Genus Larsonia Boero, Bouillon & Gravili, 1991

Stomotoca (Stomotocanna) Haeckel, 1879: 52;

type species Stomotoca pterophylla Haeckel, 1879 by monotypy.

Larsonia Boero, Bouillon & Gravili, 1991: 198; type species Stomotoca pterophylla Haeckel, 1879 by original designation.

Diagnosis: Medusa with thick apical mesoglea and usually pointed apex; manubrium large, attached to a broad gastric peduncle, extending well beyond bell margin, mouth with crenulated lips; gonads in 8 adradial rows on manubrium, complexly transversely folded; two opposite perradial tentacles, numerous rudimentary bulbs on bell margin; no ocelli.

Hydroid parasitic on fish, with plate-like naked hydrorhiza, hydranth with no tentacles; gonozooid branched, producing at ends medusa buds.

Remarks: Larson (1982) reared medusae released from a parasitic hydroid of the genus Hydrichthys Fewkes, 1887 and could identify them as Stomotoca pterophylla Haeckel, 1879 . Because Larson’s hydroid showed little host specificity and many Hydrichthys polyps are rather similar, Larson concluded that they will also produce medusae referable to the genus Stomotoca L. Agassiz, 1862 . Therefore, he suggested to synonymize Hydrichthys and Stomotoca . This was contested by Boero & Bouillon (1989) who could show that the type species of Stomotoca – S. atra L. Agassiz, 1862 – had a polyp stage that was completely different from Hydrichthys (free living, with scattered tentacles). Moreover, the polyp of a tentatively identified Hydrichthys mirus Fewkes, 1887 – type species of the genus Hydrichthys – produced a Leuckartiara - like medusa (Boero et al., 1991). Because Stomotoca pterophylla and Stomotoca atra have different polyps, Boero et al. (1991) proposed the new genus Larsonia for S. pterophylla . Boero et al. must have overlooked that Haeckel (1879) had already introduced a subgenus name for S. pterophylla, namely Stomotocanna Haeckel, 1879. As a subgenus level name automatically also becomes available at the genus level, Larsonia Boero, Bouillon & Gravili, 1991 is a junior synonym of Stomotocanna. However, because Haeckel’s name has not been used since its introduction and in the interest of nomenclatural stability, it should not be reactivated (ICZN, paragraph 23.9.1.1.)