Skeletonema comprises a planktonic genus of centric diatoms occurring in coastal and brackish waters worldwide. The most commonly reported species, Skeletonema costatum, was believed to be ubiquitous, morphologically plastic and euryhaline. Yet, recent studies showed that this perceived taxon consists of a series of species, including a more narrowly defined Skeletonema costatum sensu stricto. Here we investigated the effect of salinity on growth rate and cell morphology. Ten strains belonging

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... o six Skeletonema species were incubated at salinities between 0 and 35; their growth response was measured and their cell morphology observed using light and electron microscopy. All strains grew well between salinities of 10 and 35, whereas the lowest salinity at which growth was observed differed among species and even within the same species: Skeletonema marinoi strains deriving from distinct environments (Baltic and North Sea) showed different lower tolerance limits. Two species, Skeletonema costatum and Skeletonema subsalsum, showed growth between salinities of 0 and 35 and exhibited very short intercellular processes when cultured in freshwater. Skeletonema subsalsum underwent cell enlargement when exposed to a salinity of 35.