Hydractinia dendritica Hickson & Gravely 1907

Hydractinia dendritica Hickson & Gravely, 1907

(Fig. 3 a–f)

Material examined. ANT XV/3: 48-33, some polyps, on H. interpolatum and O. terranovae, with developing gonophores; 48-36, numerous polyps, on O. terranovae, with developing gonophores; 48-39, several polyps, on dead octocoral, with developing gonophores; 48-210, numerous polyps, on O. terranovae, with developing gonophores; 48-220, several polyps, on Sc. nana, with developing gonophores; 48-264, several polyps, on dead octocoral, with developing gonophores; ANT XVII/3: 111-5, numerous polyps, on O. terranovae, with mature gonophores; 111-7, numerous polyps, on dead octocoral, O. terranovae and T. longstaffi, with mature gonophores; 111-9, numerous polyps, on Sc. unifurcata and T. longstaffi, with mature gonophores; 111-18, numerous polyps, on O. terranovae, with mature gonophores; ANT XXI/2: PS65/248, few polyps, on Eudendrium sp.2, with developing gonophores; PS65/281, several polyps, on O. terranovae, with developing gonophores.

Remarks. The material examined here entirely coincides with the re-description of the type-series of H. dendritica provided by Peña Cantero (2015), including the presence of erect stems, dwarf gonophores (Fig. 3e), and dactylozooids (Fig. 3f). Hydractinia dendritica seems to have different growing strategies depending on the substrate on which it develops, as it has already been reported for other congeneric (Schuchert 2008a). In the material examined, some sections of the same colony (or whole colonies) develop sharp spines when gastrozooids and dactyolozooids are not yet developed or are proportionally less abundant (Fig. 3c), while other portions (or colonies) are almost deprived of spines but have abundant gastrozooids, and some dactylozooids (c. one dactylozooid per 10–20 gastrozooids). The fully mature colony, where gonozooids carrying gonophores are found, have a sort of perisarcal wall joining spines, as a kind of “room divider”, which could act as an extra-protection for the reproductive polyps (Fig. 3b, e). All this different growing morphologies can be observed in the same colony, and we suggest they correspond either to differences in the substrate used or to ontogenetic changes.

Ecology and distribution. The species had been found once, in McMurdo Sound (Ross Sea), at a depth of 18 m (Hickson & Gravely 1907); our material was collected from several locations in the Weddell Sea, at depths between 62 and 473 m, growing on different substrates (see Tab. 3). Present contribution constitutes the second report of the species, and the first evidence from both the Weddell Sea and West Antarctica, pointing to a circum- Antarctic distribution.