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

Unionid clams (Lampsilis teres) and apple snails (Pomacea bridgesi) were gr adually acclimated to media ranging in osmotic concentration from fresh wat er to 400 mOsm. There was no mortality among L. teres in all media from fre sh water to up to dilute sea water with an osmolality of 200 mOsm; mortalit y was 20% in dilute sea water with an osmotic concentration of 400 mOsm. Th e 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 osmolal ity is less than 50 mOsm and an osmotic and ionic conformer when the enviro nmental osmolarity is above 50 mOsm. The hemolymph of P. bridgesi is hypero smotic and hyperionic in animals acclimated to osmolalities below 100 mOsm; the animal is an osmoconformer in higher ambient osmolality. The concentra tion of Ca2+ is higher in hemolymph from P. bridgesi than in blood from L. teres. Tissue hydration in L. teres does not change in animals acclimated t o 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 acclimat ed to 200 mOsm. The amino acid content of the gills of both species increas es with acclimation to high salinity, but in media of 200 mOsm, the increas e in L. teres is larger than that in P. bridgesi. In L. teres, alanine, gly cine and p-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 freshwate r gastropods have a lower capacity for increasing the amino acid pool in re sponse to increases in the ambient osmolality than do freshwater bivalves. (C) 1999 Elsevier Science Inc. All rights reserved.