Comparison of salinity tolerance and osmoregulation in two closely related species of blennies from different habitats

This study compares osmoregulatory capabilities of two closely related blennies inhabiting different habitats: Salaria pavo which inhabits marine rocky coasts in the Mediterranean and the eastern coasts of the Atlantic Ocean, and Salaria fluviatilis which inhabits freshwater habitats around the Mediterranean, both in rivers connected to the sea and in lakes that for a long time have been separated from the sea. Fishes for this study were collected in Lake Kinneret (Sea of Galilee) (S. fluviatilis) and Habonim beach, Israel, East Mediterranean coast (S. pave). Both species tolerated salinities of 0, 40 and 100% seawater (0, 14.4 and 36 ppt sea salt respectively). Acclimation for a period of three months to freshwater (S. pavo) and seawater (S. fluviatilis) did not affect survival rates. After acclimation, total body water content of S. fluviatilis remained unaffected by experimental salinities but increased in S. pave as salinity decreased. The hematocrit of both species increased as salinity increased. Plasma osmolality and Naf concentrations were unaffected by experimental acclimation salinities in S. fluviatilis, but significantly decreased in S. pave as salinity decreased. The results indicate that S. fluviatilis is able to osmoregulate both in fresh and seawater. S. pavo, living in seawater, while able to tolerate freshwater environment, fails to maintain constant extracellular fluid concentration. It is suggested that S. pave, or its marine ancestor, has euryhaline capabilities that enabled it to invade freshwater habitats. S, fluviatilis, as the evolutionary result of this invasion, is thus better adapted to freshwater environment, but did not lose its ability to live in seawater.