(J.Grøntved) B.Leadb. (Leadbeater, 1978: 514)
Buck & Garrison (1983: 1264, fig. 29), Thomsen et al. (1990: 370, figs 6–8, 17, 18)
Salpingoeca natans J.Grøntved (1956: 9, fig. 6)
Cells solitary. Lorica 32–35 µm long overall; lorica chamber 9–10 µm long. Longitudinal costae 3; transverse costa 1. Anterior spines 3, 15 µm long, not continuous with longitudinal costae; pedicel to 6 µm long. Protoplast ellipsoidal to obovoid, 5 µm long.
Common at inshore and oceanic stations of Prydz Bay, East Antarctica as well as offshore in the vicinity of Casey Station; Weddell Sea (Buck & Garrison, 1983); larger cells (to 60 µm long) reported from Lützow-Holm Bay (Takahashi, 1981); Danish waters (Grøntved, 1956).
Calliacantha natans can be recognised by its 3 anterior spines which are not continuous with the longitudinal costae (cf. C. simplex), and the relatively complex arrangement of costal strips around the lorica chamber. Thomsen et al. (1990) reported “considerable variability in lorica dimensions and proportions” for C. natans and C. simplex and presented data illustrating the bimodal nature of size distributions of the two species. However, unlike Arctic specimens, the size of Antarctic C. natans appears to be constant.