---

The cold-water species Peridinium aciculiferum and Scrippsiella hangoei form a species flock with small genetic differences but large distinct morphological differences. It has been hypothesised that this species flock could be an example of a recent rapid adaptive radiation in response to environmental changes such as salinity differences. Saline water has a higher density than fresh water and therefore the selection pressure on cell shape and size to remain suspended in the water, should be much higher in freshwater than in seawater. For that we would expect differences in morphology between the two environments. Here the fine-scale morphology and sinking velocity was studied in the cold-water P. aciculiferum/S. hangoei species flock. Furthermore the freshwater P. aciculiferum and the marine-brackish S. hangoei were competed under constant and fluctuating light conditions to investigate why S. hangoei from the Baltic Sea doesn´t occur in swedish freshwater lakes. In total 8 different populations originating from Scandinavian lakes, Lake Baikal, the Baltic Sea and Antarctic lakes were analysed. For all measurements the Fluid Imaging Technology (FlowCAM©) was used.

---

Alina Wiemers, MSc. Born in Salzburg, Austria. Studied Ecology and Ecosystems with specialisation in Aquatic Ecology at the University of Vienna, Austria and at Lund University, Sweden.

---