Inorganic Osmolytes and Enzymatic Biomarkers from the Manabi Oyster (crassostrea Cf. Corteziensis) in Response to Saline Stress

Abrupt drops in salinity that occur in tropical estuaries during the equatorial rainy season led to hyposaline conditions which may reduce the populational density of oysters. To assess the effect of saline stress on physiological and metabolic responses of the Manabi oyster (Crassostrea cf. corteziensis) was exposed to 35, 30, 20,10 and 5%o concentrations during 96 h. Inorganic osmolytes, pH, salinity, haemocyanin and protein concentration in the plasma as well as the number of oysters with closed valves were recorded. Aspartate aminotransferase (AST), alanine aminotransferase (ALT), lactate dehydrogenase (LDH), alkaline phosphatase (ALP) and catalase (CAT) activity were analysed. Inorganic osmolytes and internal salinity were elevated in oysters exposed to 35, 10 and 5%o. A significant number of oysters with valve closure was observed in 10 and 5%o, which coincided with a decline in physiological pH and changes in haemocyanin concentrations. AST activity and AST/ALT ratio were reduced under 35, 10 and 5%o, and CAT increased in oysters exposed to 35%o; but protein concentration, LDH and ALP did not show significant variations. Metabolic adjustment and behavior of the Manabi oyster could explain tolerance and survival (at least for a short term) to hyposaline stress in tropical estuarine ecosystems.