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 1991 in gill tissues of two groups of juvenile oysters (Crassostrea virginica Gmelin), one transferred from a low salinity field site (8) to a field site of high salinity (20) and high Perkinsus marinus (Mackin, Owen, and Collier) prevalence, the other kept at the low salinity field site. Within 24 h, glycine levels in the oysters transferred to high salinity increased 8-fold, taurine concentrations doubled and the total FAA pool rose from 150 mol g^–1 dry wt to 400 mol g^–1 dry wt. Taurine levels reached a plateau within 20 d after transfer to high salinity and remained at that level until P. marinus infections were detected 85 d after transfer. Taurine and glycine levels declined by 40% in the high salinity 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 after Day 85. FAA levels in the low salinity group remained relatively constant throughout the experiment except for an initial rise triggered by an increase in ambient salinity from 8 to 12. The results suggest that salinity tolerance mechanisms in C. virginica may be impaired by P. marinus infection.