(Janisch) Hasle, in Hasle et al. (1971: 326, figs 1–43, 46)
Coscinodiscus tumidus var. fasciculatus Rattray (1890a: 475)Coscinodiscus trigonus G.Karst. (Karsten, 1905: 84, pl. 5, fig. 10)Coscinodiscus quinquemarcatus G.Karst. (Karsten, 1905: 85, pl. 7, figs 6, 6a)Coscinodiscus inflatus G.Karst. (Karsten, 1905: 85, pl. 7, figs 7, 7a)Coscinodiscus incurvus G.Karst. (Karsten, 1905: 85, 7, figs 8, 8a)Coscinodiscus chunii G.Karst. (Karsten, 1905: 86, pl. 7, figs 10, 10a)Coscinodiscus subtilis auct. non Ehrenberg (1841), sensu G.Karst. (Karsten, 1905: 86, pl. 7, figs 11, 11a)Coscinodiscus quadrifarius Manguin (1960: 251, pl. 24, figs 284, 285)For additional synonyms, see Hasle et al. (1971: 326).
Coscinodiscus tumidus Janisch (1878), in Schmidt et al. (1874–1959: pl. 59, figs 38, 39)
Rivera (1981: 127, figs 378–389), Johansen & Fryxell (1985: 176, figs 28–32), Kopczyńska & Ligowski (1985: 74), Fryxell & Johansen (1990: 99, pl. 11.9, fig. 14), Moisan & Fryxell (1993: 293), Hasle & Syvertsen (1997: 70)
Cells usually solitary. Valves circular, 21–137 µm diameter, often concave in the centre. Areolation linear to fasciculate; areolae 8–9 in 10 µm, loculate, opening externally via a simple foramen and internally via an evenly pored velum. Strutted processes generally scattered over valve face, apart from a ring of 6 or more near centre, positioned between areolae in heavily silicified coarse valves or taking the place of areolae in lightly silicified valves; marginal strutted processes in 2 or 3 rings, 5 processes in 10 µm, usually not visible by LM. Strutted processes operculate, usually triradiate but with 1 operculum usually absent from processes near valve margin (Fig. f). Marginal strutted processes with a small granule or spine formed on upper (near valve face) rim of external opening. Labiate processes 6 or 7, formed on margin, difficult to see by LM (especially in heavily silicified valves), with slightly flared external openings (Fig. b, e), often compressed to form an oval parallel to the direction of an internal slit that is slightly oblique to the pervalvar axis. For a complete description, see Johansen & Fryxell (1985) or Hasle & Syvertsen (1997).
Southern Ocean, 54–61°S, south of Australia; coastal waters off Syowa Station, East Antarctica (Ishikawa et al., 2001); type locality, described as “Antarktik” (Schmidt et al., 1874–1959); circumpolar within the Antarctic and sub-Antarctic (Hasle et al., 1971); Ross Sea (Watanabe, 1982); Weddell-Scotia Confluence (Garrison et al., 1987).
The degree of silicification of the cell walls of T. tumida appears to be sensitive to the availability of silica, silicification being most evident in cells found near the shore and near convergence waters and at its lowest in cells in the intervening ocean or in places where they divide rapidly in bloom conditions. Thalassiosira tumida is structurally similar to T. gravida, T. rotula A.Meunier and T. antarctica, but it lacks the cluster of central strutted processes seen in these taxa.