Amphinema biscayana

Amphinema biscayana (Browne, 1907)

(Fig. 2)

Bimeria biscayana Browne, 1907: 15, 16, 18, 21, fig. 1, pl. 1 figs 4–5; Stechow, 1923: 75; Rees, 1938: 3, 6; Le Danois, 1948: 174, 236.

Garveia biscayana — Vervoort, 1985: 277; Van der Land et al., 2001: 118.

‘Garveia’ biscayana —Altuna (Prados), 2007: 684.

Amphinema biscayana — Schuchert, 2000: 413, 415–417, fig. 3A–E; 2001: 21–22, fig. 11A–D; 2007: 317–319, fig. 51; Bouillon et al., 2006: 188.

Material examined. 9753#4, one strongly polysiphonic stem 30 mm high (broken into three fragments), with gonophores; 51416#1, four strongly polysiphonic stems up to 65 mm high, and two polysiphonic, distally truncated, stems 18 and 7 mm high, no gonophores; 51805, two stems 65 and 45 mm high, no gonophores; JC011/ 0 17, one stem 85 mm high, neither polyps nor gonophores; JC011/023, several stems up to 90 mm high, neither polyps nor gonophores; JC011/075, several stems up to 65 mm high, in bad condition, neither polyps nor gonophores.

Description. Strongly polysiphonic stems, up to 85 mm high. Branching irregular. In strongly polysiphonic portions, stem main tube completely surrounded by numerous, distinctly thinner accessory tubes (Fig. 2A). Distal part of stem and branches with main tube mostly naked, with few accessory tubes (Fig. 2B–C). Main tube with a sort of coarse network formed by thin layers of perisarc (Fig. 2B). Polyps with a basal cup of perisarc (Fig. 2C–D). Gonophores spherical, with perisarc envelop (Fig. 2E–F), developing into an actinula-like form, one per gonophore, with four tentacles provided with desmonemes (Fig. 2F–G).

Cnidome consisting of microbasic euryteles (8– 9 x 4 µm) (Fig. 2H) and desmonemes (6 x 4 µm).

Remarks. See Schuchert (2000: 415–417) for a wider description of the species. In A. biscayana, the axial tube of the side branches originates from auxiliary tubes of the main branch and not from the axial tube, in contrast with what happens in the similar Garveia arborea (Browne, 1907), in which the main tube of the branches originates from the main tube of the stem (see Schuchert 2000, 2007).

The allocation of the species into the genus Amphinema is provisional as no information on the mature medusa is available (see Schuchert 2000: 417).

According to Schuchert (2000, 2007), gonophores develop into normal medusa (he had to dissect them out of their envelope to reveal their anatomy). Schuchert (2000: 417) indicated that the oldest medusae “had a simple manubrium without appendages, four broad radial canals, two opposite strongly developed and large tentacle bulbs, elongated, conical, presumably without ocelli, each with one long tentacle without desmonemes. Other bulbs small or absent. Between larger tentacles (presumably at interradial positions) two very small tentacles (totally 4), their tips provided with desmonemes”.

The gonophores found in our material differ from those described by Schuchert (2000, 2007). Even when at first sight the general appearance and presence of four tentacles reminds a medusa, there is no sign of radial canals, manubrium or sub-umbrellar cavity. Instead, its massive structure and tentacles seems to correspond with an actinula-like form (Fig. 2E–F).

The differences with Schuchert’s (2000, 2007) material could indicate that our material belongs to a different species, but they could be related to development or sexual dimorphism. The young medusa described by Schuchert, which was still inside its perisarc envelop, could undergo a transformation into a polyp, with a loss of the manubrium and the perradial tentacles, and a further development of the four small interradial tentacles he described. According to Schuchert (2000, 2007) the tips of these four interradial tentacles are provided with desmonemes. In our material, the four tentacles of the actinula-like form are also provided with these nematocysts.

This could be a case of ontogeny reversal, with direct transformation of isolated medusa buds into polyps. This phenomenon, which has been described for several species (cf. Piraino et al. 1996), can happen either before the medusa liberation (e.g. Podocoryna carnea M. Sars, 1846, see Müller 1913) or in newly liberated medusae (e.g. in Turritopsis nutricula McCrady, 1859, see Bavestrello et al. 1992; Piraino et al. 1996). This ontogeny reversal has even been reported in Leptothecata. For example, Carré & Carré (1990) described how medusae of Eucheilota paradoxica Mayer, 1900 differentiate into one or two polyps, which develop progressively whereas the mother medusa undergoes gradual regression.

On the other hand, it is also possible that the actinula-like form observed in our material resulted from development of a fertilised egg. It is known in certain hydroids that the planula continues its development within the gonophore, either partially (producing an actinula) or completely (producing a normal polyp), for example in Tubularia Linnaeus, 1758 and Cordylophora Allman, 1844, respectively (cf. Bouillon et al. 2006). In our material, development from a fertilised egg would imply that the retained female medusa only develops an egg, because all gonophores develop a single polyp.

Browne (1907) expected that this species would have fixed sporosacs as gonophores and Schuchert (2000) indicated that it was surprising for a deep-water species forming large colonies to produce medusa, although he pointed out that there are other examples, e.g. Stegopoma plicatile (M. Sars, 1863). Our material seems to indicate that A. biscayana does not liberate medusa, but a polyp form, as it would be expected for a large deep-water species. However, our observations do not dismiss the possibility that this species could under other circumstances liberate medusa. As observed in other species, e.g. E. paradoxica (Carré & Carré 1990), different environmental conditions induce different reproductive strategies.

Ecology and distribution. This is a deep-water species, found at depths between 754 (Browne 1907) and 2074 m (Schuchert 2000). The present material ranges from 1942 to 2780 m, notably increasing its lower bathymetric limit. Collected with medusa buds in August (Schuchert 2000) and April (present material). Found growing on coral (Browne 1907).

The species is only known from the northeast Atlantic: Bay of Biscay (Browne 1907), south of Iceland (Schuchert 2000) and Rockall Plateau (Schuchert 2007). Our material comes from Porcupine Seabight and the Mid-Atlantic Ridge.