Genus Hydrichthella Stechow, 1909

Hydrichthella Stechow, 1909: 31; Hirohito, 1988: 142, 144; Bouillon et al., 2006: 153; Xu et al., 2014: 265.

Hydrichthelloides Bouillon, 1978: 55; Bouillon, 1985: 59.

Type species: Hydrichthella epigorgia Stechow, 1905 .

Diagnosis. Hydroid. Colony usually growing on sea fans, hydrorhiza encrusting, covered by naked coenosarc, or consisting of perisarc-covered reticular stolons pending substrate; gastrozooid tubular, without tentacles; hypostome studded by nematocysts; dactylozooids hollow, without mouth, with two types: one with many capitate tentacles and the other filiform, with capitate tip; gonozooids similar to gastrozooids in shape, bearing eumedusoids.

Medusa. Reduced to short-living eumedusoids, with 4 radial canals and subumbrellar cavity, with manubrium not eccentric; with 8 non-tentacular bulbs; gonads on manubrium; with or without ocelli.

Remarks. The genus Hydrichthella shares with Hydrichthelloides the hydroids colony polymorphic and dactylozooids of two types: one with many capitate tentacles and the other filiform with capitate tips. Type species of both genera show distinct from the structure of hydrorhiza: Hydrichthelloides reticula Bouillon, 1978 consists of perisarc, covered reticular stolons pending substrate; Hydrichthella epigorgia Stechow, 1909 is encrusting, covered by naked coenosarc. The systematic value of this distinguished character is questionable even at the generic level. Hirohito (1988) treated Hydrichthelloides as a synonym of Hydrichthella .

The genus Hydrichthella comprised two species: the hydroid H. epigorgia Stechow, 1909 from the East China Sea, Japan, Indonesia, Papua New Guinea and Seychelles Island (Hirohito, 1988; Tang & Gao, 2008; Xu et al., 2014) and H. reticulata (Bouillon, 1978) from Papua New Guinea (Bouillon, 1978). In the present work, a new medusa species, H. ocellata Xu, Huang & Wang, sp. nov., collected from the South China Sea with red ocelli on the 8 non-tentacular bulbs which suggests it should be a new species of the genus.