(Pl. 1 A –E, Fig. 2 A –C)
Hydractinia angusta Hartlaub, 1904: 7, pl. 4. — Naumov & Stepanjants, 1972: 35, fig. 1. — Stepanjants, 1979: 13, pl. 1 fig. 7. — Peña Cantero & Ramil, 2006: 950, fig. 2 A. — Peña Cantero, 2012: 855.
Hydronema angustum — Stechow, 1923: 67. — Totton, 1930: 138. — Briggs, 1939: 10.
Hydractinia dendritica Hickson & Gravely, 1907: 9, pl. 2 figs 7–10.
Hydronema dendriticum — Stechow, 1921: 252. — Stechow, 1923: 67, fig. F.
? Hydractinia sp. —Stepanjants, 1972: 59, fig. 4.
Material examined. Stn. SHO — 16.ii. 2011, Ant.09/ 2011 ( 43 m): fertile (female) colony on worm tube (MHNG- INVE- 79798). Stn. ARD — 28.i. 2011, Ant. 11 / 2011 ( 39 m): fertile (female) colony on worm tube (MHNG-INVE- 79799).
Description. Hydrorhiza an encrusting mat issued through the coalescence of the stolonal system, covered by naked coenosarc (Pl. 1 C). Spines present, often continuous with each other (Pl. 1 D), provided with low ridges of perisarc (Pl. 1 E), open or closed at tips. Polyps dimorphic: gastro- and gonozooids. Gastrozooids up to 5 mm high in fixed material, notably longer in life (Pl. 1 A); with 12–14 amphicoronate tentacles and dome-shaped hypostome. Gonozooids dwarfed, less than 0.5 mm high, and degenerate, with 1–5 tentacles, and possibly no mouth; 4–6 rounded gonophores basally, each carrying three eggs. Nematocysts: desmonemes (6.0– 6.5) × ( 3.6–3.8) µm and microbasic euryteles ( 8.9–9.8) × ( 3.1–3.5) µm (both measured undischarged).
Remarks. Totton ( 1930) reexamined the type of H. dendritica Hickson & Gravely, 1907 and suggested that it could reasonably be considered as conspecific with Hartlaub's ( 1904) species, an opinion which is followed here.
Geographical distribution. Circum-Antarctic ( Peña Cantero & Ramil 2006).