Oswaldella stepanjantsae El Beshbeeshy 1991

Oswaldella stepanjantsae El Beshbeeshy, 1991

(Figs 13, 16 M, 18E, 20D)

Material examined. United States Antarctic Research Program (USARP): Stn 32/2121, Eltanin, 12 February 1968, 72°27’– 72°26’S, 177°04’– 177°12’E (off Cape Adare, Ross Sea), 1883–1890 m (USNM 1003367); Stn 691/ 26, Hero, 10 February 1969, 63°26’– 63°25’S, 62°15’– 62°14’W (south of Low Island, Antarctic Peninsula), 119– 124 m (USNM 1003370); Stn 833/018-1, Hero, 1 April 1983, 62°41.50’S, 56°01.75’W (off Elephant Island, South Shetland Islands), 210–240 m (USNM 1003372). New Zealand Antarctic Expedition TAN0802: Stn 17, Tangaroa, 9 February 2008, 73.1245°S, 174.3205°E (Ross Sea, Cape Adare), 321 m (NIWA 35451).

Description. Polysiphonic, branched stems, up to 500 mm high, divided into internodes. Angle between cauline apophyses and stem ca. 45°. Cauline apophyses with two to six axillary nematophores, each emerging through perisarc hole (Fig. 13 B–C), frequently with very short collar-shaped nematotheca (Fig. 13 B), and with up to two extra nematophores, each emerging through ‘mamelon’ (Fig. 13 B–C).

Hydrocladia much branched (Fig. 13 A), with up to fourth-order hydrocladia. First hydrocladial internode bifurcated, with two similar prongs (Fig. 13 A). Mesial inferior nematophore emerging from marked swelling at proximal third of internode (Fig. 13 D–E); with rudimentary, nail-shaped nematotheca (Fig. 13 F–G). Hydrotheca elongate, placed on middle of internode. Abcauline wall roughly straight. Adcauline wall mostly adnate (Fig. 13 D– E). Hydrothecal aperture circular, perpendicular to longitudinal axis of internode (Fig. 13 E); rim even.

Gonothecae fusiform, with sub-terminal aperture (Fig. 13 H).

Remarks. Oswaldella stepanjantsae is remarkable for the high variability in the number of axillary nematophores in the cauline apophyses. Peña Cantero & Vervoort (2004) reported from two to six. Although we have only observed four, the structure of the central nematophores in some of the cauline apophyses examined seems to indicate that they might have resulted from fusion of the four central nematophores (Fig. 13 C). In addition, although the species typically has two mamelons in the cauline apophyses, sometimes one, or even both, may be absent (Peña Cantero & Vervoort, 2004).

The SEM study has also revealed the, so far unknown, frequent presence of a neck of perisarc in the axillary nematophores of the cauline apophyses (Fig. 13 B).