Distribution of free D-amino acids in bivalve mollusks and the effects of physiological conditions on the levels of D- and L-alanine in the tissues of the hard clam, Meretrix lusoria

The distribution of D-amino acids was examined on the tissues of seven species of marine bivalve mollusks belonging to two different subclasses, Pterimorphia and Heterodonta. High concentration of D-alanine was found in all tissues of only Heterodonta and not in the tissues of Pterimorphia except in gills and midgut gland. Several other D-amino acids including D-arginine also occurred in the several tissues of both groups of bivalves. Along with the salinity stress on the hard clam Meretrix lusoria in 150% seawater, only D- and L-alanine increased in adductor muscle, gills, and midgut gland. The incorporation of D-alanine from external seawater was found only in gills, hemolymph, and midgut gland of hard clam, and the levels of D- and L-alanine in adductor and foot muscles decreased in the seawater containing 50 mM D-alanine and returned to the control level after recovery in normal seawater. During the starvation of hard clam for 22 days, D- and L-alanine as well as other free amino acids decreased considerably in most tissues. The anoxia of hard clam for a week duration gave no large effect on the levels of free amino acids but resulted in the slight increases of D- and L-alanine levels after seven days.