Staurotheca pachyclada Jaderholm 1904

Staurotheca pachyclada (Jäderholm, 1904)

(Fig. 11 A–B)

Staurotheca pachyclada — Peña Cantero & Vervoort, 2003: 2698 –2702, figs 14, 15 (synonymy); Peña Cantero, 2006: 942, fig. 4D; 2008: 460; 2009: 1749; 2013: 131.

Material examined. Stn 15, one stem 180 mm high (NIWA 117537); Stn 17, eight fragments up to 100 mm long, perhaps belonging to six stems (NIWA 117538); Stn 18, one stem 83 mm high (MNCN 2.03/611); Stn 72, one stem 140 mm high (NIWA 117539); Stn 78, one fragmented stem 270 mm long (NIWA 117540); Stn 96, two stems up to 115 mm high (MNCN 2.03/612); Stn 101, one stem 175 mm high, originating from an old stem lying on bottom (NIWA 117541); Stn 108, one stem 60 mm high on pebbles (MNCN 2.03/613); Stn 124, one stem 220 mm high, basibiont of Halecium tangaroa sp. nov. and Billardia subrufa (MNCN 2.03/614); Stn 140, two stems, ca. 140 and 100 mm high (NIWA 117542); Stn 143, one stem 170 mm high (NIWA 117543); Stn 160, one stem 250 mm (MNCN 2.03/615).

Description. Stems up to 270 mm high, sparsely branched, usually with long, roughly straight, unbranched pinnae (only occasionally with few secondary ones). Branching alternate in one plane in younger stems, but irregular in several planes in older ones.

Hydrothecae arranged in decussate verticils of four to six hydrothecae, forming eight to 12 longitudinal rows; sometimes spirally arranged. Hydrothecae immersed into the branches in most of their volume; adcauline wall almost completely adnate (Fig. 11 A). In frontal view, hydrotheca typically cylindrical at distal half, but distinctly widening basally (Fig. 11 B). Hydrothecal aperture circular and sloping downwards. Hydrotheca without mushroom-shaped diaphragm (Fig. 11 A–B).

Measurements (in µm). Cnidome: larger microbasic mastigophores [range 20.0–21.5 x 4.5–5.0, mean 20.8±0.6 x 5.0±0.2 (n=10); ratio, range 4.0–4.4, mean 4.2±0.1 (n=10)]; smaller (9 x 2.5–3).

Remarks. Stems seem to start branching alternately in one plane. Nevertheless, branches are irregularly arranged in several planes in older colonies. For example, a 60 mm high stem from Stn 108 is alternately branched in one plane, a 170 mm high stem from Stn 143 is basally branched in one plane, but in several planes upwards, especially at distal part, and, finally, a 220 mm high stem from Stn 124 is irregularly branched in several planes.

This species has stems scarcely branched, usually just provided with long, roughly straight, unbranched pinnae. Sometimes, however, a few more-developed pinnae might give rise to a few secondary ones, as in colonies from Stn 108 and Stn 124. Although hydrothecae are typically deprived of mushroom-shaped diaphragm, I have observed it in some basal hydrothecae from Stn 96. The perisarc is usually smooth, but striated in material from stations 108, 124, 140 and 143.

In this species, the diameter of stem and branches is roughly constant, even if perisarc constrictions are present. In addition, the outline of stem and branches is distinctly straight.

Ecology and distribution. Reviewed by Peña Cantero & Vervoort (2003). Eurybathic species (Peña Cantero & Vervoort 2003), found at depths from 42 (Stepanjants 1979) to 1405 m (Peña Cantero & Vervoort 2003); present material at depths between 212 and 736 m, epilithic on pebbles, and basibiont for colonies of Billardia subrufa and Halecium tangaroa sp. nov.

Circum-Antarctic distribution (cf. Peña Cantero & Vervoort 2003). Recently reported from off Livingston Island (Peña Cantero 2006, 2008), Trinity Island (Peña Cantero 2008), and Low Island (Peña Cantero 2013), in West Antarctica, and from the Balleny Islands (Peña Cantero 2009), in East Antarctica. See Peña Cantero & Vervoort (2003) for previous records. In the Ross Sea, already known from the central basin, at Pennell Bank and off Cape Hallett and Cape Adare (Peña Cantero & Vervoort 2003). Present material off Cape Adare, Adare Peninsula, Possession Islands, Moubray Bay and Cape Hallett.