Halecium arcticum

Biology Found between 5 m (this study) and 40 m (Broch, 1910) depth. Colonies growing on different types of substratum, most frequently colonizing rhizoids of laminarians (Laminaria digitata, L. saccharina, Alaria esculenta) and bryozoans (Table 2). Rarely, they were found on Eudendrium annulatum Norman, 1864, ascidiacea, Balanus sp., and rock.

? Halecium reversum. — Naumov, 1969: 487, fig. 341. in part Halecium undulatum. — Calder, 1970: 1510, pl. 2 figs 5 – 9.

Description Erect, branching stems arising from creeping, ramified stolons. Stems usually short, monosiphonic, occasionally weakly polysiphonic in basal part (up to 5 tubes). Colonies without distinct primary axis, bushy, branching irregularly, with up to 40 hydranths per shoot; paired or unpaired side branches arising mostly at acute angles immediately below hydrothecae (Figs 2 – 3). Perisarc rather thick, strongly corrugated nearly throughout, with smooth stretches below hydrothecae and at origin of some branches (Fig. 2); nodes frequent, irregularly distributed, often delimiting short segments with two annular bulges but without hydrothecae (Fig. 3). No zooxanthellae observed. Primary hydrothecae either at end of side-branches or mostly on distal part of short segments, hydrophore relatively short, hydrothecae thus appearing almost sessile. Hydrotheca relatively deep, walls everted, rim rolled, diaphragm thin, ring of desmocytes present; below diaphragm regularly an annular thickening of the perisarc (pseudodiaphragm, Figs 3 – 4). Secondary hydrothecae with distinct hydrophore; hydrophore as high as its diameter or longer (Fig. 4). Hydranth with about 16 tentacles. Two types of nematocysts: (1) abundant elongate, almond shaped capsules, (6 – 7) x (2) μm, not seen discharged; (2) rare oval microbasic heteronemes, (8 – 9.5) x (3 – 3.5) μm, not seen discharged. Colonies monoecious with dimorphic gonothecae. Gonothecae lacking hydranths and arising from branches or stolons, sometimes developing from within primary hydrothecae; stalk either annulated or smooth; usually on stems, sometimes on stolons. Female gonothecae (Figs 5 – 10) circular to slightly ovoid, lentil-shaped, with two prominent, distal horns flanking a U-shaped aperture (Figs 9 – 10); horns rarely absent (Fig. 11). Perisarc rather thick, without crests, ridges or radiating creases; no internal secondary capsule; soft tissue not forming reticulate pattern; irregular pedicel of variable length. Gonotheca with usually 6 – 7 developing planulae. Male gonothecae smaller (Figs 12 – 13), ovoid lenticular, flattened for about a factor 1.6 (ratio width-thickness), surface smooth; usually without distinct horns, rarely a small one present; small, distal aperture on slight neck formation. For dimensions, see Table 1. TABLE 1. Dimensions of some characters of Halecium arcticum sp. nov.

Eudendrium. annulatum 2 (Hydrozoa) Ascidiacea 2 Balanus sp. 2 Rock 2

Diagnosis Branched stems, up to 16 mm, mono- or rarely polysiphonic, strongly annulated or corrugated, many short segments without hydrothecae; hydrothecae deep, everted; female gonotheca lentil-shaped, surface smooth, with two distinct apical processes.

Distribution Svalbard Archipelago, NE Canada. Substratum F % Algae 55 Bryozoans 10 Bivalves 3