Rhizorhagium antarcticum

Rhizorhagium antarcticum (Hickson & Gravely, 1907)

(Fig. 2 f–i)

Material examined. ANT XV/3: 48-31, some polyps, up to 4 mm high, on Oswaldella terranovae Peña Cantero & Vervoort, 1996 and Antarctoscyphus grandis (Blanco, 1977); 48-33, numerous polyps, up to 7 mm high, on Sc. unifurcata, O. terranovae and Tubularia sp.2, with gonophores; 48-36, some polyps, up to 3 mm high, on O. terranovae, with gonophores; 48-50, several polyps, up to 10 mm high, on Schizotricha nana Peña Cantero, Svoboda & Vervoort, 1996 and Campanularia hicksoni Totton, 1930; 48-194, some polyps, up to 3 mm high, on C. hicksoni and Sc. nana, with gonophores; 48-220, numerous polyps, up to 5 mm high, on Sc. nana, Sy. exochus and Symplectoscyphus anae Peña Cantero, Svoboda & Vervoort, 2002, with gonophores; 48-222, several polyps, up to 4 mm high, on Eudendrium generale von Lendenfeld, 1885, with gonophores; 48-223, some polyps, up to 3 mm high, on Sc. nana; ANT XVII/3: 111-5, some polyps, up to 5 mm high, on C. hicksoni and O. terranovae, with gonophores; 111-6, some polyps, up to 5 mm high, on Sy. exochus and C. hicksoni, with gonophores; 111-7, few polyps, up to 6 mm high, on O. terranovae and Tubularia longstaffi Hickson & Gravely, 1907, with gonophores; 111-9, numerous polyps, up to 15 mm high, on polychaete tube, A. grandis, E. generale, Zyzzyzus parvula (Hickson & Gravely, 1907), O. terranovae, Sy. weddelli, Sc. unifurcata and T. longstaffi, with gonophores; 111-18, numerous polyps, up to 5 mm high, on Billardia subrufa (Jäderholm, 1904), C. hicksoni, Halecium interpolatum Ritchie, 1907, O. terranovae, Sc. nana, Sy. weddelli and Tubularia sp.1, with gonophores; ANT XXI/2: PS65/39, few polyps, up to 6 mm high, on Sy. exochus, with gonophores; PS65/121, few polyps, up to 3 mm high, on Sc. nana; PS65/248, some polyps, up to 2 mm high, on Sy. curvatus and octocoral; PS65/276, numerous polyps, up to 5 mm high, on bryozoan, Staurotheca vanhoeffeni (Peña Cantero & García Carrascosa, 1994), Staurotheca glomulosa Peña Cantero, Svoboda & Vervoort, 1997 and Symplectoscyphus vanhoeffeni Totton, 1930, with gonophores; PS65/278, some polyps, up to 5 mm high, on Antarctoscyphus elongatus (Jäderholm, 1904), Halecium secundum Jäderholm, 1904 and sponge; PS65/279, few polyps, up to 3 mm high, on B. subrufa; PS65/280, some polyps, up to 3 mm high, on bryozoan; PS65/281, few polyps, up to 3 mm high, on O. terranovae.

Remarks. The analysis of some extraordinarily well preserved reproductive colonies allowed us to confirm the presence of a distinct pseudohydrotheca, as a thin layer of perisarc, reaching the base of tentacles, but not covering them (Fig. 2g), as noticed by Hickson & Gravely (1907) in the original description, and by other authors (Naumov & Stepanjants 1972). In contrast, according to Peña Cantero (2015: 378), “the perisarc sheet covering the polyp body is totally inconspicuous” in the type material he examined. Sampling procedures could strongly damage or even remove this delicate structure, and this could be the reason, in this specific case, of discrepancies among different authors’ observations.

Hickson & Gravely (1907) provided a detailed description of the gonophores of R. antarcticum (as Perigonimus antarcticus). However, these authors employed a terminology commonly used with reduced medusae (i.e. criptomedusoids or eumedusoids), therefore causing some misperception about the nature of the gonophore. According to the figures by Hickson & Gravely (1907) and our own observations while dissecting the gonophores (Fig. 2h), the gonosome of R. antarcticum can be recognized as fixed sporosacs with confidence, in agreement with the diagnosis of the genus (Bouillon et al. 2006).

Cnidome composed by microbasic euryteles [range 6.5–8.0 x 4.0–4.5 µm, mean 7.4±0.5 x 4.4±0.2 µm (n=22)] and desmonemes [range 5.0–6.0 x 3.0–3.5 µm, mean 5.7±0.3 x 3.3±0.3 µm (n=16)].

Ecology and distribution. Previously found between the tidal level (Hickson & Gravely 1907, as Perigonimus antarcticus) and 450 m (Totton 1930, as Gravelya antarctica); present material was collected from 62 to 287 m. In agreement with Peña Cantero et al. (2013) and Peña Cantero (2014a), a careful re-examination of the abundant records is needed in order to assign a pattern of distribution; for now, R. antarcticum can be tentatively considered as a species with an Antarctic-Kerguélen distribution. In spite of being a widely reported species in the whole Southern Ocean, its presence in the Weddell Sea sector had not been confirmed; the first evidence comes from the present study.