Salinity tolerance and osmoregulation in two blenny species from different habitats

Marine fishes live in water with higher salt concentration compare to their own body fluids. Osmotic pressure cause influx of salt into the body and of water from the water from the body to the environment. In order to maintain constant body fluid concentration and overcome this flax, marine fishes drink large amounts of seawater and actively excrete salts from their gills.

Freshwater fishes encounter the opposite problem. Since they live in water with salt concentration much lower than their body fluids, they almost not drink, secrete water from the body through their kidney and accumulate salts from their food and from the water by active transport in the gills.

Most members of the family Blenniidae are marine fishes. An exception is the freshwater blenny, Salaria fluviatilis (Asso, 1801) which inhabits freshwater habitats and is widely distributed round the northern and eastern parts of the Mediterranean. S. fluviatilis can be found in recent alluvial regions near sea level (e.g. rivers estuaries which are directly connected to the sea), lakes of postglacial age at higher altitudes and also in inner lakes, that are separated from one another by great distances, and lost their contact with the Mediterranean as long ago as the Pliocene. The latter include the Jordan Valley and Lake Kinneret (Sea of Galilee) where specimens used in this study were collected.

Salaria fluviatilis is closely related to Lipophrys pavo (Risso, 1810), a marine blenny that inhabits intertidal rocky coasts of the Mediterranean and European Atlantic coasts. Some scientists considered S. fluviatilis as a polytopic derivative of L. pavo, and others assumed an eurytherm and euryhaline ancestor of both species which tolerated not only the desalination of the Mediterranean Sea in the upper Pliocene period, but also adapted itself to the subsequent changing conditions.

Both species are considered euryhaline, e.g., able to tolerate different levels of salinity. However, it seems that most of the populations of both species live in relatively stable marine (L. pavo) or freshwater (S. fluviatilis) conditions. Based on the evolutionary and paleontologic history, S. fluviatilis population in Lake Kinneret has been isolated from other S. fluviatilis or L. pavo populations for more than one million years. Except for few short periods of partial salinization of the southern part of the lake by saline water from the Dead Sea, they lived during this period in freshwater and for the last 30000 years, did not experience salt concentration above 2 ppt.

In this study I examined the osmoregulation capabilities of L. pavo and S. fluviatilis. My working hypotheses were: (i) If L. pavo is the ancestor, or sister species, of the freshwater blenny, S. fluviatilis, it should show good osmoregulatory capabilities and tolerance for freshwater, since there are necessary conditions for invading freshwater habitats; (ii). S. fluviatilis, that recently invaded freshwater, should be osmoregulatory adapted to freshwater, but may have excellent ability to tolerate and osmoregulate in seawater as well, a remnant from the time it dwelled in the sea.

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