Abstract
Light and scanning electron microscope investigations of type material and representative samples from the Davis Strait, the North Pacific Ocean and the Antarctic show that Thalassiothrix longissima and T. antarctica deserve the rank of separate species. The main distinctive characters are the presence of serrated protrusions at both poles of T. longissima and two winged spines at one pole of T. antarctica; the location of the marginal spines at the lower border of the vela in T. longissima and in the middle of the vela in T. antarctica; and in addition, the difference in the shape of the cell, T. longissima being straight or slightly curved and T. antarctica usually sigmoid but occasionally straight. Comparison with other araphid diatoms indicates an isolated taxonomie position of Thalassiothrix, Thalassionema being the closest relative. Morphological similarities and dissimilarities between T. antarctica and Synedra reinboldii are also illustrated and discussed.
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