Hydrozoa 378569 225420188 2008-07-13T16:24:01Z J. Spencer 568400 disambig morphology {{Taxobox | name = Hydrozoa | image = ColonialHydrozoa.jpg | image_width = 240px | image_caption = Closeup of a hydrozoan colony | regnum = [[Animal]]ia | subregnum = [[Eumetazoa]] | phylum = [[Cnidaria]] | subphylum = [[Medusozoa]] | classis = '''Hydrozoa''' | classis_authority = [[Richard Owen|Owen]], 1843 | subdivision_ranks = [[Subclass]]es | subdivision = [[Leptolinae]]<br /> [[Trachylinae]] }} '''Hydrozoa''' ('''hydrozoans''') are a [[taxonomic]] [[Class (biology)|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 (biology)|phylum]] [[Cnidaria]]. Some examples of hydrozoans are the [[Freshwater Jelly]] (''Craspedacusta sowerbyi''), the freshwater polyps (''[[Hydra (genus)|Hydra]]''), ''[[Obelia]]'', the [[Portuguese Man o' War]] (''Physalia physalis''), the [[chondrophore]]s (Porpitidae), "[[air fern]]" (''Sertularia argenta'') and the [[pink-hearted hydroid]]s (''Tubularia''). ==Systematics== [[image:Portuguese Man-O-War (Physalia physalis).jpg|thumb|right|The highly [[apomorph]]ic [[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 [[order (biology)|order]]s, 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 [[plesiomorph]]ic (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 [[morphology (biology)|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 [[subclass]]es, 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.<ref name = schuchert2005>Schuchert (2005)</ref> In any case, according to this classification, the hydrozoans can be subdivided as follows, with [[taxon]] names emended to end in "-ae":<ref name = schuchert2005 /> '''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 [[cell (biology)|cell]]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]] [[DNA sequence|sequence]] analysis found it to be closer to [[Myxozoa]]. It was traditionally placed in its own [[class (biology)|class]] Polypodiozoa and this view is presently often seen to reflect the uncertainties surrounding this highly distinct animal.<ref>Zrzavý & Hypša 2003</ref> ===Other classifications=== [[Image:Flower Hat Jellyfish 1.jpg|thumb|right|[[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 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]] [[Image:Fire corals.JPG|thumb|right|[[Fire coral]]s 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]] [[Image:Porpita porpita.jpg|thumb|right|Some incorrectly 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 (Hydrozoa)|Capitata]] * Order [[Chondrophora]] * Order [[Filifera]] * Order [[Hydroida]] * Order [[Siphonophora]] ==''Hydra'', a freshwater genus== The most widely-known and researched freshwater hydrozoan is [[Hydra (genus)|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 [[nematocyst]]s, or "stinging cells," to disable its prey. Hydra eat small [[crustacean]]s (such as brine shrimp), [[insect]] [[larva]]e, 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. [[Zygote]]s become free-swimming [[planula|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== {{Reflist}} ==References== * {{aut|Schuchert, Peter}} (2005): [http://www.ville-ge.ch/mhng/hydrozoa/hydrozoa-directory.htm The Hydrozoa Directory] - [http://www.ville-ge.ch/mhng/hydrozoa/intro/phylogeny.htm Hydrozoan Phylogeny and Classification]. Retrieved 2008-JUL-08. * {{aut|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. <small>{{doi|10.1111/j.1096-0031.2003.tb00305.x}}</small> (HTML abstract) ==External links== {{commonscat|Hydrozoa}} * [http://www.icm.csic.es/scimar/68s2.html 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).] * [http://etic.univ-reunion.fr/hydroids/ Hydroids from Reunion Island and Indian Ocean] * http://zygote.swarthmore.edu/intro6.html * [http://www.psat.wa.gov/you_can_do/creature/hydroid.htm Puget Sound Online] * [http://www.vattenkikaren.gu.se/fakta/arter/cnidaria/hydrozoa/obellive.html Aquascope] * [http://www.ville-ge.ch/musinfo/mhng/hydrozoa/hydrozoa-directory.htm The Hydrozoa Directory] * [http://www.freeweb.hu/tarrdaniel/images/hydrozoa.htm Hydrozoa Picture Gallery] [[Category:Hydrozoa| ]] {{Link FA|uk}} [[ca:Hidrozou]] [[cs:Polypovci]] [[de:Hydrozoen]] [[et:Hüdraloomad]] [[es:Hydrozoa]] [[fr:Hydrozoa]] [[gl:Hydrozoa]] [[ko:히드라충강]] [[hr:Obrubnjaci]] [[it:Hydrozoa]] [[he:הידרתים]] [[lt:Hidragyviai]] [[nl:Hydroidpoliepen]] [[ja:ヒドロ虫綱]] [[no:Småmaneter]] [[pl:Stułbiopławy]] [[pt:Hydrozoa]] [[ru:Гидроидные]] [[sk:Polypovce]] [[sr:Hydrozoa]] [[fi:Polyyppieläimet]] [[sv:Hydrozoer]] [[th:ไฮโดรซัว]] [[tr:Polip (zooloji)]] [[uk:Гідроїдні]] [[zh:水螅纲]]