Evaluation of hemolymph biochemical properties, clearance rate, bacterial microbiota and expression of HSP genes of gulf pearl oyster Pinctada radiata in response to salinity changes

Abstract

Hypo-salinity exposure is a serious concern for bivalve aquaculture, with profound effects on the animals’ physiological condition. The present study aimed to evaluate the influence of low-salinity exposure on the clearance rate, total heterotrophic marine bacteria and total Vibrio count in flesh, as well as the biochemical parameters of hemolymph and heat shock protein (HSP) gene expression, in Pinctada radiata. To this end, the oysters were reared for 14 days at salinities of 40 (control group), 35, 30, 25 and 20 PSU. Results indicated that low salinity led to a significantly lower clearance rate, with the lowest clearance rate observed at 20 PSU. However, no significant differences were found among the groups in the total heterotrophic marine bacteria, calcium concentration, aspartate aminotransferase (AST), alanine aminotransferase (ALT) or alkaline phosphatase (ALP) levels. In addition, the results demonstrated that the total Vibrio count increased significantly in oysters exposed to the hypoosmotic condition. A reduction in glucose concentration was observed in the 25 and 20 PSU groups compared to the other experimental groups. In addition, the highest survival rate was found in the 40 and 35 PSU groups. Based on q-RT-PCR results, HSP70 and HSP90 mRNA levels were significantly up-regulated by low-salinity exposure, while the expression of HSP20 mRNA was remarkably down-regulated in the 35 and 30 PSU salinity groups. The present study shows that pearl oysters tolerate a wide range of salinities (40–20 PSU), whereas optimal salinity for oyster culture ranges from 30 to 40 PSU. These results also provide a better understanding of the feeding behavior and physiological changes of P. radiata in response to salinity changes.