Halecium tangaroa CANTERO, 2017, sp. nov.

Halecium tangaroa sp. nov.

(Figs 17–18)

Material examined. Stn 124, a few stems up to 3 mm high on Staurotheca pachyclada (MNCN 2.03/653); Stn 130, a few tiny stems up to 7 mm high on S. densa, with gonothecae (NIWA 117601); Stn 139, a few stems up to 3 mm high on S. nonscripta, with female gonothecae (MNCN 2.03/654); Stn 140, a few stems up to 3 mm high on S. nonscripta, with male? gonothecae (NIWA 117602); Stn 178, two colonies with several stems up to 4 mm high on Staurotheca compressa, with female gonothecae (holotype, NIWA 115619; paratype, NIWA 115620).

Description. Monosiphonic stems, up to 7 mm high. Stems usually with series of annulations basally (Figs 17 A–B, 18A–B), either starting with internode provided with distal hydrotheca (Fig. 17 A), usually with several of lower order (Fig. 17 B), or with regular internode provided with long hydrophore (Fig. 18 B); in the former situation, regular internodes originating from hydrophore of lower-order hydrothecae (Fig. 17 C). Stems unbranched or scarcely branched, irregularly giving rise to primary branches, either originating from hydrophore of primary hydrotheca or, more frequently, from hydrophore of lower-order hydrothecae (Figs 17 C, 18C). Secondary branches usually originating from hydrophore of lower-order hydrothecae. Stem and branches divided into internodes by alternately arranged oblique nodes, usually with series of annulations at basal part. Internodes arranged in zigzag (Fig. 17 C). Hydrothecae alternately arranged in one plane and placed at end of free hydrophores (Figs 17 C–F, 18A–B, D); ratio adcauline length of hydrophore:diameter at diaphragm 0.6–1.6. Abcauline side of hydrophore straight or slightly convex; adcauline side straight or slightly concave (Figs 17 C–F, 18D). Hydrothecae exceeding distal node of internode; typically without pseudodiaphragm. Hydrotheca relatively high, widening distally, particularly at distal part; rim everted (Figs 17 A–F, 18A–E). Up to seventh-order hydrothecae (Fig. 18 C).

Gonothecae (Figs 17 G–H, 18F) flattened, bivalve-shaped on hydrorhiza and/or stem. Female gonothecae with about eight eggs (Fig. 18 G–H).

Measurements (in µm). Hydrothecae: diameter at aperture 180–235, diameter at diaphragm 100–140, height 65–100, adcauline length of hydrophore 70–200. Gonothecae: female, height 600–950, width 500–600; male?, height 470–500, width 400. Cnidome: microbasic euryteles?, with blunt ends [range 10.0–12.0 x 6.0–7.0, mean 11.1±0.6 x 6.6±0.4 (n=7); ratio, range 1.6–1.8, mean 1.7±0.1(n=7)] and microbasic mastigophores?, with sharp ends [range 6.0–7.0 x 1.5–2.0].

Remarks. Halecium tangaroa sp. nov. is characterized by tiny stems, up to 7 mm high, unbranched or with sparse and irregular ramification, with relatively high, flared hydrothecae on free hydrophores and with gonothecae both on stem and hydrorhiza. It is allied with several Antarctic species characterized by having hydrothecae on free hydrophores (e.g. H. antarcticum, H. pseudodelicatulum), but particularly with H. interpolatum, with which it shares the shape and size of the hydrotheca and the size of the microbasic euryteles. However, there are important differences between both species. Whereas H. tangaroa sp. nov. has tiny, up to 7 mm high, monosiphonic stems, H. interpolatum has much larger, up to 35 mm high, basally polysiphonic stems. They also differ in the branching pattern, as H. interpolatum has paired branches originating from hydrophore of primary hydrotheca, whereas stems are irregularly branched in H. tangaroa sp. nov., and the single branches usually originate from hydrophores of lower-order hydrothecae. In addition, they are clearly distinguishable because internodes in H. interpolatum have a very characteristic, long and straight basal part (cf. Peña Cantero 2014b). Finally, they also differ in the fact that gonothecae develop from within hydrothecae in H. interpolatum, whereas they originate from the stem and hydrorhiza in H. tangaroa sp. nov.

Ecology and distribution. Halecium tangaroa sp. nov. was collected at depths from 120 to 348 m, off Cape Adare, Adare Peninsula and Moubray Bay. It was found epibiotic on several species of Staurotheca (i.e. S. compressa, S. densa, S. nonscripta and S. pachyclada). Gonothecae were present in colonies collected in February.

Etymology. The specific name “ tangaroa ” is a tribute to the New Zealand research vessel RV Tangaroa.