Sibogaster Döderlein, 1924

Sibogaster Döderlein 1924: 63; Mah 2016: 129

Diagnosis

Abactinal plates smooth, bare, weakly convex, round to polygonal in outline. Embedded glassy bosses present or absent. Marginal plates elongate, mostly smooth, bare ( S. digitatus has fewer than 6 granules on one or two interradial superomarginals). Inferomarginals adjacent to actinal surface covered with variably abundant granules. Actinal plates covered by granules. Adambulacral plates rectangular in shape. Furrow spines numerous, 6 to 15. Subambulacrals. thickened relative to furrow spines, remaining granulation identical to others on actinal surface.

Comments

Sibogaster includes 3 species, the low latitude tropical Pacific S. digitatus Döderlein, 1924, the Atlantic S. bathyheuretor Mah, 2020 and the widely occurring S. nieseni Mah, 2016, which have been recorded from 640 to 4175 m, but primarily from bathyal and abyssal depths (> 1000 m).

Mah (2016) observed morphological affinities with Plinthaster, Astroceramus and Pergamaster . Okeanosaster Mah 2022 also displays affinities with Sibogaster in that it shows smooth, bare abactinal and marginal plates-but with much smaller peripheral granules and lacking glassy granules embedded in the abactinal plate surface.