Rhizorhagium antarcticum Hickson & Gravely 1907

Rhizorhagium antarcticum (Hickson & Gravely, 1907)

(Figs 1 C, D, 2E, F, 3F)

Perigonimus antarcticus Hickson & Gravely, 1907: 4 –6, pl. 1 figs 1–3, pl. 4 fig. 32; Rees, 1938: 7; 1956: 338, 347; Naumov & Stepanjants, 1972: 33, 35; Stepanjants, 1972: 56–57, fig. 1; 1979: 10, pl. 1 fig. 1.

Atractylis antarctica — Vanhöffen, 1910: 283, fig. 8;

Atractylis antarcticus — Ritchie, 1913: 10, 11.

Gravelya antarctica — Totton, 1930: 139, fig. 1a, b.

? Gravelya antarcticum — Schuchert, 1996: 41 –42, fig. 21a–d.

Rhizorhagium antarcticum — Stechow, 1919: 21; Rees, 1956: 338, 347; Rees & Thursfield, 1965: 55; Millard, 1971: 401, fig. 3; Branch & Williams, 1993: 8, fig.; Peña Cantero, 2004: 768; Bouillon et al., 2006: 137, fig. 79I, J; Peña Cantero et al., 2013: 735 –736, figs 2c–d, 3a–b.

Material examined. Discovery Antarctic Expedition 1901-04: Type (NHM 1907.8.20.1), McMurdo Bay (Ross Sea), 1–20 fms, several stems, with gonophores, on Hydrodendron arboreum.

Description. Usually stolonal (Fig. 1 C, D), but stems with up to three polyps present. Stems covered with perisarc (Figs 1 C, D, 2E, F), smoothly disappearing on hydranth body (Fig. 2 E, F). No distinct pseudohydrotheca present. Polyps 800–900 µm high and 250–300 µm in maximum diameter, with conical hypostome and a distal crown of nine to 12 filiform tentacles (Fig. 2 E, F).

Gonophores c. 950 µm high and c. 680 µm in maximum diameter, on short pedicels, either on hydrorhizal stolon or at basal part of largest stems (Figs 1 C, D, 3F). “The colonies appear to be invariably dioecious. In both sexes the gonophore is a degenerate medusa... protected by a thin layer of perisarc. It is larger in the female than in the male (1.1 mm. x 0.9 mm. in the female, and 0.9 mm. x 0.7 mm. in the male). In the female gonophore there is a large sub-umbrella cavity (fig. 3, su.c), the manubrium is well developed and has a well-marked endoderm cavity. In the young gonophore there is a distinct endodermal layer of cells and mesoglea in the umbrella, but in the adult gonophore (fig. 32) these are reduced to a non-cellular mesoglea except at the margin, where a cord of cells represents the ring canal. There are no radial canals in the adult gonophore. In the male gonophore the subumbrella cavity is completely filled with sperm cells (figs. 2 and 32, sp.)” (Hickson & Gravely, 1907: 5).

Cnidome consisting of two categories of nematocysts, Type I (microbasic euryteles?) and desmonemes.

Measurements (in µm). Cnidome: Type I [range 7.0–7.5 x 4.0–4.5, mean 7.3±0.2 x 4.1±0.2 (n=10); ratio, range 1.6–1.9, mean 1.8±0.1 (n=10)], desmonemes [range 5.5–6.0 x 3.0–3.5].

Remarks. This is a well-characterized species, the only important character missing from the original description was information related to the cnidome. The examination of the type material has allowed me to complete this partially. The heteronemes could not be identified because they could not be seen discharged. However, it is worth mentioning that Peña Cantero et al. (2013), in material that perfectly agrees with this species, identified them as microbasic euryteles. Millard (1971) also indicated the presence of microbasic euryteles.

Hickson & Gravely (1907) gave an excellent description of the gonophores (see above).

Problems with this species were related with its genus allocation. Originally described as Perigonimus antarcticus, it was later considered belonging to Atractylis by Vanhöffen (1910). Totton (1930) even erected the new genus Gravelya to allocate it. Rees (1938) re-erected the genus Rhizorhagium M. Sars, 1874 to include all unbranched colonial bougainvilliids with pseudohydrotheca not enveloping the tentacles, with one crown of filiform tentacles and with fixed sporosacs. Calder (1988) separated members with nipple-shaped hypostomes from this group, including the type species of Rhizorhagium (R. roseum). Consequently, Schuchert (1996) saw it necessary to move those species that lack nipple-shaped hypostomes into other genera, in particular to Aselomaris Berrill, 1948 and Gravelya Totton, 1930, the latter harbouring the present species. More recently, however, Calder’s division based on the shape of the hypostome was not supported (cf. Bouillon et al. 2006: 126) and the species is again considered belonging to Rhizorhagium.

Schuchert’s (1996) material seems to correspond to a different species. The perisarc and the pseudohydrotheca are contaminated with detritus and the size of polyps is smaller. Female gonophores are also distinctly smaller (0.5 x 0.3 mm) than those studied by Hickson & Gravely (1907). He also indicated that there are up to 20 eggs per gonophore; Millard (1971) and Branch & Williams (1993) found 30– 40 eggs.

According to the diagnosis of the genus provided by Bouillon et al. (2006), Rhizorhagium should have ‘perisarc firm, continued over polyp base as a pseudohydrotheca, but never investing tentacle bases’. According to Hickson & Gravely, (1907), ‘the perisarc is continued as an exceedingly thin film over the hydranth as far as the base of the tentacles’. Naumov & Stepanjants (1972: 35) also indicated that ‘the polyp’s body is covered by a fine sheet of perisarc reaching the base of tentacles’. In the material examined the perisarc is not firm and it does not form a distinct pseudohydrotheca; the perisarc sheet covering the polyp body is totally inconspicuous.

Ecology and distribution. Rhizorhagium antarcticum has been found at depths from the tidal level (Hickson & Gravely 1907) to 450 m (Totton 1930), epibiotic on hydroids (Hartlaub 1904; Ritchie 1913; Totton 1930; Naumov & Stepanjants 1972; Stepanjants 1972, 1979; Peña Cantero et al. 2013) and tube of polychaete (Peña Cantero et al. 2013) and epilithic on stones (Millard 1971). It is used in turn as substrate by other species of hydroids (Peña Cantero et al. 2013). Gonophores have been found in colonies collected in January (Totton 1930; Stepanjants 1979; Peña Cantero et al. 2013), February (Totton 1930; Stepanjants 1979), from May to July (Stepanjants 1979), August (Ritchie 1913; Stepanjants 1979) and in December (Stepanjants 1979).

Probably Antarctic-Kerguelen distribution, although a careful re-examination of previous records is needed. In Antarctic waters, recorded from the Ross Sea (Hickson & Gravely 1907; Ritchie 1913; Totton 1930; Peña Cantero et al. 2013), Adélie Land (Naumov & Stepanjants 1972), Davis Sea (Vanhöffen 1910; Stepanjants 1972, 1979), and Sodruzestva Sea (Emery glacier) (Stepanjants 1979), in East Antarctica, and from the Bellingshausen Sea (Hartlaub 1904), in West Antarctica. In sub-Antarctic waters, it is known from off Marion and Prince Edwards Islands (Millard 1971; Branch & Williams 1993) and Crozet (Stepanjants 1979). Schuchert’s (1996) record is from New Zealand.