Rhizogeton L. Agassiz, 1862

Calder, Dale R., 2010, Some anthoathecate hydroids and limnopolyps (Cnidaria, Hydrozoa) from the Hawaiian archipelago 2590, Zootaxa 2590 (1), pp. 1-91 : 13

Genus Rhizogeton L. Agassiz, 1862

Rhizogeton L. Agassiz, 1862: 224 .

Type species. Rhizogeton fusiformis L. Agassiz, 1862 , by monotypy.

Diagnosis. Oceaniid hydroids with stolonal colonies. Hydranths arising from hydrorhiza, with thin perisarcal collar at base, varied in shape but nearly cylindrical when extended; tentacles filiform, scattered over much of hydranth; hypostome conical.

Gonophores fixed sporosacs, borne on hydrorhiza.

Remarks. The genus Rhizogeton L. Agassiz, 1862 is distinguished from other oceaniid hydroids in having stolonal colonies with fixed gonophores that arise only from the hydrorhiza, and in lacking nematophores. Rhizodendrium Calder, 1988 has been considered a synonym ( Schuchert 2004; Bouillon et al. 2006), and I agree. Rhizodendrium was established after comparing R. sterreri Calder, 1988 from Bermuda with incompletely described and poorly preserved type material of Rhizogeton fusiformis L. Agassiz, 1862 , type species of Rhizogeton , from Massachusetts. The diagram and description of the hydranth of R. fusiformis in L. Agassiz (1862: 224–226, Plate 20) must be considered misleading based on subsequent observations of the species. Abundant fertile colonies identified as R. fusiformis from Passamaquoddy Bay, Canada (ROMIZ B3101, ROMIZ B3102) generally overlap Rhizodendrium sterreri in important characters such as hypostome shape, tentacle numbers and arrangement, and gonophore morphology, the reasons originally given for establishing Rhizodendrium as a distinct genus.

Five species are currently recognized in the genus Rhizogeton ( Schuchert 2009) . Possible undescribed species have been noted in both the Indian Ocean ( Gravier-Bonnet & Mioche 1996) and in European waters ( Schuchert 2008a: 273–274). Hydroids of Rhizogeton are mostly inconspicuous and poorly known, but they now appear to be notably widespread. Representatives have been reported from the Atlantic, Pacific, Indian, and Arctic oceans, and some are known to raft on phoretic substrates such as pelagic Sargassum ( Calder 1988, 1995).