Hydrozoa

(invertebrate zoology) A class of the phylum Cnidaria which includes the fresh-water hydras, the marine hydroids, many small jellyfish, a few corals, and the Portuguese man-of-war.

(Hydroids)

Phylum: Cnidaria

Class: Hydrozoa

Number of families: 114

Thumbnail description
Invertebrates with a body plan that is comprised of a medusa with velum, a muscular projection from the subumbrellar margin that partially closes the subumbrellar cavity, and polyps; life cycles always involve the presence of a planula larva

Evolution and systematics

Considered a superclass by some experts, this group has representatives since the Precambrian (velellids). Because of the rarity of skeletal structures, however, the fossil record is fragmentary. Recent interpretations of the medusary nodule (the structure that, from a polyp, gives rise to a medusa by a special type of budding) suggest that the Hydrozoa are triploblastic. The subumbrellar cavity of the hydromedusae, and the layer of striated muscle that lines it, are originated by a morphogenetic pattern very similar to schizocoely, with a third layer of tissue that is formed between ecto- and endo-derm and that becomes hollow. The subumbrellar cavity, therefore, is interpreted as a coelom that later becomes open, with the origin of the velar opening. Molecular evidence is now available, showing, for instance, that the genes coding for mesodermic structures in the Bilateria is also present in the Hydrozoa. The polyp stage, thus, is diploblastic, whereas the medusae are triploblastic. If this view is confirmed by further evidence, the transition from diploblasts to triploblasts occurs every time a medusa is budded from a polyp, and one of the great mysteries of metazoan evolution is solved.

Hydrozoa (characterized by medusae with direct development or produced by lateral budding from polyps) are comprised in the subphylum Medusozoa, including also the Scyphozoa (characterized by medusa production by strobilation from polyps), and the Cubozoa (characterized by medusa production by complete metamorphosis of the polyp).

The classes of the superclass Hydrozoa include Automedusa, with the subclasses Actinulidae (one order, two families), Narcomedusae (one order, three families), and Trachymedusae (one order, five families); and Hydroidomedusa, with the subclasses Anthomedusae (two orders, 49 families), Laingiomedusae (one order, one family), Leptomedusae (two orders, 34 families), Limnomedusae (one order, three families), Siphonophorae (three orders, 16 families), and Polypodiozoa (one order, one family).

Physical characteristics

The Automedusa class is represented only by medusae; there are no polyp stages. Development is usually direct, sexes are separated; each fertilized egg leads to a planula that develops into a single medusa, except in some Narcomedusae, in which parasitic stages issued from the egg may give rise to several medusae by asexual budding. Medusae are not formed through a medusary nodule: the subumbrellar cavity and velum are formed by folding and deepening of the oral embryonic ectoderm, being only analogous to the subumbrellar cavity and velum of the Hydroidomedusa. The primary marginal tentacles are always formed before the subumbrellar cavity and the gastro vascular system. The marginal tentacles are deprived of tentacular bulbs. Sensory organs are in the form of ecto-endodermal statocysts, with an endodermal axis, growing out from the circular canal, with sensory cells characterized by numerous kinocilium-lacking rootlets, surrounded by stereocilia, innervated by the upper nerve ring; statoliths are of endodermal origin. Asexual reproduction is present only in "actinula"-like larvae and adults of Narcomedusae. The Actinulidae are all members of the interstitial fauna; they look like actinula-larvae, and the statocysts are the most distinctive medusan feature of these highly specialized medusae. The Narcomedusae have a flattened exumbrella, with lobed margin, incised by deep grooves. Usually with no radial canals, gametes are carried on the wide manubrium. Their intermediate tentaculated post-embryonic stages are juvenile medusae and are inappropriately called "actinulae," like the juvenile polyps of some Anthomedusae. The Trachymedusae have a bell-shaped umbrella, with circular and radial canals. Gametes ripen on the radial canals. The manubrium is often on a peduncle.

The Hydroidomedusa class is represented by a succession of three stages during indirect development. The planula is a ciliated motile gastrula; it typically develops into a benthic, modular, larval stage, polyp or hydroid (except in the Porpitidae, Margelopsis and Pelagohydra, where the hydroid is floating). Hydroids can be solitary, but generally form modular colonies by simple budding. Polyps can be specialized for different functions (defensive dactylozooids, reproductive gonozooids, nutritive gastrozooids, etc.). Polyps give rise, by asexual budding, to planktonic, free-swimming, and solitary hydromedusae, representing the sexual adult. The sense organs of pelagic hydroidomedusae, when present, are ocelli (Anthomedusae, some Leptomedusae), or statocysts (Leptomedusae, Limnomedusae); sometimes cordyli of unknown function are also present (Leptomedusae). The Siphonophores have no visible sense organs. Medusae are often reduced to sporosacs (fixed gonophores), so that hydroids, by paedomorphosis, secondarily become the sexual stages. The Hydroidomedusa may also form pelagic, highly polymorphic colonies (Siphonophores). Medusa budding occurs via a medusary nodule or entocodon, forming a coelomlike cavity, the subumbrellar cavity, lined by striated muscle cells; primary marginal tentacles always develop after the subumbrellar cavity and the gastro vascular system. Both embryonic and larval stages, the planula and the polyp, are typically diploblastic; the adult sexual stages, the hydromedusae, acquire a triploblastic kind of organization during embryonic development (medusary nodule formation). Hydroidomedusae are frequently seasonal; the hydroid stage may develop several types of resting stages (frustules, propagules, cysts, stolon system) to overcome unfavorable ecological conditions.

The hydroids of the subclass Anthomedusae do not have a protective perisarc sheath around the polyps and are said athecates; they are usually colonial (but the most famous hydroid,

Hydra, is a solitary paedomorphic anthomedusa). The colonies can be either monomorphic or polymorphic, while the structure of the tentacles is in two forms: filiform tentacles do not present particular aggregations of cnidocysts, whereas capitate tentacles have cnidocyst knobs. The medusae are typically bell shaped; their gonads (aggregates of gametes commonly referred to as gonads) are confined on manubrium, sometimes extending on the most proximal parts of the radial canals. Their marginal sense organs, if present, are ocelli; the marginal tentacles are peripheral, hollow, or solid, usually with tentacular bulbs; sexual reproduction occurs through a complex planula. The hydroids of the subclass Laingiomedusae are unknown. The medusae have an almost hemispherical umbrella, with lobed margin, divided by peronial grooves or similar structures. The radial canals are four; there is no typical circular canal but a solid core of endodermal cells around the umbrella margin. The tentacles are solid, inserted above the exumbrellar margin, on the exumbrella. The manubrium is simple, quadrangular, tubular, or conical; the mouth opening is simple, quadrangular to circular; gametes are in four masses on the manubrium or as epidermal lining of interradial pockets of the manubrium. The hydroids of the subclass Leptomedusae are thecate: all parts of the colonies are typically protected by a chitinous perisarcal structure. The hydranth is protected by a hydrotheca, the nematophore by a nematotheca, and the gonophore by a gonotheca. Rarely, hydranths are naked. The medusae are typically with hemispherical or flattened umbrella; the masses of gametes are confined to radial canals, exceptionally extending onto the proximal part of manubrium; when present, the marginal sense organs are ectodermal velar statocysts, rarely cordyli, occasionally adaxial ocelli. The marginal tentacles are peripheral and hollow (except in Obelia), with tentacular bulbs. Sexual reproduction occurs through a complex planula. The hydroids of the subclass Limnomedusae are very simple; solitary or colonial; small, sessile; with or without tentacles; often close to planula structure and budding planula-like structures or frustules; there are no perisarcal thecae, but cysts and stolons are covered by chitin. The medusae usually have gamete masses along the radial canals or, exceptionally, on the manubrium. The marginal tentacles are peripheral, hollow, without a true basal bulb; their base is usually with a parenchymatic endodermal core embedded in umbrellar mesoglea. The marginal sense organs are internal, enclosed ecto-endodermal statocysts that are embedded in the mesoglea near the ring canal or in the velum. Exceptionally, medusae can be reduced medusoids. Sexual reproduction leads to simple planulae, without embryonic glandular cells.

The subclass Siphonophorae comprises generally pelagic, free-swimming, or floating species, forming highly polymorphic modular colonies of polypoid and medusoid zooids attached to a stem or stolon supported by a floating and swimming system or nectosome (pneumatophores and nectophores).

The Polypodiozoa class is represented by a single species, Polypodium hydriforme Ussow, 1885, the only known metazoan adapted to intracellular parasitism. Polypodium has a unique lifecycle, having a succession of a free-living stage and of an intracellular parasitic stage of some Acipenseridae and Polyodontidae eggs. The earliest parasitic stage known is a binucleate cell observed in previtellogenetic fish oocytes. Further parasitic development leads to a didermic stolonal structure, with inverted germ layers, growing at the expense of the egg's yolk and forming numerous inverted buds. Before becoming free at fish spawning, eversion takes place and the germ layers take their normal position (ectoderm out, endoderm inside). Once liberated, the stolon becomes fragmented into individual buds, each originating a free-creeping globular stage that can multiply by longitudinal fission. These stages can move and feed, having an oral mouth-cone and tentacles. Germ cells are endodermal; the females have two kinds of gonads, each with a gonoduct opening in the gastral cavity; the gonads of the males form gametophores carrying cnidocysts. The free-living stage presumably represents the sexual medusae, the parasitic stages being considered as polypoid.

They differ, however, from all other Hydrozoa by unique features: bilateral symmetry; presence of gonoducts; aberrant gametogenesis; unique structure of cnidocil apparatus; inversion of germ layers during parasitic life; and the complete separation of epidermal and muscle cells.

The umbrella of Hydroido- and Automedusae generally measures between 0.02 in (0.5 mm) and 2 in (50 mm), but in numerous species the size may be greater, reaching 3.9–7.8 in (100–200 mm) (Aequorea) and even, exceptionally, 15.7 in (400 mm) of diameter (Rhacostoma atlanticum). The colonies of the Hydroidomedusae usually have a reduced size. Most of them do not exceed a few inches (centimeters to a few decimeters) (i.e., Cladocarpus lignosus is 27.5 in [70 cm]); the hydranths are usually very tenuous, not exceeding a few inches (millimeters), but there are exceptions (i.e., Hydrocoryne miurensis: 2.3 in [6 cm]; Corymorpha nutans: 4.7 in [12 cm]; Monocoryne gigantea: 15.7 in [40 cm]; Candelabrum penola: 33.4 in [85 cm]; and Branchiocerianthus imperator: more than 6.5 ft [2 m]). The smallest polyps are those of the Microhydrulidae (Limnomedusae): they are reduced to a spherical or irregular body, ranging from 20 to 480 µm. The longest siphonophore is Apolemia uvaria, with colonies reaching 98.4 ft (30 m). The medusae of the hydrozoa are usually diaphanous, as are their polyps. Colored species, however, are frequent. Pigments derive from the diet or are produced directly. The most common color is reddish, deriving from crustaceans; other colors for medusae can be green, white, or orange; whereas polyps can be, according to the species, reddish, pink, white, or blue.

The medusae have typical and easily recognizable body architecture; the main feature that distinguishes them from other medusae is the velum. For this reason, they have also been called Craspedotae (with velum), as opposed to the medusae of the Scypozoa and Cubozoa, which were called Acraspedae (without velum). The polyps are quite varied in architecture, ranging from coral-like colonies (Millepora and the stylasterids) to gigantic polyps resembling those of the Anthozoa (Branchiocerianthus) to microscopic polyps reduced to a simple ball of tissue (Microhydrula).

Distribution

The Hydrozoa are cosmopolitan, and can be found in all water masses of the world, both in marine and in fresh waters. The Hydrozoa are known since the beginning of the modern study of animals; many species are Linnean and were described in the eighteenth century.

Habitat

The medusae of the Hydrozoa and the siphonophores are mostly planktonic; they are seasonal in occurrence and can be present in swarms, transported by the currents. Some medusae and some siphonophores, however, can be benthic. The polyp stages are usually benthic and live attached to the bottom, even though some species can be planktonic, such as the well-known Velella velella.

The Hydrozoa occur in all aquatic habitats, from anchialine caves to deep-sea trenches, from lakes and ponds to rocky coasts and the interstices among sand grains. The polyp stages of many species live exclusively on certain types of substrate, usually other organisms such as fishes, tunicates, polychaetes, bryozoans, mollusks, crustaceans, sponges, cnidarians, algae, sea-grasses, etc., with which they have symbiotic relationships ranging from simple epibiosis to commensalisms, mutualism, and parasitism.

Hydrozoa are mostly carnivores, using their habitat to acquire favorable positions to catch their prey. Planktonic stages are transported by currents, but can also move within water masses, searching for food. The position of benthic forms is decided by the planula, at the moment of settlement. Colonies are positioned at locations that will ensure a supply of new, fresh water around the hydranths, enhancing the transport of potential prey.

Behavior

The medusae are sharply individual; they can be gathered by winds and currents to form extensive swarms, but it is not known if they have any kind of social interaction while being in close contact. The colonies of Hydrozoa, especially polymorphic ones, have been compared to superorganisms because of the complexity of the functions that they perform through the different types of zooids. The zooids of a colony usually derive from a single planula, thus being interconnected members of a single clone. It is possible, however, that different colonies merge their tissues or that different planulae aggregate to form coalescent colonies. In these cases, different individuals can be in such close connection that they become a single individual, possibly one of the extreme forms of social organization.

Most Hydrozoa have separate sexes. Fertilization is usually internal, with no copulation. Males spawn in the water and the sperm actively swim toward the eggs that are still on the maternal body (either a medusa or a polyp colony). The Hydrozoa are the first animals in which sperm attractants have been demonstrated, with species-specific attraction of sperms by the eggs. For many medusan species, both males and females spawn in the water, where fertilization occurs. Also in this case, however, sperm attractants facilitate gamete encounter.

The members of the same colony perform coordinated behaviors that surely involve communication. In Thecocodium brieni, for instance, the dactylozooids catch the prey with their tentacles, while the gastrozooids perceive that a prey is available and stretch towards the dactylozooids, detaching the prey from their tentacles and ingesting it. This division of labor, involving great coordination, is frequent in polymorphic colonies.

Planktonic animals do not show particular territoriality, being, by definition, transported by currents. It is probable, however, that medusae actively prevent being in too close contact to avoid competitive interactions while foraging. Territoriality is very strong in sessile organisms, where competition for the occupation of space is very evident. The arrangement of dactylozooids on the edge of many colonies is related to the defense of the territory from overgrowth by nearby animals. The feeding polyps can eat the settling larvae of potentially competing species, thus preventing competition for space.

When unfed, both polyps and medusae are always in search for food, with species-specific activity patterns. When the coelenteron is full of food, tentacles are usually contracted and do not catch prey, showing some control of cnidocyst discharge. Many medusae perform daily migrations through the water column.

Feeding ecology and diet

Hydrozoa are usually thought to feed on planktonic crustaceans such as copepods. Under laboratory conditions, they mostly survive with a diet based on Artemia nauplii. The medusae are mostly voracious carnivores and, when feeding on fish eggs and larvae, can be considered as being at the apex of trophic chains. The polyps are more varied in their food preferences. Recent investigations showed that the few species studied thoroughly feed on a great variety of prey, ranging from gelatinous plankton for the medusae to phytoplankton for the polyps. Some species have symbiotic zooxanthellae and are functionally photosynthetic.

Some medusae can remain immobile in the water, with their tentacles outstretched across the water column, performing ambush predation, whereas others can move across the water to contact prey, performing cruising predation. Polyps can simply extend their tentacles to catch passing prey, but they can also use special sense organs to perceive approaching prey and grab it actively, or they can form currents by moving their tentacles to direct food particles toward the mouth. Symbiotic species are very specialized in their feeding behavior; the most extreme cases are Halocoryne epizoica that feed on bryozoan tentacles, and Polypodium hydriforme that feed on sturgeon eggs from the inside, being the only intracellular parasitic metazoan.

Hydrozoa use cnidocysts as the main organelles to catch their food. The superclass has the richest variety of cnidocyst types of the whole phylum, with a fine range of adaptations to catch from tiny prey like the single cells of phytoplankton to the crustaceans and larvae of the zooplankton to the animals that live in the sand and mud, such as nematodes. Many species have very restricted diets, being specialized for just one type of food.

Both polyps and medusae are mostly carnivorous, feeding on almost all animals of proper size. In no other metazoan group (with the possible exception of parasitic trematodes) is the lifecycle of such paramount importance in defining the properties of a given species, and this is valid also for the type of prey. Tiny medusae that feed on fish eggs and larvae, sometimes impairing the success of recruitment, can be the most voracious predators of fish. The propensity of feeding on almost all types of larvae (both for polyps and for medusae) includes the Hydrozoa within the predators of almost all Metazoa with an indirect lifecycle.

Reproductive biology

Nothing is known regarding the courtship and mating. The eggs are brought in masses on female medusae or in female gonophores. According to the species, the eggs can be small and in great numbers, or they can be large and few, even a single one per gonophore.

In the Automedusae, with no polyp stages, there is little or no asexual reproduction in larval medusae (known as actinulae in the Narcomedusae) so that each fertilization event leads to one or a few adult medusae. In the Hydroidomedusae, the widespread asexual reproduction of the polyp stage can be considered as a polyembryony or as a larval amplification, as it happens in some parasites such as trematodes. Each fertilization event, therefore, leads to a single planula that produces a polyp colony that, in its turn, will produce many adult medusae. Since the lifespan of polyp colonies can last many years, each planula can lead to the production of hundreds or thousands medusae.

The so-called planula larva is nothing more than a gastrula, thus being more an embryo than a larva. Since gametes are shed before or soon after fertilization, embryonic development takes place outside the maternal body. Planulae can be solid (stereogastrula) or hollow (coeloblastula); usually the species with medusae in the lifecycle have hollow planulae that live part of their life in the water column, swimming with cilia or flagella to reach the settling sites. The species with suppressed medusae usually produce solid planulae that fall to the bottom and settle near the parent colony. Adults, by definition, are the sexually reproductive morphs in a lifecycle; if a medusa is present, it is the adult in the lifecycle. The polyp stage, in this framework, is a specialized (and perennial) larva that produces a great number of adults throughout its long life. In many species, however, the medusa stage can be reduced or even suppressed, so that the larvae, by paedomorphosis, become the sexually mature morphs of the lifecycle. Almost half of the species of Hydroidomedusae have suppressed or reduced medusae; the group is, therefore, the most paedomorphic of the whole animal kingdom.

Some medusae (e.g., Eleutheria) have special brood pouches where they safeguard developing medusae. Some hydroids have gonothecae with apical brood chambers that retain the planulae for a certain period.

Many species are sharply seasonal, being active only in narrow periods of time. The medusae can be present for a few weeks or months, completely disappearing from the water column and being represented by the correspondent polyps in the benthos. Polyp colonies can regress to resting hydrorhizae for long periods, reactivating at the onset of favorable conditions. Planulae can become encysted and remain dormant just as the resting hydrorhizae, being covered by a chitinous sheath.

Conservation status

No species are listed by the IUCN Red List. For most Hydrozoa, the distribution and the abundance are not known and only the few remaining specialists know about their presence. Many species are endemic simply because they are not searched for and only the areas of activity of specialists are covered. The only groups mentioned in regional or national red lists are the calcified, coral-like Milleporidae and Stylasteridae, which are also listed by CITES. Inclusion of these taxa on conservation lists is linked to the assessment of species value according to appearance.

Milleporidae and Stylasteridae have been subjected to trade, as are some hydroids (known as "white weeds" in the North Sea). Their decline is linked to habitat degradation.

Significance to humans

The Hydrozoa are mostly inconspicuous, both in the polyp and in the medusa stage, and are generally overlooked. The famous treatise by Tremblay, describing transplants in Hydra, inspired the fantastic novel Frankenstein by Mary Shelley. The illustrations of some monographs, especially those on medusae by Haeckel, are renowned for their beauty. The modern music composer, Frank Zappa, wrote a song on a hydromedusa that has been named after him: Phialella zappai. "White weeds" (the colonies of hydroids of the genera Hydrallmania and Sertularia) had been used as decoration before the sharp reduction of their populations. Some Hydrozoa are used as laboratory animals for experimental biology; Hydra is the most popular one, but others include Aequorea victoria (for the production of the labeling enzyme aequorein), Hydractinia spp., Laomedea spp., and Tubularia spp.

Hydroids are important members of fouling communities, inhibiting the functioning of power plants by clogging their pipes and reducing the velocity of ships by settling on their hulls. Some species have been reported as pests in aquaculture, feeding on the larvae of the reared species or on their food. Polypodium hydriforme is a threat to the production of caviar, being a parasite of sturgeon eggs.

Some species of medusae (e.g., Gonionemus) can inflict severe stings on humans, as do some hydroid colonies such as the species of Millepora (fire corals) and some aglaopheniids. When present in swarms, even small medusae like those of Clytia can inflict slight stings on swimmers. The most important threat to human activities is the predation of some medusae (e.g., Aequorea victoria) and floating hydroids (e.g., Clytia gracilis) on the eggs and larvae of commercially exploited fish. This kind of predation can reduce the success of fish recruitment, reducing the yield of fisheries.

Species accounts

Resources

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[Article by: Ferdinando Boero; Jean Bouillon]

A class of the phylum Coelenterata which includes the fresh-water hydras, the marine hydroids, many of the smaller jellyfish, a few special corals, and the Portuguese man-of-war. The Hydrozoa may be divided into six orders: the Hydroida, Milleporina, Stylasterina, Trachylina, Siphonophora, and Spongiomorphida. See separate article on each order.

The form of the body varies greatly among the hydrozoans. This diversity is due in part to the existence of two body types, the polyp and the medusa. A specimen may be a polyp, a medusa, a colony of polyps, or even a composite of the first two. Polyps are somewhat cylindrical, attached at one end, and have a mouth surrounded by tentacles at the free end. Medusae are free-swimming jellyfish with tentacles around the margin of the discoidal body.

In a representative life cycle, the fertilized egg develops into a swimming larva which soon attaches itself and transforms into a polyp. The polyp develops stolons (which fasten to substrates), stems, and other polyps to make up a colony of interconnected polyps. Medusae are produced by budding and liberated to feed, grow, and produce eggs and sperm.

Most hydrozoans are carnivorous and capture animals which come in contact with their tentacles. The prey is immobilized by poison injected by stinging capsules, the nematocysts. Most animals of appropriate size can be captured, but small crustaceans are probably the most common food. See also Coelenterata.

The class of aquatic jellyfishes to which the Portuguese-man-of-war belongs.

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Hydrozoa
Closeup of a hydrozoan colony
Scientific classification
Kingdom: Animalia Subkingdom: Eumetazoa Phylum: Cnidaria Subphylum: Medusozoa Class: Hydrozoa Owen, 1843
Subclasses
Leptolinae Trachylinae

Hydrozoa (hydrozoans) 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. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.

Some examples of hydrozoans are the Freshwater Jelly (Craspedacusta sowerbyi), the freshwater polyps (Hydra), Obelia, the Portuguese Man o' War (Physalia physalis), the chondrophores (Porpitidae), "air fern" (Sertularia argenta) and the pink-hearted hydroids (Tubularia).

Contents 1 Systematics 1.1 Other classifications 2 Hydra, a freshwater genus 3 Life cycles 4 Footnotes 5 References 6 External links

Systematics

The highly apomorphic Siphonophorae - like this Portuguese Man o' War (Physalia physalis) - have long misled hydrozoan researchers.

Hydrozoan systematics 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 orders, according to their mode of growth and reproduction. Most famous among these was probably the assemblage called "Hydroida", but this group is apparently paraphyletic, united by plesiomorphic (ancestral) traits. Other such orders were the Anthoathecatae, Actinulidae, Laingiomedusae, Polypodiozoa, Siphonophora and Trachylina.

As far as can be told from the molecular and morphological 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 subclasses, the Leptolinae (containing the bulk of the former "Hydroida" and the Siphonophora) and the Trachylinae, containing the others (including the Limnomedusae). The monophyly of several of the presumed orders in each subclass is still in need of verification.^[1]

In any case, according to this classification, the hydrozoans can be subdivided as follows, with taxon names emended to end in "-ae":^[1]

CLASS HYDROZOA

• Subclass Leptolinae
  • Order Anthomedusae (= Anthoathecata(e), Athecata(e), Stylasterina(e)) - includes Laingoimedusae but monophyly requires verification
  • Order Leptomedusae (= Leptothecata(e), Thecaphora(e), Thecata(e))
  • Order Siphonophorae
• Subclass Trachylinae
  • Order Actinulidae
  • Order Limnomedusae - monophyly requires verification; tentatively placed here
  • Order Narcomedusae
  • Order Trachymedusae - 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 cells. 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 analysis found it to be closer to Myxozoa. It was traditionally placed in its own class Polypodiozoa and this view is presently often seen to reflect the uncertainties surrounding this highly distinct animal.^[2]

Other classifications

Limnomedusae like the Flower Hat Jelly (Olindias formosa) were long allied with Anthomedusae and Leptomedusae in the "Hydroida".

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 with the currently available data. Especially the presumed phylogenetic distinctness of the Siphonophora is a major flaw that was corrected only recently.

The obsolete classification mentioned above was as follows:

• Order Actinulidae
• Order Anthoathecatae
• Order Hydroida
  • Suborder Anthomedusae
  • Suborder Leptomedusae
  • Suborder Limnomedusae
• Order Laingiomedusae
• Order Polypodiozoa
• Order Siphonophora

Fire corals were initially considered a separate order. They are actually a family of the Anthomedusae.

• Order Trachylina
  • Suborder Narcomedusae
  • Suborder Trachymedusae

A very old classification that is sometimes still seen is:

• Order Hydroida
• Order Milleporina
• Order Siphonophorida
• Order Stylasterina (= Anthomedusae)
• Order Trachylinida

Catalogue of Life uses the following:

• Order Actinulida
• Order Anthoathecata (= Anthomedusae)
• Order Hydroida

Some place the anthomedusan family Porpitidae in a separate order "Chondrophora".

• Order Laingiomedusae
• Order Leptothecata (= Leptomedusae)
• Order Limnomedusae
• Order Narcomedusae
• Order Siphonophora
• Order Trachymedusae

Animal Diversity Web uses the following:

• Order Actinulida
• Order Capitata
• Order Chondrophora
• Order Filifera
• Order Hydroida
• Order Siphonophora

Hydra, a freshwater genus

This section does not cite any references or sources. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (January 2009)

The most widely-known and researched freshwater hydrozoan is Hydra, 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 crustaceans (such as brine shrimp), insect larvae, and annelid worms. Hydra may reproduce sexually, through the spawning of sperm (and thus insemination of eggs on the female body column), or through asexual reproduction (budding).

Life cycles

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Some colonial hydrozoans have both a medusa stage and a polyp 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 colonies are usually dioecious, 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) bud off asexually-produced medusae. These tiny, new medusae (which are either male or female) mature and spawn, releasing gametes freely into the sea in most cases. Zygotes become free-swimming planula larvae or actinula larvae that either settle on a suitable substrate (in the case of planulae), or swim and develop into another medusae or polyp directly (actinulae). Colonial hydrozoans include siphonophore colonies, Hydractinia, Obelia, 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". Most hydromedusae have shorter life spans than the larger scyphozoan jellyfish. Some species of hydromedusae release gametes shortly after they are themselves released from the hydroids (as in the case of fire corals), living only a few hours, while other species of hydromedusae grow and feed in the plankton for months, spawning daily for many days before their supply of food or other water conditions deteriorate and cause their demise.

Footnotes

1. ^ ^{a b} Schuchert (2005)
2. ^ Zrzavý & Hypša 2003

References

• Schuchert, Peter (2005): The Hydrozoa Directory - Hydrozoan Phylogeny and Classification. Retrieved 2008-JUL-08.
• Zrzavý, Jan & Hypša, Václav (2003): Myxozoa, Polypodium, and the origin of the Bilateria: The phylogenetic position of "Endocnidozoa" in light of the rediscovery of Buddenbrockia. Cladistics 19(2): 164–169. doi:10.1111/j.1096-0031.2003.tb00305.x (HTML abstract)

External links

Wikimedia Commons has media related to: Hydrozoa

• J. Bouillon, M.D. Medel, F. Pagès, J.M. Gili, F. Boero and C. Gravili. 2004. Fauna of the Mediterranean Hydrozoa. Scientia Marina, 68 (Suppl. 2).
• Hydroids from Reunion Island and Indian Ocean
• http://zygote.swarthmore.edu/intro6.html
• Puget Sound Online
• Aquascope
• The Hydrozoa Directory
• - A general page about hydromedusae

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