Thalassiosira mala Takano
Living cells are discoid or drum-shaped, with the pervalvar axis frequently larger than the diameter (Fig. 1; living cells in vitro) and often imbedded in an amorphous gelatinous mass of indeterminate size when in vivo. Valve diameter is reported as 4-10µm (Hasle & Syvertsen 1997). Areolae are arranged in eccentric or linear tangential rows (Fig. 2, TEM of valve), about 35 in 10µm, but denser near the margin. There is a single marginal ring of fultoportulae (7-9 in 10µm) with a rimoportula (Fig. 2, arrow) between two fultoportulae in the ring. A single subcentral fultoportula is displaced from the valve center, and always on the side of the marginal rimportula.
This species has a cosmopolitan distribution (Hasle 1976), except for boreal and polar waters, and has been reported more frequently in the Atlantic. On the U.S. East coast, it is found as far north as the Gulf of Maine (PE Hargraves, personal observation). In the IRL it has rarely been seen in winter near the inlets, suggesting its advection from coastal waters. In its type locality, Tokyo Bay, it was implicated in mass mortality of shellfish (Takano 1965). Other Thalassiosira species in the IRL that may occur in gelatinous masses are T. minuscula and T. subtilis, but they are morphologically quite different.