Rhizogeton sterreri

Description. Colony stolonal, with creeping, tortuous hydrohriza from which arise individual hydranths and gonophores. Perisarc thin, variably covering bases of hydranths, extending up 1 mm or more over body; perisarc generally infested with various microscopic algae, mainly diatoms. Hydranths much elongated, up to 6 mm high in extension, nearly cylindrical, tapering imperceptibly but gradually towards hypostome; width 0.3 – 0.4 mm. Hypostome long, cylindrical, with rounded apex in relaxed hydranths, short and dome-shaped in contracted specimens. Hydranths with up to 28 tentacles; one whorl of 4 – 6 oral tentacles held vertically, plus 5 – 6 additional, more or less distinct lower whorls, each with variably 2 – 5 tentacles attached obliquely to horizontally; whorls scattered over 2 / 3 of hydranth body; all tentacles filiform, length decreasing from distal towards proximal part of hydranth body; distal tentacles also more robust than proximal ones. Male gonophores, sporosacs without radial canals, borne on hydrorhiza by means of short pedicels; covered by thin layer of slightly corrugated periderm; pear-shaped to ovate, tapering more abruptly at base and gradually from middle part towards apex. Female gonophores not seen. Nematocysts (undischarged capsules): microbasic euryteles (6.7 – 7.6) × (2.6 – 2.8) µm (in tentacles and gonophores); desmonemes (4.6 – 5.2) × (2.8 – 3.0) µm (in tentacles). Color: portion below hypostome and gonophore content reddish; gastrodermis of tentacle bases bright white; remaining of body translucid.

Remarks. Calder (1988 b) introduced the new genus Rhizodendrium in order to accommodate several species of Rhizogeton L. Agassiz, 1862 with short, dome-shaped hypostome, more than 20 tentacles (arranged in both a distinct, oral whorl and several additional lower, more or less distinct whorls), and spherical gonophores. These species were: Rhizogeton nudus Broch, 1909, Rhizogeton ezoense Yamada, 1964, and Rhizodendrium sterreri Calder, 1988 b. Following this point of view, only Rhizogeton fusiformis L. Agassiz, 1862 could be retained in Rhizogeton. However, the available type material of R. sterreri was relatively scarce (two colonies on Sargassum sp., see Calder 1988 b) and sterile. Therefore, no thorough comparison with its congeners could be made when the species was first described. The examination of the present material showed that the mature (male) gonophores of R. sterreri are fusiform rather than spherical, and therefore different from those of its congeners. Some additional inconsistencies are also illustrated by two other species, as follows. The hydranths of Rhizogeton conicum Schuchert, 1996 have up to 20 tentacles, and its gonophores are spherical. A likely new record of R. fusiformis from Korea (Park 1997), proved that the number of tentacles per hydranth could be as much as 16 – 20, not only 10 – 12 as in the original description given by L. Agassiz (1862). This difference is most probably due to a combination of both geographical and ecological factors. Therefore, the tentacle number in these hydroids should not be regarded as having a great taxonomical importance. Taken together, these facts suggest the inappropriateness of the genus Rhizodendrium, which should be included in the synonymy of Rhizogeton L. Agassiz, 1862. Although originally found on pelagic Sargassum (Calder 1988 b), R. sterreri was presently found growing on a variety of substrates: algae, sponges, crevices in both scleractinians and hydrocorals, and mineral concretions.

Distribution. Previously reported from Bermuda (Calder 1988 b).

Material examined. Stn. 3: 01.04.2008 — numerous colonies without gonophores, on various algae and concretions. Stn. 4: 22.03.2008 — one sterile, badly preserved colony, on coral. Stn. 5: 31.03.2008 — one small, sterile colony, on hydrocoral. Stn. 7: 25.03.2008 — numerous sterile colonies, on various algae, concretions, and sponge; 27.03.2008 — numerous colonies, some with male gonophores, on various algae, sponges, and hydrocoral (MHNG INVE 60976); 29.03.2008 — a few sterile colonies, on hydrocoral and algae. Type locality. Whalebone Bay, St. George’s Island, Bermuda.