Osmotic regulation and salinity tolerance in the freshwater snail Pomacea bridgesi and the freshwater clam Lampsilis teres

Abstract

Unionid clams ( Lampsilis teres) and apple snails ( Pomacea bridgesi) were gradually acclimated to media ranging in osmotic concentration from fresh water to 400 mOsm. There was no mortality among L. teres in all media from fresh water to up to dilute sea water with an osmolality of 200 mOsm; mortality was 20% in dilute sea water with an osmotic concentration of 400 mOsm. The survival of P. bridgesi was 80–100% in media ranging from fresh water to 200 mOsm. All snails exposed to media of 400 mOsm died within a few days. L. teres is a hyperosmotic and hyperionic regulator when the ambient osmolality is less than 50 mOsm and an osmotic and ionic conformer when the environmental osmolality is above 50 mOsm. The hemolymph of P. bridgesi is hyperosmotic and hyperionic in animals acclimated to osmolalities below 100 mOsm; the animal is an osmoconformer in higher ambient osmolality. The concentration of Ca 2+ is higher in hemolymph from P. bridgesi than in blood from L. teres. Tissue hydration in L. teres does not change in animals acclimated to higher salinities, suggesting that the animals are good volume regulators. Tissue hydration is unchanged in P. bridgesi gills in animals acclimated to osmolalities from fresh water to 150 mOsm, but falls in animals acclimated to 200 mOsm. The amino acid content of the gills of both species increases with acclimation to high salinity, but in media of 200 mOsm, the increase in L. teres is larger than that in P. bridgesi. In L. teres, alanine, glycine and β-alanine are responsible for most of the increase; glutamic acid, alanine and glycine show the largest increases in gills from P. bridgesi. Our analysis of these data is consistent with the hypothesis that freshwater gastropods have a lower capacity for increasing the amino acid pool in response to increases in the ambient osmolality than do freshwater bivalves.