Order Limnomedusae Kramp, 1938

diagnosis

Hydroid small, simple, mostly solitary, some forming colonies; sessile; with or without tentacles; without theca but having a mucoprotein periderm, cysts and stolons can be covered by perisarc.

Medusa mostly with 4 complete radial canals, 6 also possible, often also with incomplete centripetal canals not reaching manubrium; with or without marginal nematocyst ring; gonads along radial canals or exceptionally on manubrium (Armorhydra and Limnocnida); marginal tentacles peripheral, hollow, without true basal bulb, marginal sense organs as internal enclosed statocysts of endo-ectodermal origin, embedded in the mesogloea near ring canal or in the velum; no ocelli; exceptionally reduced medusoids (Monobrachium).

Some species also in freshwater.

higher classification

Cnidaria, Medusozoa, Hydrozoa, Trachylinae

species

about 54

terms and forms

Classification

Family Olindiasidae

Medusa 1-30 mm, with or without centripetal canals; internal epi-gastrodermal statocysts;  unbranched radial canals; gonads on radial canals or exceptionally on manubrium (Limnocnida); no ocelli.

Hydroid usually solitary, rarely forming  colonies; generally minute; no tentacles, one tentacle, or several tentacles in a single whorl, sometimes with dactylozooids; no theca; active asexual reproduction by buds or frustules; usually with free medusae, exceptionally with free or fixed eumedusoids.

Family Microhydrulidae

Microscopic hydroids living in biofilms, resembling polyp stage of some Olindiasidae, reduced to a spherical or irregular body;  without tentacles and mouth, body covered by mucus, with a basal lamella of periderm; sexual reproduction unknown, asexual reproduction by mobile frustules.

Family Monobrachiidae

Hydroid colony stolonal, living on bivalve shells; hydrorhiza creeping reticulated or incrusting or both reticulated and incrusting; hydranth sessile, claviform, only a single filiform tentacle below large hypostome; sometimes dactylo- zooids in form of stalked nematocyst knobs; gonophores arising from hydrorhiza, developing into fixed or free eumedusoids without tentacles, gonads along radial canals; with or without statocysts.  

 of unclear affinity: Anthomedusae?

Family Armorhydridae

Medusa phase reduced, microscopic, creeping within the interstices  of coarse sand sediments; umbrella margin with a whorl of two kind of solid tentacles, filiform and adhesive; manubrium voluminous, linked to subumbrella by longitudinal septa containing endodermal tubes; gonads on manubrium; velar opening narrow; no radial canals, no statocysts or any other visible sense organs. Nematocysts: microbasic euryteles.

Remarks

The Limnomedusae as currently conceived could be polyphyletic.   Some groups that have occasionally been included in the Limnomedusae, like the Proboscidactylidae, the Moeresidae, and the genus Tiaricodon, do not belong to here. They are now all referred to the Anthomedusae.

Regarding only the medusa phase, then the core group of the Limnomedusae, the Olindiasidae, are hardly separable from some Trachymedusae, i. e. the Geryioniidae. The Olinidiasidae were actullay included in the Trachymedusae, before Kramp separated them based on the presence of a polyp stage. Considering the extensive flexiblity of the life-cycle evolution within the Hydrozoa, it is easily imaginable that the Olindiasidae are actually a subgroup the Trachymedusae.  Only molecular investigations could resolve the dilemma.

Although the name Limnomedusae suggests that this group is mainly occurring in freshwater, the majority of species is strictly marine.

References

Hand, C. 1957. The systematics, affinities, and hosts of the on-tentacled commensal hydroid Monobrachium with new distributional records. J. Wash. Acad. Sci. 47: 84-88.

Lacassagne, M. 1968. Anatomie et histologie de l'hydroméduse bentique: Armorhydra janowiczi Swedmark et Teissier. Cahiers de Biologie Marine 9: 187-200.

Kramp, P. L. 1959a. The Hydromedusae of the Atlantic Ocean and adjacent waters. Dana Report  46: 1-283.

Russell, F. S. 1953.The medusae of the British Isles. Cambridge University Press, London,  pp.  530, 35 pls