LEVELS OF INTRACELLULAR FREE AMINO-ACIDS USED FOR SALINITY TOLERANCE BY OYSTERS (CRASSOSTREA-VIRGINICA) ARE ALTERED BY PROTOZOAN (PERKINSUS-MARINUS) PARASITISM

Free amino acid (FAA) levels were measured from May through October 19 91 in gill tissues of two groups of juvenile oysters (Crassostrea virg inica Gmelin), one transferred from a low salinity field site (8 parts per thousand) to a field site of high salinity (20 parts per thousand ) and high Perkinsus marinus (Mackin, Owen, and Collier) prevalence, t he other kept at the low salinity field site. Within 24 h, glycine lev els in the oysters transferred to high salinity increased 8-fold, taur ine concentrations doubled and the total FAA pool rose from 150 mu mol g(-1) dry wt to 400 mu mol g(-1) dry wt. Taurine levels reached a pla teau within 20 d after transfer to high salinity and remained at that level until P. marinus infections were detected 85 d after transfer. T aurine and glycine levels declined by similar to 40% in the high salin ity population as infection intensity increased between 70 and 105 d. Total FAA declined by approximately 33% over this period. The oysters kept at low salinity were not infected and continued to grow while the infected high salinity oysters showed no increase in shell length aft er Day 85. FAA levels in the low salinity group remained relatively co nstant throughout the experiment except for an initial rise triggered by an increase in ambient salinity from 8 to 12 parts per thousand. Th e results suggest that salinity tolerance mechanisms in C. virginica m ay be impaired by P. marinus infection.