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 Closeup of a hydrozoan colony
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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).
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Systematics
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.
Eventually is turned out that 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
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 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
- 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
- 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
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).
Colonial hydrozoans
Colonial hydrozoans typically have both a medusa stage and a polyp stage in their lifecycle. They have a base, a stalk, and one or more polyps. Hydrozoan colonies are composed of a number of specialized polyps (or "zooids") - including feeding, reproductive, and protective zooids. Reproductive polyps, known as gonozooids (or "gonotheca" in thecate hydrozoans) bud off sexually-produced medusae. These medusae mature and spawn, producing gametes. 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 is typically the dominant sexually-reproductive phase in hydrozoans that alternate between a polyp and a medusa. The medusa often has a limited lifespan, though, and may die shortly after releasing gametes (as in the case of fire corals).
Footnotes
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
- 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
- Hydrozoa Picture Gallery
