The effect of fluctuating salinity on the concentrations of free amino acids and ninhydrin-positive substances in the adductor muscles of eight species of bivalve molluscs

Abstract

Eight species of bivalve molluscs were exposed both to gradual and abrupt salinity fluctuations and the changes in free amino acids and ninhydrin-positive substances in their adductor muscles measured. In all the species there was an initial rise in the concentration of ninhydrin positive substances when exposed to decreasing salinities. After acclimation for one week (14 cycles) to a 30 % sea-water minimum sinusoidal salinity regime there was no difference in the concentration of ninhydrin positive substances at high and low salinities in the adductor muscles of Mytilus edulis L. Together, the changes in taurine and non-essential amino acids alanine, aspartic acid, glutamic acid, and glycine largely accounted for the changes in the free amino-acid pool. It was found that ‘shell-closing’ mechanisms may result in changes in the free amino-acid pool brought about by reductive amination of Krebs cycle and other keto-acids under anaerobic conditions. It is suggested that ninhydrin-positive substances and free amino acids are used as osmotic effectors in marine bivalves exposed to constantly lowered salinities, but are not used for the same purpose in animals exposed to cyclic salinity changes.