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The valve morphology of the brackish-water plankton diatom Skeletonema subsalsum is dependent on salinity. In cells grown at salinities of 3‰ or less, the valve faces were flat and the connecting processes much reduced in length. Cells grown at salinities of 5‰ or higher had dome-shaped valves and extended connecting processes. The effect of an increase in salinity could be duplicated by the addition of sucrose to the growth medium. The growth rate was nearly independent of salinity in the range of 1–10‰. It was concluded that the osmotic pressure of the medium exerts a direct effect on cell-wall morphogenesis in S. subsalsum.

Type material of Skeletonema subsalsum has been examined by light and electron microscopy. The species belongs to Skeletonema and is not conspecific with Stephanodiscus subsalsus. Skeletonema subsalsum is distinguished from S. costatum morphologically mainly by structure of connecting processes, those of S. subsalsum being tubular with a basal circular hole while those of S. costatum are trough-shaped with a wide basal opening. In addition to the Stockholm Archipelago (type locality), S. subsalsum was recorded from the Gulf of Finland and the river Wumme in the Bremen area.