Order Siphonophora
Siphonophora

Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....
Order Trachylina

Suborder Narcomedusae
Suborder Trachymedusae
Trachymedusae

Trachymedusae are an order in the cnidarian class Hydrozoa. They contain some 50 species, divided among about 30 genera and 5 family , of which the Rhopalonematidae are by far the most diverse....

A very old classification that is sometimes still seen is:

Order Hydroida
Hydroida

Hydroida is an obsolete cnidarian order which united such animals as hydra , hydromedusae, and many marine attached Hydrozoa. However, the group is paraphyletic and not composed from close relatives....
Order Milleporina
Order Siphonophorida
Order Stylasterina (= Anthomedusae)
Order Trachylinida

Catalogue of Life

The Catalogue of Life is planned to become a comprehensive catalogue of all known species of organisms on Earth by the year 2011. Rapid progress has been made recently and this, the eighth edition of the Annual Checklist, contains 1,105,589 species....
uses the following:

Order Actinulida
Order Anthoathecata (= Anthomedusae)
Order Hydroida
Hydroida

Hydroida is an obsolete cnidarian order which united such animals as hydra , hydromedusae, and many marine attached Hydrozoa. However, the group is paraphyletic and not composed from close relatives....

* Order Laingiomedusae

Order Leptothecata (= Leptomedusae)
Order Limnomedusae
Order Narcomedusae
Order Siphonophora
Siphonophora

Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....
Order Trachymedusae
Trachymedusae

Trachymedusae are an order in the cnidarian class Hydrozoa. They contain some 50 species, divided among about 30 genera and 5 family , of which the Rhopalonematidae are by far the most diverse....

Animal Diversity Web

Animal Diversity Web is an online database that collects the natural history, Scientific classification, species characteristics, conservation biology, and Range information of thousands of species of animals....
uses the following:

Order Actinulida
Order Capitata
Order Chondrophora
Order Filifera
Order Hydroida
Hydroida

Hydroida is an obsolete cnidarian order which united such animals as hydra , hydromedusae, and many marine attached Hydrozoa. However, the group is paraphyletic and not composed from close relatives....
Order Siphonophora
Siphonophora

Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....

Hydra, a freshwater genus

The most widely-known and researched freshwater hydrozoan is Hydra

Hydra (genus)

Hydra is a genus of simple fresh-water animals possessing symmetry #Radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa....
, which is found in slow-moving waters.

Hydra has a pedal disc composed of gland cells that helps it attach to substrates, and like all cnidarians uses nematocysts, or "stinging cells," to disable its prey. Hydra eat small crustacean

Crustacean

Crustaceans are a large group of arthropods, comprising almost 52,000 described species , and are usually treated as a subphylum . They include various familiar animals, such as crabs, lobsters, crayfish, shrimp, krill and barnacles....
s (such as brine shrimp), insect

Insect

Insects are the biggest class of arthropods and the only ones with wings. They are the most diverse group of animals on the planet. They are most diverse at the equator and their diversity declines toward the poles....
larva

Larva

A larva is a young form of animal with indirect developmental biology, going through or undergoing metamorphosis .The larva can look completely different from the adult form, for example, a caterpillar differs from a butterfly....
e, and annelid

Annelid

The annelids, collectively called Annelida , are a large Scientific classification of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches....
worms. Hydra may reproduce sexually, through the spawning of sperm (and thus insemination of eggs on the female body column), or through asexual reproduction

Asexual reproduction

Asexual reproduction is reproduction which does not involve meiosis, ploidy reduction, or fertilization. Only one parent is involved in asexual reproduction....
(budding).

Life cycles

Some colonial hydrozoans have both a medusa

Medusa (biology)

In biology, a medusa is a form of cnidarian in which the body is shortened on its principal axis and broadened, sometimes greatly, in contrast with polyps....
stage and a polyp

Polyp

In zoology, a polyp is one of two forms of individuals found in many species of cnidarians. The two are the polyp or hydroid and the medusa . Polyps are approximately cylindrical, elongated on the axis of the body....
stage in their life cycle (but more hydrozoan species do not have the medusa stage). Each colony has a base, a stalk, and one or more polyps. Hydroid

Hydroid

Hydroid may refer to:...
colonies are usually dioecious

Dioecious

Dioecious - from Greek language for "two households" - refers to species of seed-bearing plants having separate male and female plants. That is, no individual plant of the species produces both microspores and megaspores; individual plants are either male or female ....
, which means that they have separate sexes - all the polyps in each colony are either male or female, but not usually both sexes in the same colony. Hydrozoan colonies are composed of a number of specialized polyps (or "zooids") - including feeding, reproductive, and sometimes, protective zooids. In some species, the reproductive polyps, known as gonozooids (or "gonotheca" in thecate hydrozoans

Leptomedusae

Leptomedusae or Leptomedusa, commonly called thecate hydroids, are a cnidarian order in the subclass Leptolinae. They were formerly placed at suborder rank in the paraphyletic "Hydroida"....
) bud off asexually-produced medusae. These tiny, new medusae (which are either male or female) mature and spawn, releasing gamete

Gamete

A gamete is a Cell that fuses with another gamete during fertilization in organisms that sexual reproduction. In species which produce two morphologically distinct types of gametes, and in which each individual produces only one type, a female is any individual which produces the larger type of gamete?called an ovum ?and a male produces th...
s freely into the sea in most cases. Zygote

Zygote

A zygote is a cell that is the result of fertilization. That is, two ploidy cells—usually an ovum from a female and a sperm cell from a male—merge into a single ploidy cell called the zygote ....
s become free-swimming planula larvae

Planula

A planula is the free-swimming, flattened, cilium, symmetry #Bilateral symmetry larva of various cnidarian species. In all cases, the planula forms directly 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....
or actinula larvae that either settle on a suitable substrate

Substrate (biology)

In biology a substrate is the surface a plant or animal lives upon. The substrate can include biotic or abiotic materials. For example, encrusting algae that lives on a rock can be substrate for another animal that lives above the algae on the rock....
(in the case of planulae), or swim and develop into another medusae or polyp directly (actinulae). Colonial hydrozoans include siphonophore colonies, Hydractinia, Obelia
Obelia

Obelia 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....
, and many others.

The medusa stage, if present, is the sexually-reproductive life cycle phase (that is, in hydrozoan species that have both polyp and medusa generations). Medusae of these species of Hydrozoa are known as "hydromedusae

Anthomedusae

Anthomedusae or Anthomedusa, the athecate hydroids, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria....
". Most hydromedusae have shorter life spans than the larger scyphozoa

Scyphozoa

Scyphozoa is a class of jellyfish.Scyphozoans are members of the Phylum Cnidaria and are referred to as the "true jellyfish". Scyphozoans range from the Ediacarian time period through the Recent....
n jellyfish. Some species of hydromedusae release gametes shortly after they are themselves released from the hydroids (as in the case of fire coral

Fire coral

Fire corals are colonial marine organisms that look rather like real coral. However they are technically not corals; they are actually more closely related to jellyfish and other stinging Sea anemone....
s), living only a few hours, while other species of hydromedusae grow and feed in the plankton

Plankton

Plankton consist of any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. Plankton are defined by their ecological niche rather than their Phylogenetics or taxonomy classification....
for months, spawning daily for many days before their supply of food or other water conditions deteriorate and cause their demise.

Hydrozoa

Systematics

Other classifications

Hydra, a freshwater genus

Life cycles

Footnotes

External links