Hydridae Dana, 1846

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 43-44

Family Hydridae Dana, 1846

Hydridae Dana, 1846: 116 .

Diagnosis. Aberrant capitate freshwater hydroids with solitary hydranths, occasionally appearing colonial because of asexual budding from lower walls. Hydranths varied in shape, elongate and cylindrical or with central bulge when extended, barrel-shaped when contracted, with pedal disc basally and whorl of tentacles distally; pedal disc with central pore; tentacles hollow, filiform to slightly moniliform distally, in one whorl around hypostome; hypostome dome-shaped; perisarc absent except on embryothecae.

Gonophores completely reduced and medusa suppressed; gametes produced within wart-like protuberances in ectoderm of hydranth wall; planula stage absent; sexes hermaphroditic or gonochoric; when hermaphroditic, ‘testis’ occurring distally and ‘ovary’ proximally on hydranth column.

Remarks. The family Hydridae Dana, 1846 is now generally thought to contain a single genus, Hydra Linnaeus, 1758 ( Petersen 1990; Bouillon et al. 2006; Daly et al. 2007; Hemmrich et al. 2007; Schuchert 2010). Of some 80 described species, fewer than 15 are considered valid ( Jankowski et al. 2008).

Polyps of species of Hydra resemble filiferans, particularly in having tentacles that appear filiform. However, their affinities with Capitata Kühn, 1913 are reflected in the cnidome (comprising stenoteles, desmonemes, and haplonemes) and in overall development ( Petersen 1990). They were regarded by Petersen as being closely related to Moerisiidae Poche, 1914 , with the two having similar cnidomes and aflagellate planulae. Bouillon et al. (2006) suggested that Hydridae might warrant classification as a distinct order, but included them, together with Moerisiidae and four other families, in the capitate suborder Moerisiida Poche, 1914 . Collins et al. (2005) included Hydridae in Aplanulata Collins et al., 2005.

The taxonomy and nomenclature of Hydridae have long been in disarray. Although Hydra was established by Linnaeus (1758) with 11 included nominal species, none of the original names is in current use ( Campbell 1989). Some of the originally included taxa have been assigned to other groups, while any conforming with the current concept of the genus have unfortunately been given replacement names that have become more familiar than now-abandoned original ones. Campbell (1989) discussed some of the confusion over names and identifications in the group.