Living across an estuarine transition zone: ecophysiological response of the cryptic species complex, Eurytemora affinis (Copepoda)

The St. Lawrence estuarine transition zone includes the interface between the freshwater and the brackish. The euryhaline and dominant copepod Eurytemora affinis occurs throughout the entire transition zone, but E. affinis is a sibling species complex with two morphologically similar but genetically distinct clades. The Atlantic and the North-Atlantic clades show differences in their distribution pattern across the fresh and brackish water interface. We hypothesized that niche separation occurs between the two clades based on their feeding capacity and/or ecophysiological performance potential resulting in spatial segregation of the two clades. To test this hypothesis we 1) characterized food sources and physiological condition of the copepods in each habitat and 2) performed “in situ” reciprocal transplant experiments between habitats. To quantify the ecophysiological condition of both copepods clades, we determined their fatty acids composition and concentration, their mortality rate and reproduction efficiency. Results showed similar food composition between habitat, but suggest different feeding behaviour for each clade. The Atlantic clade showed similar results in the transplant experiment between both habitats whereas the North-Atlantic clade showed high mortality in the freshwater habitat. Despite the fact that total fatty acid content did not change significantly during the transplantation, the North-Atlantic clade showed a strong decrease of their reproduction rate in the freshwater habitat, suggesting a trade-off between survival and reproduction in order to maximize available energy for survival. Thus the Atlantic clade had a larger performance range across the interface compared to the North-Atlantic clade which was much more limited to its brackish water habitat. The Atlantic clade can be considered as a generalist whereas the North-Atlantic clade acts as specialist.