Ionic Regulation in Estuarine and Freshwater Palaemonid Shrimp

Hyper-regulation requires animals to absorb ions against concentration gradients. Although ion absorption would involve different pathways in estuarine and freshwater organisms, current models remain incomplete for euryhaline palaemonid shrimp. This study examines the hyper-regulatory performance of two closely related euryhaline palaemonid species in response to a similar environmental salinity gradient. We measured the function of key branchial ion transport-related enzymes, critical hemolymph parameters, and muscle moisture following 3-week exposure to high and low salinity treatments: 20‰S and 5‰S for the marine/estuarine shrimp Palaemon macrodactylus; 17‰S and 2‰S for the 'freshwater' species Palaemon argentinus. Both shrimp species exhibited weak and strong osmoregulatory capacities against high and low salinities, respectively. At a salinity of 5‰S, the estuarine shrimp P. macrodactylus experienced hemolymph sodium and chloride leakage as well as water accumulation in muscle tissue. These parameters remained unchanged in P. argentinus, whereas gill H⁺-ATPase (VHA) activity was 9.4 times higher at the low salinity treatment. An apical VHA secreting protons would facilitate the transepithelial influx of Na⁺ through the gills of the freshwater shrimp. The unexpected increase in total protein concentration may have obscured the levels of specific VHA activity in P. macrodactylus kept at the low salinity. Overall, tissue swelling, ion leakage, and increased protein concentration indicate sublethal effects in P. macrodactylus following long-term exposure to 5‰S. Such physiological dysfunctions could severely impact the survival of the estuarine species in salinities below 5‰S.