Oswaldella bifurca Hartlaub 1904

Oswaldella bifurca Hartlaub, 1904

(Figs 1, 16 A, 17A, 19A)

Material examined. United States Antarctic Research Program (USARP): Stn 32/2095, Eltanin, 3 February 1968, 76°04’– 76°05’S, 164°46’– 164°51’W (Ross Sea, center of sea), 513–550 m (USNM 1003302).

Description. Monosiphonic, usually unbranched stems, up to 60 mm high, divided into internodes. Angle between cauline apophyses and stem ca. 70°. Cauline apophyses with one or two axillary nematophores emerging through simple perisarc holes (Fig. 1 B–C).

Hydrocladia much branched (Fig. 1 A); branching symmetrical, up to the fourth-order. First hydrocladial internode bifurcated, with two similar prongs (Fig. 1 A–B). Mesial superior nematophore emerging above adcauline hydrothecal wall. Mesial inferior nematophore located on a distinct elevation at lower third of internode; without nematotheca (Fig. 1 D–F). Distal hydrocladial internodes with sharp distal end (Fig. 1 A).

Hydrothecae placed on basal half or third of hydrocladial internodes. Hydrotheca low, about as high as wide. Adcauline hydrothecal wall either completely adnate to internode (Fig. 1 D) or with tiny free portion (Fig. 1 E). Abcauline wall roughly straight. Hydrothecal aperture usually circular (Fig. 1 E), although frequently frontally depressed (Fig. 1 D); rim usually rising laterally.

Gonothecae immature, inverted cone-shaped (Fig. 1 A, G).

Remarks. Peña Cantero & Vervoort (2004) already stated that the number of axillary nematophores on the cauline apophyses was variable, finding apophyses with a single axillary nematophore and others with two, even on the same stem. We have also observed that variability and, as those authors indicated, there is a clear tendency in the axillary nematophores to coalesce (Fig. 1 C).