Blood shell, Scapharca broughtonii, contains large quantities of free
D-aspartate comparable to free
L-aspartate in its tissues. When the shell was reared in hypoxic seawater,
D-aspartate as well as
L-aspartate in the foot muscle decreased rapidly, and their total level became about one-fourth within 24 hr. None of the other
amino acids examined showed a similar behavior, but many of them rather increased during the same period. The increase in
L-alanine was especially remarkable and was almost equal to the sum of the decrease in
aspartate enantiomers. When the shell that had been acclimated to hypoxic seawater for 96 hr was transferred to normoxic seawater, all the
amino acid levels mostly returned to the control levels within 96 hr. In contrast to these effects of hypoxic stress, hyperosmotic stress of 150% seawater had no effect on the D- and
L-aspartate levels in the same tissue. These results suggest that
D-aspartate is involved in anaerobic energy metabolism of this bivalve as well as
L-aspartate, whose vital role in
anoxia-tolerant bivalves is well known.