Animal Diversity Web

Di­ver­sity

The Hy­dro­zoa is a sub­group of cnidar­i­ans con­taing­ing ap­prox­i­mately 3700 species. It is a di­verse group with a va­ri­ety of life cy­cles, growth forms, and spe­cial­ized struc­tures. Like many cnidar­i­ans, hy­dro­zoans have both polyp and medusa stages in their life cycle. They are dis­tin­guished from other groups by their com­plex life cycle, by the growth of medusae from buds rather than stro­bi­lae or from meta­mor­pho­sis, by the pres­ence of a velum in­side the bell of the medusa, and by the pro­duc­tion of ga­metes from ec­to­der­mal, rather than en­do­der­mal, tis­sue. Most hy­dro­zoans are ma­rine, and hy­dro­zoan species are found in nearly every ma­rine habi­tat type; a very few species live in fresh­wa­ter. Most hy­dro­zoans form colonies of asex­ual polyps and free-swim­ming sex­ual medusae. Colonies are usu­ally ben­thic, but some, no­tably the siphonophores, are pelagic floaters. Colo­nial polyps often have some di­vi­sion of func­tion, with cer­tain polyps spe­cial­ized for de­fense, feed­ing, or re­pro­duc­tion. Most hy­dro­zoans are preda­tors or fil­ter-feed­ers, though a few have sym­bi­otic algae (zoox­an­thel­lae), in the same way that other other groups of cnidar­i­ans do.

Bet­ter-known hy­dro­zoans in­clude Por­tuguese man-o-wars (Physalia physalis), the fresh­wa­ter genus Hydra, fire coral (Mille­pori­dae), and by-the-wind sailors (Velella velella). (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003; Jankowski, et al., 2008; Mills, 2009)

Ge­o­graphic Range

Hy­dro­zoans are found in all the oceans, at all lat­i­tudes. A few species occur in fresh and brack­ish water on all con­ti­nents ex­cept Antarc­tica. (Bouil­lon, et al., 2006; Jankowski, et al., 2008)

• Biogeographic Regions
• nearctic
  • native
• palearctic
  • native
• oriental
  • native
• ethiopian
  • native
• neotropical
  • native
• australian
  • native
• oceanic islands
  • native
• arctic ocean
  • native
• indian ocean
  • native
• atlantic ocean
  • native
• pacific ocean
  • native
• mediterranean sea
  • native

• Other Geographic Terms
• cosmopolitan

Habi­tat

Hy­dro­zoans are found in nearly all ma­rine habi­tats, ex­cept per­haps heavy surf zones. They are most abun­dant and di­verse in warm shal­low wa­ters, prob­a­bly as a re­flec­tion of food abun­dance. The small num­ber of fresh­wa­ter species occur in both lotic and lentic habi­tats, and are more abun­dant in eu­trophic and mesotrophic wa­ters. (Bouil­lon, et al., 2006; Jankowski, et al., 2008)

• Habitat Regions
• temperate
• tropical
• polar
• saltwater or marine
• freshwater

• Aquatic Biomes
• pelagic
• benthic
• reef
• oceanic vent
• lakes and ponds
• rivers and streams
• temporary pools
• coastal
• abyssal
• brackish water

• Wetlands
• marsh
• swamp

• Other Habitat Features
• estuarine
• intertidal or littoral

Phys­i­cal De­scrip­tion

Most hy­dro­zoan species have a plank­tonic lar­val stage called a plan­ula. Plan­u­lae are ra­di­ally sym­met­ric ovoids, often cov­ered with fla­gel­late cells for swim­ming. They may be very sim­ple em­bryos or have cells dif­fer­en­ti­ated into sev­eral types. Plan­u­lae most often set­tle onto a ben­thic sub­strate and de­velop into a polyp.

Polyps are ra­di­ally sym­met­ric, and may be urn-shaped, con­i­cal, cylin­dri­cal, or club-shaped. In most species they are only a few mil­lime­ters tall, though the largest grow up to many cen­time­ters, and one, Bran­chio­ce­ri­anthus im­per­a­tor can be 2 me­ters tall. At their base hy­dro­zoan polyps have basal disks or elon­gate processes for at­tach­ing to sub­strate, or they may be at­tached to other polyps. Often there will also be con­nec­tions here to hol­low tubes (called stolons) that con­nect the polyp to oth­ers in its colony, and allow the ex­change of food be­tween polyps. Above the base is a ring of con­trac­tile cells called the sphinc­ter. These can con­tract to iso­late the con­tents of the polyp from the stolons, pre­vent­ing undi­gested food from en­ter­ing the stolons. Above this is the gas­tric col­umn, which usu­ally con­tains a di­ges­tive cham­ber with a sin­gle open­ing, a mouth at the apex of the col­umn. A ring of ten­ta­cles is at­tached to the col­umn below the apex and above the sphinc­ter. The num­ber, shape and size of ten­ta­cles varies greatly, but there are usu­ally be­tween and 8 and 50 on a sin­gle polyp (some have many more, and some spe­cial­ized polyps may have fewer). Most colo­nial hy­dro­zoans are poly­mor­phic, with dif­fer­ent struc­tures re­flect­ing dif­fer­ent func­tions. Some are armed with large spines ten­ta­cles for de­fense but have no mouth, some have ten­ta­cles and func­tional mouths for feed­ing, and some are only re­pro­duc­tive, with no ten­ta­cles or mouth, and pro­duce medusae (see below) or ga­metes.

Like all cnidar­i­ans, hy­dro­zoans have spe­cial ec­to­der­mal cells called cnido­cytes, each con­tain­ing a sin­gle in­tra­cel­lu­lar struc­ture called a cnida (aka ne­ma­to­cyst). Cnidae are unique to the Cnidaria. Each cnida, when trig­gered by a me­chan­i­cal or chem­i­cal stim­u­lus, shoots out a tiny hol­low tube at high speed. Some cnidae are is equipped with sharp spines, and/or ven­omous or acidic com­pounds, but some are ad­he­sive and have nei­ther spines nor tox­ins. Hy­dro­zoans use dif­fer­ent types of cnidae to cap­ture prey, to repel preda­tors, and to at­tach to sub­strate.

Most hy­dro­zoan species are colo­nial. A found­ing polyp pro­duces new polyps by bud­ding, and these grow a net­work of in­ter­con­nect­ing hol­low tubes (stolons) formed of liv­ing tis­sue, col­lec­tively called the coenosarc. Colony growth forms vary be­tween species, some may form a sin­gle layer of polyps spread­ing across the sub­strate, oth­ers grow­ing as erect stems, with polyps grow­ing off the stems. Polyps and the coenosarc may se­crete chiti­nous sheaths, or stems, or cal­care­ous coat­ings (the lat­ter form­ing struc­tures sim­i­lar to the an­tho­zoan Scle­r­ac­tinia, the stony corals). In many colonies, polyps are poly­mor­phic, with dif­fer­ent struc­tures re­flect­ing dif­fer­ent func­tions. Some have no mouth, but are armed with large spines or cnidae-equipped ten­ta­cles for de­fense, some have ten­ta­cles and func­tional mouths for feed­ing, and some, with nei­ther mouth nor ten­ta­cles, are strictly re­pro­duc­tive, and pro­duce medusae (see below) or ga­metes.

The medusa is the sex­u­ally re­pro­duc­ing stage in most hy­dro­zoans. They are often formed by bud­ding from polyps, and are usu­ally soli­tary free-swim­ming or­gan­isms. They are sim­i­lar in struc­ture to an in­verted polyp, ra­di­ally sym­met­ric, and often have four-fold sym­me­try. Their main body part is the um­brella, a bell or cone shaped gelatin-filled struc­ture, which floats with the open­ing down. Medusa are usu­ally small, usu­ally 1-50 mm in di­am­e­ter, though a few are larger, the largest (genus g. Rha­cos­toma) grow to 400 mm in di­am­e­ter. Around the in­side of the open­ing is a mus­cu­lar ring of tis­sue called the velum. The velum can con­tract and relax, chang­ing the di­am­e­ter of the open­ing, and play­ing an im­por­tant role in swim­ming The pres­ence of the velum is a di­ag­nos­tic char­ac­ter for Hy­dro­zoa, only one genus, Obelia, has lost it. Around the out­side of the open­ing of the um­brella is a ring of ten­ta­cles, which vary greatly among species in num­ber, shape, and de­gree of arm­ing with cnido­cytes. In­side the um­brella, sus­pended like the clap­per of a bell, is the manubrium, which con­tains the gas­tric cav­ity, and ends in a mouth. Struc­tures that pro­duce ga­metes form on the sides of the manubrium. Most species have dioe­cious medusae, each in­di­vid­ual pro­duc­ing only eggs or sperm. Some are mo­noe­cious, but usu­ally not si­mul­ta­ne­ously her­maph­ro­dite. In some species sex is de­ter­mined by en­vi­ron­men­tal con­di­tions, mainly tem­per­a­ture.

Both polyps and medusae have net­works of nerves, but no brain or cen­tral gan­glion. Some have light-sen­si­tive struc­tures called ocelli, and many have sta­to­cysts that allow them to de­tect grav­ity and their ori­en­ta­tion.

These struc­tural pat­terns are com­mon, but there is great vari­a­tion in the life cy­cles of hy­dro­zoans. Some have sup­pressed or re­duced one or more stages. In the Siphonophora and a few other groups of hy­dro­zoans, colonies of polyps are pelagic, and float at the sur­face by means of a gas-filled tis­sue. They often re­tain medusae as part of the colony. (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003; Jankowski, et al., 2008)

• Other Physical Features
• ectothermic
• heterothermic
• radial symmetry
• polymorphic
• venomous

De­vel­op­ment

Hy­dro­zoans have a com­plex life cycle, usu­ally with two or three mor­pho­log­i­cally dif­fer­ent stages. The clas­sic cycle starts with fer­til­ized eggs de­vel­op­ing into small, free swim­ming lar­vae called plan­u­lae, which may be able to enter a dor­mant rest­ing state to re­sist un­suit­able en­vi­ron­men­tal con­di­tions. Plan­u­lae trans­form into ses­sile polyps, usu­ally at­tached to sub­strate, but free-float­ing in some groups. Polyps du­pli­cate them­selves asex­u­ally by bud­ding, often pro­duc­ing colonies of hun­dreds or thou­sands of poly­mor­phic in­di­vid­ual polyps. Polyps pro­duce "adult" sex­u­ally-re­pro­duc­ing medusae by bud­ding. Medusae are soli­tary, free-swim­ming, dieo­cious. They re­lease sperm and eggs into the water, where fer­til­iza­tion oc­curs. This is the basic cycle, but there is an enor­mous range of vari­a­tions. In nearly half of species (e.g. Hydra) the the medusa stage is en­tirely sup­pressed; polyps pro­duce ga­metes di­rectly. In oth­ers the medusa are formed, but never de­tach from the par­ent polyp, and pro­duce ga­metes while still at­tached. In some cases these fused com­bi­na­tions form elab­o­rate struc­tures. In other taxa the polyp stage is sup­pressed, and plan­u­lae trans­form di­rectly into tiny medusae, or form a polyp, pro­duce a medusa, and re­sorb the polyp. Nu­mer­ous taxa have sup­pressed the plan­ula as well. (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003)

• Development - Life Cycle
• metamorphosis
• colonial growth

Re­pro­duc­tion

Hy­dro­zoans are mostly broad­cast spawn­ers. In some species only sperm is shed, and eggs are re­tained on the par­ent. Eggs re­lease sperm-at­tract­ing com­pounds.

• Mating System
• polygynandrous (promiscuous)

Hy­dro­zoan polyps re­pro­duce asex­u­ally by bud­ding, cre­at­ing daugh­ter polyps, medusae, or both. In some species medusae re­pro­duce asex­u­ally as well, by fis­sion or bud­ding. Medusae (if pre­sent in the life cycle) or polyps pro­duce ga­metes. Most hy­dro­zoan species are dioe­cious, a few are se­quen­tial her­maph­ro­dites. Eggs and sperm are most often re­leased into the water col­umn and fer­til­iza­tion is ex­ter­nal. In some species eggs are re­tained and fer­til­ized in­ter­nally, in which case em­bryos may be re­leases as lar­vae or re­tained until even more de­vel­oped. (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003)

• Key Reproductive Features
• iteroparous
• seasonal breeding
• year-round breeding
• gonochoric/gonochoristic/dioecious (sexes separate)
• sequential hermaphrodite
• sexual
• asexual
• fertilization
  • external
  • internal
• ovoviviparous
• oviparous

Most hy­dro­zoan species have min­i­mal parental in­vest­ment. Eggs and sperm are re­leased into the water, and left to sur­vive on their own. In a few species, eggs are re­tained in spe­cial struc­tures on the par­ent, and the em­bryos are re­tained as brood, de­vel­op­ing to the plan­ula or even young polyp stage. In the lat­ter case we have no in­for­ma­tion on whether the young are nour­ished by their par­ent, or just pro­tected. (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003)

• Parental Investment
• no parental involvement
• female parental care
• pre-fertilization
  • provisioning
  • protecting
    • female
• pre-hatching/birth
  • protecting
    • female

Be­hav­ior

Hy­dro­zoans com­bine ses­sile or seden­tary polyp stages and free-swim­ming soli­tary stages in their life cy­cles. Polyps may be soli­tary or colo­nial. Some can move by crawl­ing, but most are ses­sile. The Siphonophora form float­ing colonies of at­tached polyps medusae. Many free-swim­ming hy­dro­zoans fol­low the diel mi­gra­tion pat­tern com­mon to many pelagic ma­rine or­gan­isms. They spend day­light hours in deep water where light does not pen­e­trate, and rise to the sur­face after sun­set. (Br­usca and Br­usca, 2003; Mills, 2009)

• Key Behaviors
• natatorial
• diurnal
• nocturnal
• crepuscular
• sessile
• motile
• sedentary
• solitary
• colonial

Com­mu­ni­ca­tion and Per­cep­tion

All hy­dro­zoans have tac­tile and chem­i­cal sens­ing struc­tures. Some also have eye­spots that de­tect light, and/or sta­to­cysts that de­tect grav­ity. They com­mu­ni­cate mainly by chem­i­cal sig­nals. Some free-swim­ming hy­dro­zoans, in­clud­ing many siphonophores have bi­o­lu­mi­nes­cent struc­tures. It's not known what func­tion these serve. It is un­likely that they com­mu­ni­cate with other hy­dro­zoans (their light sen­sors are too sim­ple for this). Pos­si­bly they are lures for prey or have some preda­tor de­fense func­tion. (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003; Dunn, 2009; Mills, 2009)

• Communication Channels
• visual
• chemical

• Other Communication Modes
• photic/bioluminescent
• pheromones

• Perception Channels
• visual
• tactile
• chemical

Food Habits

Hy­dro­zoans vary in their feed­ing habits. Many trap small zoo­plank­ton with their ten­ta­cles. Some fil­ter sus­pended par­ti­cles (such as fish eggs and fecal pel­lets) from the water col­umn. Some con­sume phy­to­plank­ton. A few groups con­tain sym­bi­otic algae, and may get most of their nu­tri­tional needs from their sym­biotes.

Pelagic hy­dro­zoans, in­clud­ing siphonophore colonies and medusae, are known to show some se­lec­tiv­ity in prey types, some tak­ing mainly fish lar­vae, oth­ers tak­ing soft-bod­ied in­ver­te­brates, oth­ers mi­cro-crus­taceans. They are also sen­si­tive to chem­i­cals pro­duced by prey, and will move to­wards higher con­cen­tra­tions of these chem­i­cals.

Large pop­u­la­tions of hy­drom­e­dusae may be sig­nif­i­cant eco­log­i­cal fac­tors in pelagic ma­rine ecosys­tems. (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003; Dunn, 2009; Mills, 2009; Pur­cell, 1997)

• Primary Diet
• carnivore
  • eats eggs
  • eats non-insect arthropods
• planktivore
• detritivore

• Foraging Behavior
• filter-feeding

Pre­da­tion

De­spite their pro­tec­tive sting­ing cells, hy­dro­zoans are prey for many types of preda­tors. A va­ri­ety of snails and worms graze on polyps and stolons, as do some fish and crus­taceans. Fish also con­sume medusae and pelagic colo­nial hy­dro­zoans, as do some sea tur­tles (es­pe­cially leatherbacks), ctenophores, and other cnidar­i­ans, in­clud­ing larger hy­dro­zoans.

A va­ri­ety of preda­tors have the abil­ity to con­sume the sting­ing cells of hy­dro­zoans with­out trig­ger­ing them. These preda­tors then se­quester the sting­ing cells in their body to de­fend them against their own preda­tors. Nudi­branchs are par­tic­u­larly well known for this abil­ity, but some species of ctenophores, turbel­lar­ian flat­worms, and pri­a­pulids can store cnido­cysts as well.

Nearly all hy­dro­zoans pro­tect them­selves with their cnido­cysts. Some colo­nial species have spe­cial­ized polyps that grow large ten­ta­cles armed with dense bat­ter­ies of these sting­ing cells or grow large rigid spines. Many colo­nial polyps se­crete a rigid pro­tec­tive layer over stolons and polyp tubes. This layer is often made of chitin, some groups pro­duce a min­eral skele­ton. Free-swim­ming medusae can­not use rigid pro­tec­tion, but do de­fend them­selves with sting­ing cells. There is ev­i­dence that some also con­tain toxic com­pounds that dis­cour­age preda­tors from eat­ing them. Most hy­dro­zoan medusae also fol­low the diel mi­gra­tion pat­tern com­mon to many plank­tonic or­gan­isms -- sink­ing below the limit of light pen­e­tra­tion to avoid vi­sual preda­tors dur­ing the day, and then ris­ing to­wards the sur­face at night in pur­suit of prey. (Bouil­lon, et al., 2006; Br­usca and Br­usca, 2003; Dunn, 2009; Mills, 2009; Pur­cell, 1997)

• Known Predators

  • leatherback sea turtle (Dermochelys coriacea)
  • Nudibranchia
  • Ctenophora

Ecosys­tem Roles

Hy­dro­zoans are both preda­tors and prey for many ma­rine or­gan­isms, and large sea­sonal blooms of medusae may strongly af­fect local fish and zoo­plank­ton pop­u­la­tions. Some species of polyps are hosts for sym­bi­otic algae, and many large pelagic forms have sym­bi­otic hy­per­iid am­phipods liv­ing on or in them. There is even a small species of fish, Nomeus gronovii, that lives in as­so­ci­a­tion with Por­tuguese man-o-wars. Some polyp colonies grow on the shells of her­mit crabs, pro­vid­ing them pro­tec­tion. (Br­usca and Br­usca, 2003; Dunn, 2009; Mills, 2009; Pur­cell, 1997)

Eco­nomic Im­por­tance for Hu­mans: Pos­i­tive

Hy­dro­zoans are im­por­tant parts of many ma­rine food chains, and so di­rectly or in­di­rectly sup­port de­sir­able food sources. Species in two fam­i­lies that have colonies with cal­care­ous ex­oskele­tons (Mille­pori­dae, Sty­las­teri­dae) have been har­vested com­mer­cially, and prob­a­bly still are. There are now lim­its on their trade and some are con­sid­ered en­dan­gered (see Con­ser­va­tion Sta­tus). (Br­usca and Br­usca, 2003; Mills, 2009; Pur­cell, 1997; Schuchert, 2009)

Eco­nomic Im­por­tance for Hu­mans: Neg­a­tive

The stings of some hy­dro­zoans (most fa­mously the Por­tuguese man-o-war, Physalis physalis), are dan­ger­ous to hu­mans. Also, the fire corals (Mille­pori­dae) get their name from the painful sen­sa­tion divers get if they touch them.

Hy­dro­zoans are com­mon mem­bers of "foul­ing com­mu­ni­ties" -- the ben­thic or­gan­isms that nat­u­rally at­tach to hard sub­strates, and so grow on the hulls of ships and on sub­merged water pipes, in­ter­fer­ing with their func­tion. (Br­usca and Br­usca, 2003; Dunn, 2009; Mills, 2009)

• Negative Impacts
• injures humans
  • bites or stings
  • venomous

Con­ser­va­tion Sta­tus

The con­ser­va­tion sta­tus of the vast ma­jor­ity of hy­dro­zoan species is un­known. Species in two fam­i­lies, the fire corals (Mille­pori­dae) and the lace corals (Sty­las­teri­dae), have been com­mer­cially har­vested, and in some places over-ex­ploited. They are now listed in Ap­pen­dix I of CITES, the in­ter­na­tional treaty lim­it­ing trade in wildlife. Also the IUCN has eval­u­ated many species of Mille­pora and rated sev­eral of them En­dan­gered. (Br­usca and Br­usca, 2003; Mills, 2009)

• IUCN Red List [Link]

  Not Evaluated

Con­trib­u­tors

George Ham­mond (au­thor), An­i­mal Di­ver­sity Web.

Ref­er­ences

Bouil­lon, J., C. Grav­ili, F. Pagès, J. Gili, F. Boero. 2006. An In­tro­duc­tion to Hy­dro­zoa. Paris, France: Pub­li­ca­tions Sci­en­tifiques du Muséum.

Br­usca, R., G. Br­usca. 2003. In­ver­te­brates. Sun­der­land, Mass­a­chu­setts, USA: Sin­auer As­so­ci­ates, Inc..

Dunn, C. 2009. "Siphonophores" (On-line). Ac­cessed July 09, 2009 at http://​www.​siphonophores.​org/​index.​php.

Jankowski, T., A. Collins, R. Camp­bell. 2008. Global di­ver­sity of in­land water cnidar­i­ans. Pp. 35-40 in E Balian, C Lévêque, H Segers, K Martens, eds. Fresh­wa­ter An­i­mal Di­ver­sity As­sess­ment. Dor­drecht, The Nether­lands: Springer Nether­lands.

Mills, C. 2009. "Bi­o­lu­mi­nes­cence and other fac­toids about Ae­quorea, a hy­drom­e­dusa" (On-line). Ac­cessed July 10, 2009 at http://​faculty.​washington.​edu/​cemills/​Aequorea.​html.

Mills, C. 2008. "Hy­drom­e­dusae" (On-line). Ac­cessed July 10, 2009 at http://​faculty.​washington.​edu/​cemills/​Hydromedusae.​html.

Pur­cell, J. 1997. Pelagic cnidar­i­ans and ctenophores as preda­tors: se­lec­tive pre­da­tion, feed­ing rates, and ef­fects on prey pop­u­la­tions. An­nales de l'In­sti­tute Oceanographique, 72/2: 125-137.

Schuchert, P. 2009. "The Hy­dro­zoa Di­rec­tory" (On-line). Ac­cessed July 09, 2009 at http://​www.​ville-ge.​ch/​mhng/​hydrozoa/​hydrozoa-directory.​htm.

Wro­bel, D. 2009. "The Jel­lies Zone - Jel­ly­fish And Other Gelati­nous Zoo­plank­ton" (On-line). Ac­cessed July 10, 2009 at http://​jellieszone.​com/​.