(Figs 3 A, 4; Table 2)
Material examined. Stn 126, one polyp, c. 13 mm long (holotype, NIWA 115613).
Description. 13-mm-long polyp (Fig. 3 A). Basal 9 mm devoid of tentacles, enveloped by thin perisarc. Basal 3 mm provided with anchoring filaments. Capitate tentacles, usually in groups of three, one of them usually more developed.
Gonophores absent.
Cnidome consisting of stenoteles in two size groups (Fig. 4 A –B), desmonemes (Fig. 4 A, D), fusiform microbasic mastigophores? (Fig. 4 B, D) and an unidentified, relatively large, tear-shaped type (Fig. 4 C).
Measurements (in µm).Cnidome: larger stenoteles [range 33– 36 x 30 –33, mean 34.8±1.0 x 31.6±0.8 (n=10); ratio, range 1.0–1.2, mean 1.1±0.1 (n=10)], smaller stenoteles [range 19– 21 x 15–16, mean 19.9±0.7 x 15.6±0.4 (n=10); ratio, range 1.2–1.4, mean 1.3±0.1 (n=10)], desmonemes [range 20–22 x 13.5–17, mean 20.8±0.7 x 14.6±0.9 (n=10); ratio, range 1.3–1.5, mean 1.4±0.1 (n=10)], microbasic mastigophores? [range 27– 30 x 13–15, mean 28.6±1.0 x 13.6±0.7 (n=10); ratio, range 1.9–2.3, mean 2.1±0.1 (n=10)], unidentified type [range 43– 48 x 20–25, mean 45.6±1.7 x 22.4±1.6 (n=12); ratio, range 1.9–2.2, mean 2.0±0.1 (n=12)].
Remarks. Notwithstanding the absence of gonophores, Monocoryne antarcticasp. nov. is recognizable by the cnidome (cf. Table 2). Only Monocoryne bracteata (Fraser, 1943) also has two size categories of stenoteles (Schuchert et al. 2016), although in Fraser’s species they are smaller, particularly the larger ones. The unidentified type of nematocysts in M. antarcticasp. nov. could correspond to the microbasic euryteles observed in Monocoryne gigantea (Bonnevie, 1899) and Monocoryne colonialis Brinckmann-Voss & Lindner, 2008, but if so they are larger in M. antarcticasp. nov. (cf. Table 2).
Monocoryne minor (4) Monocoryne sp.(3) Monocoryne antarcticumsp. nov.
Apart from the differences in the cnidome, there are geographical reasons to consider M. antarcticasp. nov. a different species. Monocoryne colonialis and M. bracteata are known from the North Pacific, M. gigantea from the North Atlantic and Monocoryne minor Millard, 1966 from South Africa. Only a species of Monocoryne, described as Monocoryne sp. by Stepanjants (1979), had been reported from Antarctic waters. Monocoryne antarcticasp. nov. can be distinguished from Stepanjants’ material by the cnidome, as only one category of stenoteles was described by her, microbasic euryteles or equivalent were not described, and the remaining shared nematocysts are distinctly larger in M. antarcticasp. nov. This species also differs from the other geographically close species, M. minor from South Africa, by the presence of only three categories of nematocysts, and their distinctly smaller size, in Millard’s species.
Ecology and distribution. Material collected at a depth of 159–161 m, off Cape Adare.
Etymology. The specific name “ antarctica ” refers to the Antarctic region where this species was found.