Halecium pseudoincertus Cantero, 2014, sp. nov.

Halecium pseudoincertus sp. nov.

(Figs 10 B, 12)

Halecium incertus — Peña Cantero, 2008: 454 –455, fig. 1b; 2009: 1747; 2012: 857.

Material examined. Spanish Antarctic Expedition Bentart 95: Stn 22R, two stems, up to 90 mm high, on pebbles; Stn 31A, one stem, c. 110 mm high. Spanish Antarctic Expedition Bentart 2006: Stn 34, a stem, c. 165 mm high (Holotype, MNCN 2.03/445, Museo Nacional de Ciencias Naturales, Madrid, Spain), and a stem, 75 mm high (Paratype), basibiont of Stegopoma plicatile; Stn 34-2, a distal stem fragment, c. 43 mm long. New Zealand Antarctic Expedition TAN0402: Stn 232, one stem, c. 310 mm high; Stn 277, one stem, c. 75 mm high, on gravel.

Diagnosis. Long and thin, polysiphonic, mainly unbranched stems, up to 310 mm high. Stem giving rise every third hydrotheca and alternately in two planes to monosiphonic branches at distal part. Branches originating from hydrophore of primary hydrotheca, monosiphonic, unbranched or with up to two secondary branches. Hydrothecae alternately arranged in two planes. Hydrotheca at the end of short, mostly adnate hydrophore provided with a strongly marked pseudodiaphragm. Hydrotheca distinctly widening distally; rim everted. Adcauline hydrothecal wall free; distinctly larger than abcauline one. Hydrothecal aperture directed downwards. Up to five-order hydrothecae present. Gonothecae unknown. Cnidome consisting of isorhizas, larger microbasic euryteles?, smaller microbasic euryteles and microbasic mastigophores.

Description (Holotype). Stem polysiphonic, except for last 17 mm, mostly unbranched (Fig. 10 B), tortuous and relatively long (c. 165 mm high) and thin (c. 1 mm at basal part).

Stem distally provided with 16 primary branches and eight secondary ones. Branching regular, alternate at every third hydrotheca in two planes, forming an obtuse angle of c. 90° in distal part of stem (where branches are present). Probably, angle slightly larger at basal part. Length of lateral branches decreasing upwards.

Branches monosiphonic, unbranched, or with a single secondary branch (occasionally two). Secondary branches directed inwards (i.e., towards space delimited by primary branches). Lower-order branches resting on large apophysis formed at the side of primary hydrothecal hydrophore just below hydrotheca.

Hydrothecae alternately arranged in two planes, making an angle of c. 90°. Hydrothecae resting on hydrophores adnate in most of its adcauline length (Fig. 12 A–D); ratio between adcauline length of hydrophore and diameter at diaphragm 0.8–1.3. Hydrotheca low, distinctly widening from diaphragm upwards, particularly at adcauline side (Fig. 12 A, B, D). Adcauline wall distinctly higher than abcauline one (Fig. 12). Adcauline hydrothecal wall free to internode (Fig. 12 A–F). Hydrothecal aperture directed downwards (Fig. 12). Rim everted. With up to five lower-order hydrothecae on smooth hydrophores (Fig. 12 A). Lower-order hydrothecae symmetrical (Fig. 12 A–C, E). A strongly marked pseudodiaphragm present (Fig. 12).

Gonothecae unknown.

Measurements (in µm). Hydrothecae: diameter at aperture 220–225, diameter at diaphragm 160–180, height 25–45. Hydrophore: adcauline length 120–230. Internodes: length 750–1070, diameter at distal part 130–170. Cnidome: isorhizas [range 9.5– 11 x 3.0–3.5, mean 10.2±0.5 x 3.1±0.2 (n=10); ratio, range 2.7–3.7, mean 3.2±0.3 (n=10)], smaller microbasic euryteles [range 7 x 3, mean 7.0±0.0 x 3.0±0.0 (n=6), ratio, range 2.3, mean 2.3±0.0 (n=6)], microbasic mastigophores c. 7.5 x 2, and larger microbasic eurytele? c. 14 x 6.

Remarks. Although the first 120 mm of the stem are deprived of branches, their origin is visible through a careful examination, clearly indicating that they are lost as the colony grows.

Halecium pseudoincertus sp. nov. is similar to H. incertus in the hydrothecal shape and the presence of pseudodiaphragm. However, they are easily distinguishable by the colonial structure. Whereas in H. pseudoincertus sp. nov. the stem is mostly unbranched, being provided with lateral monosiphonic branches, originating alternately in two planes, at its most distal part only, in H. incertus the colonies have thick and polysiphonic stems, repeatedly branched irregularly in several planes, and primary and secondary branches also polysiphonic. In addition, the hydrothecae, and particularly the isorhizas, are distinctly smaller in H. pseudoincertus sp. nov. Internodes are also thinner and longer in H. pseudoincertus sp. nov. Furthermore, the hydrophore is completely adnate in H. incertus, whereas it has a distinct free portion in H. pseudoincertus sp. nov. Finally, whereas Millard (1977) indicated that secondary hydrophore, with or without pseudodiaphragm, are rather rare in H. incertus, up to five-order hydrothecae are present in H. pseudoincertus sp. nov. Because of all these differences, it is easy to identify both species even in the absence of gonothecae, which justifies the description of the new species to science despite the absence of information on gonothecae.

Ecology and distribution. Halecium pseudoincertus sp. nov. has been collected at depths from 96 (Peña Cantero 2009) to 1019 m (Peña Cantero 2008), epilithic on pebbles (Peña Cantero 2008) and gravel (Peña Cantero 2009). It is used as substratum by colonies of other hydroids (Peña Cantero 2009, 2012).

Halecium pseudoincertus sp. nov. seems to have a circum-Antarctic distribution. It has been reported from the Bellingshausen Sea (Peña Cantero 2012) and off Deception Island and the north of Livingston Island (Peña Cantero 2008), in West Antarctica, and off Sturge and Young islands, the Balleny Islands (Peña Cantero 2009), in East Antarctica.

Etymology. The specific name pseudoincertus refers to the fact that the shape of the hydrotheca in this species is reminiscent of that of H. incertus.