Suborder Capitata Kühn, 1913

diagnosis

Anthoathecata which have always stenoteles in polyp and medusa phase. Polyp tentacles capitate, moniliform, or filiform; if filiform then often in two well separated whorls. Mouth of medusa mostly simple, circular, sometimes cruciform, without crenulated lips, oral arm, oral tentacles.

higher classification

Metazoa, Cnidaria, Medusozoa, Hydrozoa, Anthoathecata

Remarks

Members of this taxon are always quite easy to identify: all Anthoathecata possessing stenoteles belong to it. Stenoteles are otherwise only found in some Trachymedusae, the Actinulidae, and some Siphonophorae.

The name Capitata derives from the frequent occurrence of capitate tentacles. However, not all Capitata have them, some only during juvenile phases.

Rarely, also some Filifera can have capitate tentacles, e. g. in the Ptilocodiidae. The absence of stenoteles places them within the Filifera.

Thecocodium brieni, a member of the Filifera (Family Ptilocodiidae), has dactylozooids with capitate tentacles (modified after Brinckmann-Voss 1970)

Recent molecular investigations have shown that Capitata actually comprises two unrelated clades, the Aplanulata and Capitata sensu stricto (see Collins et al., 2006; Nawrocki et al, 2010). It will thus be necessary to revise the Hydrozoa at the order level, which implies quite drastic changes. As the positions of some families are currently somewhat unlclear, this has not yet been done here.

References

Bouillon, J. 1985. Essai de classification des hydropolypes-hydroméduses (Hydrozoa-Cnidaria). Indo Malayan Zoology 2: 29-243.

Bouillon, J., & Boero, F. 2000b. Synopsis of the families and genera of the Hydromedusae of the world, with a list of the worldwide species. Thalassia Salentina 24: 47-296.

Brinckmann-Voss, A. 1970. Anthomedusae/Athecata (Hydrozoa, Cnidaria) of the Mediterranean. Part I. Capitata. Fauna e Flora Golfo di Napoli 39:1-96, pls 1-11.

Collins, A. G., Schuchert, P., Marques, A. C., Jankowski, T., Medina, M. , & Schierwater, B. 2006. Cnidarian Phylogeny and Character Evolution Clarified by New Large and Small Subunit rDNA Data and an Assessment of the Utility of Phylogenetic Mixture Models. Systematic Biology 55: 97-115.

Millard, N. A. H. 1975. Monograph on the Hydroida of southern Africa. Annals of the South African Museum 68: 1-513.

Nawrocki, A. M., Schuchert, P., & Cartwright, P. 2010. Phylogenetics and evolution of Capitata (Cnidaria: Hydrozoa), and the systematics of Corynidae. Zoologica Scripta 39(3): 290-304.

Petersen, K. W. 1990. Evolution and taxonomy in capitate hydroids and medusae(Cnidaria: Hydrozoa). Zoological Journal of the Linnean Society 100: 101-231.

Rees, W.J. 1957. Evolutionary trends in the classification of capitate hydroids and medusae. Bull. Br. Mus. nat. Hist., Zool. 4: 455-534.

Schuchert, P. 1996. The marine fauna of New Zealand: athecate hydroids and their medusae (Cnidaria: Hydrozoa). New Zealand Oceanographic Institute Memoir 106: 1-159. 

Classification

Family Moerisiidae

Hydroid with moniliform or modified moniliform tentacles, scattered or in one whorl around middle part of the hydranth body; medusa buds between or just under the tentacles; polyp buds produced from lower part of hydranth; hydrocaulus short, ending in pedal disc forming podocysts, or with short stolon-like tubes ending in podocysts or hydranths.
Medusa with prismatic manubrium, manubrium with basal radial lobes giving into radial canals; no gastric peduncle; with or without centripetal canals; mouth simple, cruciform; gonads on manubrium and surrounding manubrial lobes or only on manubrial lobes overlying the radial canals; with either four to several hundreds moniliform or modified moniliform hollow marginal tentacles with adnate bulbs; abaxial ocelli present.

Family Sphaerocorynidae

Hydroid colony stolonal or erect, creeping stolons; hydrocaulus long, unbranched or slightly branched; hydranth vasiform with bulbous base and long, conical hypostome; no oral tentacles but numerous solid, single or trifid capitate tentacles in 3-5 closely set whorls around broadest part of hydranth body; gonophores borne singly or on short, branching blastostyles between or below tentacles, free medusae or eumedusoids.
Medusa umbrella bell-shaped, apical mesoglea thick, apical projection conical or dome-shaped, with broad apical chamber; manubrium either flask-shaped or cruciform, base quadrate; mouth round or cruciform; in non mature specimens gonads interradial, may be divided in adradial masses by longitudinal median grooves, gonads adnate along perradii in adult specimens; four hollow, marginal tentacles with either adaxial or spirally-arranged nematocyst clusters and terminating in an ellipsoid nematocyst knob; marginal bulbs clasping exumbrella, with an adaxial expansion; one abaxial ocellus per tentacle.

Family Corynidae

The least inclusive clade comprising the species Coryne pusilla Gaertner, 1774; Stauridiosarsia producta (Wright, 1858); and Sarsia tubulosa (M.Sars, 1835).

[comprises also Polyorchidae, more information]

Family Hydrocorynidae

Hydroid colony  forming a chitinized hydrorhizal stolonal plate; hydranths columnar, with thickened, ridged mesogloeal lamella; tentacles hollow, capitate, in one or five to six close-set whorls around a conical hypostome; gonophores in clusters on proximal part of hydranth body or developing from from hydrorhiza. Gonophores free medusae or sessile sporosacs.
Medusa umbrella evenly rounded; with or without gastric peduncle; four marginal tentacles with scattered nematocyst knobs and a small capitation; tentacular bulbs clasping, with ocelli; manubrium broadly flask-shaped or tubular, quadrate or cruciform; oral part of manubrium prismatic, ending in a cruciform mouth with or without nematocyst clusters; gonads interradial, without longitudinal groove, surrounding nearly the whole manubrium. 

Family Halimedusidae

Hydroid: polyps small (<0.2 mm), solitary; 3-8, generally 4 distal whorls of capitate tentacles, with a few scattered nematocysts along their length; medusa buds single, just below tentacles.
Medusa usually with a low gastric peduncle and with distinct interradial peaks in jelly above manubrium base; manubrium cruciform, with basal perradial lobes; mouth quadratic to cruciform, with lips lined by nematocysts; four radial canals; either with four perradial marginal tentacles or with four perradial marginal tentacles and four interradial groups of tentacles, all hollow at base; gonads either on manubrium or on manubrium and perradial lobes; no mesenteries; marginal bulbs cylindrical, with abaxial ocelli.
 

Family Cladonematidae

Hydroid colony stolonal; hydranth with one whorl of oral capitate tentacles, with or without aboral whorl of filiform tentacles; mouth giving into a preoral chamber formed by epidermal gland cells; medusa buds at base of hydranth or above filiform  tentacles if such are present.
Medusa adapted to benthic mode of life and able to walk, some species still able to swim freely; with or without a thickened continuous or broken ring of nematocysts around umbrellar margin, with or without brood pouch above manubrium; manubrium cylindrical, with or without perradial pouches; mouth either with short lips, or with oral nematocyst clusters, or with ramified oral tentacles; gonads either completely surrounding manubrium, or on manubrial extrusions, or in special brood pouches; number of radial canals variable, some may be branched; marginal branched, with upper branches bearing nematocyst clusters and lower branches having adhesive organs; with abaxial ocelli on tentacle base.  

Family Boeromedusidae

Medusa with apical projection, manubrium cylindrical, mouth simple, tubular; four radial canals; four conical marginal bulbs; four simple, hollow tentacles with many nematocyst clusters including a terminal ovoid cluster; gonads on manubrium as four large perradial pouches hanging freely in subumbrellar cavity; no ocelli. Hydroid: unknown.

Family Hydridae

Hydroid solitary, with hollow filiform tentacles, but often moniliform distally, in one whorl under hypostome; lower part of hydranth with simple pedal disc, no perisarc except on encysted embryos. Without medusa phase or gonophore development, eggs and sperm developed directly in epidermis of polyps in wart-like protuberances; asexual reproduction by lateral buds, leading only to temporary colonies

Family Candelabridae

Hydroid solitary or forming pseudo-colonies. Hydrocaulus short, with tubular or root-like adhesive processes, with or without perisarc. Hydranth large and elongated, covered in distal half by numerous scattered, hollow capitate tentacles, tentacles solitary or grouped adnate basally; gonophores fixed, developing on aboral part of hydranth below tentacle covered region, either directly on hydranth or on spindle-shaped blastostyles. Gonophores fixed sporosacs.

Family Acaulidae

Hydroid solitary, body divided into basal and distal part. Basal part conical to cylindrical, covered or not by gelatinous perisarc which may form anchoring filaments. Distal part with tentacles, tentacles capitate or indistinctly capitate. Upper part of hydranth with one or two oral whorls of capitate tentacles and scattered capitate tentacles below; below capitate tentacles with or without one whorl of thick filiform tentacles Gonophores fixed sporosacs in the lower or middle part of the hydranth, sometimes with asexual reproduction by transverse fission.

Family Boreohydridae

Hydroids solitary, small, with one whorl of reduced tentacles, capitate or not, located in the oral or median part of column; perisarc covering of base filmy or absent; gametes in body wall. 

Family Tubulariidae

Hydroid solitary or colonial; hydrocaulus divided into distal neck region covered by thin perisarc, and proximal stem which may be either short and thick with tuber-like aboral processes, or long and cylindrical, or cone-shaped with basal disc, or with stolons covered by thicker perisarc; neck perisarc secreted from a groove on the hydranth proper; hydranth broadly vasiform, tentacles in two sets, oral ones filiform or pseudofiliform in one to several close-set whorls, sometimes capitate or moniliform (especially transiently in juveniles); aboral tentacles long, in one whorl, pseudofiliform or filiform, sitting on a more or less developed parenchymatic cushion of gastrodermal tissue; gonophores develop above aboral tentacles, free medusae or fixed sporosacs; embryonic development without planula but an actinula larvae.
Medusae with or without exumbrellar nematocyst tracks, bell margin oblique or straight, radial radial canals; mouth usually circular; gonads covering manubrium entirely; one to four marginal tentacles; with or without asexual medusae budding from marginal bulbs; ocelli absent.

Family Corymorphidae (sensu Petersen 1990)

Hydroids solitary, hydrocaulus long, hollow or more or less filled with parenchymatic gastrodermis, lower part of caulus with short papillae or/and longer anchoring filaments; tentacles arranged into two separate sets; oral tentacles composed either of one whorl of moniliform, one whorl of capitate tentacles, or several whorls of filiform tentacles; aboral tentacles arranged in one to three whorls of moniliform or filiform tentacles; gonophores borne above aboral tentacles, either directly issuing from hydranth wall or on blastostyles, gonophores develop into free medusae or fixed sporosacs.
Medusa with or without apical projection; manubrium usually not extending beyond umbrella margin, cylindrical or exceptionally with sac-like processes; mouth simple, circular; gonads undivided surrounding manubrium completely over its entire length, exceptionally also on sac-like processes of manubrium; one to four hollow, usually filiform tentacles, tentacles all uniform or differentiated into two types, differing in size and structure, tentacles exceptionally branched or rudimentary.

Family Tricyclusidae

Hydroid solitary, attached by small pedal disk; hydrocaulus thin, as long as hydranth, perisarc covering hydrocaulus inflated, gelatinous; hydranth pear-shaped; with one oral whorl of up to six capitate tentacles and two widely spaced whorls each of 8-14 stout, solid aboral, imperfectly moniliform tentacles; hydroid buds produced from lower part of hydranth; actinuloid larvae arising from under aboral tentacles; gonophores as fixed sporosacs, only male observed.

Family Pennariidae

Hydroid colony pinnate, occasionally bushy, stem monosiphonic, giving rise alternately from opposite sides to two series of hydrocladia; hydrocaulus and hydrocladia with terminal hydranths (monopodial); hydranths on short pedicels on upper side of the hydrocladia; hydranths pear-shaped; tentacles of two types: in distal half of hydranth more or less capitate tentacles in one oral whorl and more in indistinct whorls below, on lower par of hydranth one aboral whorl of semifiliform to slightly capitate aboral tentacles; gonophores developing above aboral tentacles, eumedusoids, liberated or not. Medusa a simple eumedusoid; manubrium not extending beyond umbrella margin; gonads completely surrounding manubrium; four radial canals; four permanently rudimentary tentacles, usually reduced to mere bulbs, without ocelli.

Family Margelopsidae

Capitata with solitary, pelagic hydroids; hydrocaulus absent or reduced to a small process; hydranth vasiform, tentacles filiform to moniliform, arranged into two separate sets, oral tentacles in one or several whorls; aboral tentacles either in two to three alternating whorls or numerous and scattered over most of body; medusa buds among the tentacles or over the aboral tentacles. Medusa manubrium with simple mouth; gonads surrounding manubrium entirely; four radial canals; tentacles solid, generally moniliform, two or more tentacles per marginal bulb, in some genera tentacles also issuing at different levels on exumbrella; without ocelli.

Family Paracorynidae

Hydroid with polymorphic hydranths, differentiated into gastrozooids, gonozooids, and dactylozooids; colony flat, basal plate divided into upper layer of broad gastrodermal cavities and basal layer of large, parenchymatic gastrodermal cells continuous with those in dactylozooids, transversed by mesogloeal lamellae, all enveloped by a layer of epidermis, perisarc absent; gastrozooid short, stout, with one to four whorls of solid capitate tentacles; gonozooids short, lacking tentacles and mouth, bearing cryptomedusoid gonophores; eggs develop into actinulae within gonophore  or into encysted resting stage; dactylozooids around edge of colony, long, finger-shaped, filled with parenchymatic gastrodermis. 

Family Cladocorynidae

Hydroid colonial, stolonal simple or sparingly branched, hydranth club-shaped, one whorl of moniliform or capitate oral tentacles, aboral tentacles moniliform or branched capitate, scattered or in several whorls; with nematocysts on body wall arranged in conspicuous pads or scattered around the base of oral and aboral tentacles; gonophores carried singly or on short, branched pedicels on lower or middle part of hydranth; gonophores developing into free medusae or fixed sporosacs.
Medusa with two opposite perradial tentacles possessing stalked cnidophores and two non-tentacular bulbs; above each atentaculate marginal bulb an exumbrellar pads containing macrobasic euryteles; tentaculate marginal bulbs large, without nematocyst pads; gonads interradial on manubrium.

Family Zancleidae

Hydroid colonial, stolonal, pedicels present or very short; stolons creeping; polyps monomorphic or polymorphic; gastrozooids cylindrical or spindle-shaped, either with one whorl of oral capitate tentacles and many scattered capitate or moniliform tentacles, or with reduced capitate tentacles, or without tentacles; gonozooids and dactylozooids - when present - with capitate tentacles or these reduced to nematocyst patches.
Medusa umbrella evenly rounded; four perradial exumbrellar nematocyst pads, pads either oval, clavate, elongate or linear, usually containing stenoteles; manubrium cylindric, mouth simple, circular; gonads usually interradial, rarely in a single mass around manubrium; four radial canals; marginal tentacles 0, 2 or 4, hollow, each with numerous abaxial stalked cnidophores containing macrobasic euryteles; without ocelli.

Family Zancleopsidae

Hydroid phase so far unknown.
Medusa umbrella conical or dome-shaped, without apical chamber; manubrium broadly flask-shaped, base quadrate or cruciform, mouth square or circular, with or without faint lips; gonads either surrounding manubrium or in four interradial pads, gonads have deep interradial furrows which may divide them into eight adradial patches; two to four capitate marginal tentacles, with or without lateral capitate branches; marginal bulbs clasping umbrella margin and with adaxial hemispherical projection armed with nematocysts; marginal bulbs with or without ocelli either on marginal bulbs, or on proximal part of tentacles.

Family Teissieridae

Colony forming a basal plate or upright, branching hydrorhiza consisting of basal or central periderm provided with spines that penetrate overlying coenosarc, with or without peridermal ridges, overlaid by gastrodermal tubes enclosed in common epidermis and covered externally by thin, filmy periderm; hydranths polymorphic; broad, sausage-shaped, with numerous scattered capitate tentacles, with or without a clear whorl of oral tentacles; with or without simple dactylozooids with scattered capitate tentacles; medusae bud carried singly among tentacles on proximal part of gastrozooids; cnidome with macrobasic heteronemes.
Medusa with or without apical projection; four radial canals; four perradial exumbrellar nematocyst patches on base of radial canals, containing stenoteles, each pouch with an apical ocellus; two opposite perradial tentacles with large bulbs without ocelli, abaxial cnidophores on tentacles, non tentaculate perradial bulbs small or absent;gonads on interradial of simple manubrium; one ocellus in the most apical part of the exumbrellar pouches.

Family Porpitidae

Hydroid colonial, mobile, when mature floating on water surface, with a chitinous internal skeleton, forming a floating chamber, covered by mantle, hydranths polymorphic on underside of float, one central, large, atentaculate gastrozooid; surrounded by gastro-gonozooids and dactylozooids; tentacles capitate or reduced to nematocyst patches.
Medusa with evenly-rounded bell; four or eight radial canals and an equal number of exumbrellar nematocystcyst tracks containing stenoteles, nematocyst tracks originating from marginal bulbs; circular canal present; manubrium short, conical; mouth circular; gonads perradial or irregularly arranged perradially and interradially; two opposite thin marginal tentacles terminating in large spherical nematocyst knob, with or without an additional adaxially oriented short tentacle developed from tentaculiferous bulbs; two or six small marginal bulbs without tentacles; ocelli lacking; cndome comprising spherical stenoteles, macrobasic euryteles and atrichous haplonemes; zooxanthellae generally present.

Family Solanderidae

Hydroid forming large, branching colony, with chitinous internal anastomosing skeleton formed by coalescence and modification of adjacent hydrocauline tubes; coenosarc covering entire colony and penetrating skeletal interstices; hydranths over whole colony surface, uniform in structure, cylindrical, with a single whorl of capitate tentacles around mouth and numerous similar tentacles scattered over body; gonophores, where known, cryptomedusoid or eumedusoid, arising directly from coenosarc.

Family Milleporidae

Hydroid colony forming massive, calcareous exoskeletons (= coenosteum) of varied shape; coenosteum with an internal complex network of coenosarcal tubes and covered externally by a thin ectodermal layer, coenosteal surface perforated by pores; margins of pores not elevated; large gastropores surrounded by smaller dactylopores, forming indistinct cyclosystems; polyps polymorphic; gastrozooids relatively short and stout, with an oral whorl of four to seven short capitate tentacles, arising from gastropores; dactylozooids long, slender, mouthless, with scattered capitate tentacles, arising from dactylopores; cnidome with stenoteles and macrobasic mastigophores; gonophores arising from coenosarc within chambers embedded entirely in the coenosteum.
Medusa free swimming eumedusoids with exumbrellar nematocyst patches, radial and circular canals present, without tentacles and sense organs; gonads occupying the place of an indistinct manubrium.

Family Protohydridae

Solitary, small, elongated hydroids usually living in brackish-waters; without tentacles and gonophores, pedal disc formed by epidermal tissue only; gametes bulging into gastric cavity, nematocysts: stenoteles and isorhizas.