Osmoregulation and salinity tolerance in two species of bivalve mollusc: Limnoperna fortunei and Mytilopsis leucophaeta

Abstract

This paper reports on salinity tolerance and osmotic regulation in two bivalve molluscs: Mytilopsis leucophaeta Conrad, an oligohaline species (subclass Heterodonta) and Limnoperna fortunei Dunker, a freshwater species (subclass Pteriomorphia). The range of osmolality tolerated by Mytilopsis is 5–400 mOsm. Limnoperna survives in deionized water and in osmolalities of ≤400 mOsm. Both species are osmotic and ionic conformers in media with concentrations of > 70 mOsm; at lower ambient osmolalities, both are hyperosmotic regulators. The major hemolymph ions are Na + and Cl −; acclimation to dilute media results in hypercalemia in both species. The amino acid content of Mytilopsis soft tissues is 182 μmol · g −1 dry wt in animals acclimated to 10 mOsm. The tissue levels of amino acids increase to 405 μmol · g −1 in Mytilopsis acclimated to 400 mOsm. The free amino acid content of Limnoperna mantle tissue is 9.6 μmol · g −1 dry wt in animals acclimated to 10 mOsm and increases to 35 μmol · g −1 in animals acclimated to 200 mOsm. The differences in osmoregulatory behavior between the two species appear to be quantitative rather than qualitative, despite the phyletic distance between them.