Symplectoscyphus densus CANTERO, 2017, sp. nov.

Symplectoscyphus densus sp. nov.

(Figs 3 C, 12D–E, 14)

Material examined. Stn 7, fragment 5 mm long (paratype, NIWA 115618); Stn 97, stem 7 mm high, slightly polysiphonic basally (holotype, NIWA 115617).

Description (Holotype). Stem 7 mm high, slightly polysiphonic basally (Fig. 3 C). Stem with side branch (Fig. 3 C). Hydrothecae closely packed, alternately arranged, approximately planar (Fig. 12 D, 14A–B); almost opposite distally. Plane of hydrothecae of side branch roughly perpendicular to that of stem. Hydrotheca elongate (Fig. 12 D, 14A–D), diameter distinctly decreasing distally from median part (Fig. 12 D, 14A–D), sometimes looking like a distal neck (Fig. 12 D). Hydrotheca straight at basal half, curving abcaudally at distal half (Fig. 12 D, 14C–D). Hydrotheca free over half its adcauline length (Fig. 12 D, 14C–D). Free part of adcauline wall roughly straight or slightly convex; abcauline wall straight or slightly convex basally, concave distally. Rim of hydrothecal aperture with three well-developed, blunt cusps (Fig. 14 E); adcauline one frequently longer than abcauline ones (Fig. 12 D, 14C–E), projecting farther.

Measurements (in µm). Hydrothecae: length of abcauline wall 380–400, length free part of adcauline wall 250–275, length adnate part of adcauline wall 230–250, length adcauline wall 480–525, diameter at aperture 120–130, diameter at diaphragm 100–120, maximum diameter 150–160. Cnidome: larger microbasic mastigophores, range 9.0–10.0 x 2.0–2.5, mean 9.8±0.4 x 2.4±0.2 (n=5); ratio, range 4.0–4.5, mean 4.1±0.2 (n=5).

Remarks. There is a side branch in the paratype, similar to the holotype, but the hydrothecae of the side branch of the paratype are in the same plane of those of the corresponding stem.

The material studied differs from all known Antarctic species of Symplectoscyphus and is considered here to represent a new species to science. Notwithstanding the scarcity of material available, this species is perfectly recognizable, among the Antarctic species of Symplectoscyphus, by the shape and arrangement of the hydrothecae. The morphologically closest species are S. aggregatus, S. frondosus and S. liouvillei, all having highly packed hydrothecae. However, only in S. aggregatus the hydrothecal distal part covers the basal part of following hydrotheca on the same side, as in Symplectoscyphus densus sp. nov. However, they are clearly distinguishable by the shape and size of the hydrothecae. In S. frondosus and S. liouvillei the hydrothecae are not so closely packed and, in addition, they also differ in the shape and size of the hydrothecae. Finally, the hydrothecae of Symplectoscyphus densus sp. nov. are more distinctly directed outwards and have a larger free portion of the adcauline wall.

Ecology and distribution. Material collected from depths between 536 and 630 m, off Cape Adare and Possession Islands.

Etymology. The specific name densus refers to the density of hydrothecae.