On the morphology and predatory behavior of the dinoflagellate Oxyrrhis marina exposed to reduced salinity.

Changes in salinity are known to alter the morphology of protists, and we hypothesized that these changes subsequently alter also the predatory behavior of the dinoflagellate Oxyrrhis marina. Oxyrrhis was grown in media of 33, 25, 20, and 10% of the regular salinity of f/2 medium (31-32‰). In all cases, the cells discharged trichocysts and swelled. Cell surfaces and volumes increased with decreasing salinity, such that cell surface area at least doubled at 10% and the cell volume increased approximately fourfold. After 1 h, the cells started to regain their regular shape, which was almost completed after 24 h. Oxyrrhis immediately regained its regular shape when culture medium was added 5-10 min after the osmotic stress. When incubated with Pyramimonas grossii as prey, those short-term stressed cells showed no significant different prey uptake in comparison to non-stressed cells. In contrast, 24 h after the addition of prey, short-term stressed Oxyrrhis cells had, with weak statistical significance, more Pyramimonas cells engulfed than non-stressed cells. These results indicated that (1) trichocysts were most likely not involved in prey capture and (2) salinity-stressed Oxyrrhis either enhanced its capability to capture more prey, or its digestion apparatus was hampered.