Antarctoscyphus fragilis Pena Cantero, Svoboda and Vervoort 1999

Antarctoscyphus fragilis Peña Cantero, Svoboda and Vervoort, 1999

(Figures 5 (e,f), 9)

Antarctoscyphus fragilis Peña Cantero et al., 1999, p. 1748 –50, fig. 3.

Material examined

Eltanin: 21/282, one stem, 33 mm high, with only one paired branch left, and a highly fragmented stem (longest fragment c. 53 mm long), on gravel.

Diagnosis

Erect stems, up to 140 mm in height. Largest stems basally polysiphonic. Stem internodes long, limited by slight perisarc constrictions, usually arranged in a marked zigzag. Cauline apophyses far below distal node, forming a large angle with stem. Paired branches divided into long internodes. Paired branches slightly branched. Each paired branch branching at second and seventh internodes, giving rise to secondary branches; first second-order branch usually branching at third internode, forming one tertiary branch that, occasionally, gives rise to one fourth-order branch. Hydrothecae widely separated, alternately arranged in two planes, with strong tendency to unilateral disposition. Hydrotheca cylindrical, slightly curved outwards. Adcauline wall completely free, convex basally, roughly straight distally, as it happens to abcauline wall. Hydrothecal aperture cusps of similar development and separated by shallow embayments. Gonothecae unknown.

Description

Stems basally polysiphonic, up to 140 mm high. Material much fragmented; the largest fragment, distally and basally broken, 53 mm long, and slightly polysiphonic, except for the most distal 14 mm. Diameter of stem c. 300 µm.

Stems indistinctly divided into internodes, with slight perisarc constrictions. Stem internodes relatively long (c. 800 µm) and arranged in a marked zigzag (little marked in some stems). Cauline apophyses placed far below distal node, forming a large angle with stem. Paired branches divided into long and thin internodes, each with one hydrotheca (Figure 9 (a)).

Paired branches slightly branched; sometimes unbranched (Figure 9 (a–c)). Branches up to fourth order present; each paired branch branching at its second and seventh internodes, giving rise to two secondary branches, the first to the right and the second to the left. First second-order branch branching to the left at its third internode, giving rise to a branch of third order that gives rise to a branch of fourth order to the right at its third internode.

Hydrothecae alternately arranged in two planes, with strong tendency to unilateral disposition (Figure 9 (a)). Hydrotheca almost cylindrical and slightly directed outwards (Figures 5 (e,f) and 9(b,d)). Adcauline hydrothecal wall roughly straight, slightly convex basally (Figures 5 (f), 9(b)). Abcauline wall mostly straight (Figures 5 (e,f), 9(b,d)). Adcauline wall completely free. Hydrothecae widely separated, non-overlapping (Figure 9 (a)). Hydrothecal aperture rim with one adcauline and two abcauline cusps, equally developed (Figures 5 (e,f) and 9(d)).

Gonothecae unknown.

Measurements (in µm)

Distance between cauline apophyses: 4–11 mm. Hydrotheca: free adcauline length, range 850–1050, mean 960 ± 62 (n = 7); abcauline length, range 800–950, mean 890 ± 52 (n = 7); diameter at aperture, range 260–300, mean 280 ± 14 (n = 7). Cnidome: larger size group, range 12.0–14.5 × 3.0–3.7, mean 13.7 ± 1.5 × 3.5 ± 0.6 (n = 12).

Bathymetric and geographic distribution

Hitherto, A. fragilis was only known off McDonald Ice Rumples, on the east coast of the Weddell Sea, where it was collected at a depth of 460–470 m (Peña Cantero et al. 1999). Our material was collected at 1896–1920 m off the Pacific entrance of the Magellan Strait, considerably increasing its known geographic and bathymetric distribution, which allows us to define a West Antarctic–Patagonian distribution for the species. This is a remarkable discovery, as it constitutes the first record of a species of Antarctoscyphus from the Magellan region, which makes it possible to redefine the pattern of geographic distribution of the genus as Pan-Antarctic (see General discussion below).