Hypoosmotic stress might endanger survival of the mysid Neomysis integer despite activation of ion transport ATPases

Animals in coastal habitats are exposed to strong environmental variability. Especially in shallow aquatic systems, salinity changes occur within hours and require osmotic and metabolic adjustments of the affected organisms. We aim to understand how marine invertebrates can adjust to sudden changes in salinity, and studied the physiological responses of the mysid Neomysis integer from a brackish estuary in the southern Baltic Sea. The salinity of the habitat was 7.5 PSU. In several laboratory experiments we measured enzyme activity, respiration and survival. Salinities ranged from 2 to 33 and exposure times from a few hours to 3 weeks. Enzyme activities of Na + /K + -ATPase and V-ATPase were highest at a salinity of 2. The immediate reaction of the mysids to the transfer to a final salinity of 2 was a transient decrease in respiration rate, which then continuously increased during the next 96 h. In contrast, mysid transfer to a salinity of 13 did not result in immediate changes in respiration rates but resulted in a slight increase in respiration rate over the following 96 h. Survival was highest at a salinity of 7.5 and 13 and lowest at 2. Our results show that exposure of animals to a salinity of 2 induces a stressful condition that requires individual physiological responses. We suggest that potential desalination of coastal water bodies in the Baltic Sea driven by climate change may have critical implications for mysid population development.