Oxygen Consumption of the River Nerite <i>Theodoxus fluviatilis</i> in Different Salinities

The river nerite Theodoxus fluviatilis (Linneaus, 1758) is an euryhaline osmoconformer found in freshwater (FW) and brackish water (BW) habitats across Europe and western Asia. In northern Germany, T. fluviatilis forms regional subgroups, the FW and the BW ecotypes. Members of these ecotypes differ in shell morphology and in shell size as well as in their metabolic pathways of accumulating organic osmolytes under hyperosmotic stress. Oxygen consumption rates were measured, as a noninvasive fitness-indicator, of animals exposed to external media with different salinities. The BW water snails were exposed to hyper- and hyposaline conditions, while the FW snails were naturally only exposed to hyperosmotic conditions. Respiration rates in specimens of the BW ecotype stayed more or less constant overall in all salinities tested. While the BW snails' respiration rates were not affected by the salinity treatment per se, they were affected by the sequence of salinity treatments that they were exposed to (hyperosmotic conditions first vs. hypoosmotic conditions first). Respiration rates strongly declined in the FW ecotype individuals with increasing medium salinity. We suggest that the hypersaline conditions pose greater stress for the animals than the hyposaline conditions because of the higher metabolic activities required for tissue volume regulation.