Thalassiosira profunda (Hendey) Hasle
This species is too small for accurate determination by light microscopy. The structure of the valve surface (areolae or pseudoloculi in a radial arrangement) is indistinct in the light microscope. There are three to six fultoportulae around the margin and a single fultoportula in or near the valve center, adjacent to which is a distinctly enlarged areola (Fig. 1, SEM exterior). A single rimoportula is found close to one of the marginal fultoportulae, opposite to the central fultoportula (Fig. 2, SEM interior; Fig. 3, TEM 3, arrows). The reported size range is 1.5-5µm in diameter; in the IRL the diameters observed were was 2.3-2.8µm.
The miniscule size makes it easy to overlook; it is both coastal and oceanic in habitat with a wide global distribution, based on the information in Hasle (1976). It appears occasionally in electron microscope preparations from the IRL, but distribution and abundance are cryptic. It may be confused with T. proschkinae, but the latter is somewhat larger, has more distinct areolae, and more fultoportulae. Additional discussions on its ultrastructure are found in Hasle (1973) and Hallegraeff (1984).