Energetics of Osmoregulation: Organism-Level Comparison of the Osmotic Stress Response in Euryhaline Estuarine and Freshwater Palaemonids.

The ecophysiological profiles of decapods adapted to estuarine and freshwater environments are remarkably diverse, likely reflecting independent colonization processes. There are no clear patterns in the energy expenditure associated with inhabiting these environments or general physiological processes, such as nitrogenous waste production, water regulation, or responses to salinity changes. We compared energy expenditure, measured as oxygen consumption, total ammonia production, and muscle water content in two euryhaline shrimp species of the genus Palaemon: the estuarine P. macrodactylus and the freshwater P. argentinus, under normal conditions and after exposure to salinity changes over periods ranging from 6 hours to 3 weeks. We also assessed the behavior of both species along a salinity gradient to understand if behaviorally mediated salinity selection may play a role in tuning the energy balance. The oxygen consumption profiles differed between species, both under normal conditions and in response to salinity changes. The freshwater species exhibited lower and stable oxygen consumption. Both species tended to eliminate more ammonia immediately after being transferred from a concentrated to a more diluted condition. While the estuarine species maintained effective water content regulation, the freshwater species experienced immediate dehydration at high salinity and failed to recover over the long term. Freshwater species actively selected low salinity water, whereas the other showed no clear salinity preference. The results suggest that the observed differences in energy expenditure and volume regulation between species are related to full adaptation to freshwater. Behavior may act reinforcing the divergence.