15.10 Wednesday, Jan. 4 Molecular basis of low salinity limits in euryhaline decapod crustaceans HENRY, RP*; SERRANO, L; Auburn University; Auburn University email@example.com
Euryhaline crustaceans make the transition from osmoconformity to osmoregulation at about 26 ppt salinity, and this ability allows them to survive in very dilute estuarine waters. But even these organisms have a lower limit of salinity tolerance, and it varies among species. Blue crabs, Callinectes sapidus, can survive direct transfer down to fresh water (0 ppt); but green crabs, Carcinus maenas, have a lower lethal salinity limit near 5 ppt and are rarely found in nature below 7 pp. To test the molecular basis of extreme low salinity tolerance, we measured acute and acclimated changes in carbonic anhydrase (CA) gene expression and enzyme induction in response to multiple step-wise reductions in salinity in both species. Blue crabs survived all transfers down to 5 ppt but showed the maximum increases of both CA gene expression and enzyme induction in posterior, ion-transporting gills at 15 pp. Neither value increased in the lower salinities. Interestingly, at 5 ppt, there was a dramatic increase in CA gene expression in anterior gills, but this was not accompanied by an increase in CA activity. Green crabs also showed maximal CA gene expression and enzyme induction at 15 ppt, but both values then decreased as salinity was progressively lowered. Direct transfer to 5 ppt resulted in 50% mortality at 4 days (acute) and 100% mortality at 7 days (chronic) exposure. In the acute phase, there was a significant increase in CA gene expression in anterior gills but no increase in enzyme induction. Furthermore, there was no significant CA induction in posterior gills at 4 days post-transfer to 5 ppt. These results suggest that the molecular mechanism of transport-related protein induction breaks down near the lower lethal salinity limit, and fails to confer adequate systemic physiological adaptation for continued survival.