β-glucan-stimulated activation of six enzymes in the haemocytes of the scallop Chlamys farreri at different water temperatures

High water temperatures are associated with immune suppression and elevated mortality in economically important marine bivalves. To determine whether the stimulatory effects of β-glucan on enzymes associated with the immune system were depressed at higher temperatures, scallops Chlamys farreri were treated with 0.5, 1.0 or 2.0 mg/ml of β-glucan suspensions at 15 °C or 25 °C, and the activities of six enzymes (phenoloxidase (PO), acid phosphatase (ACP), superoxide dismutase (SOD), peroxidase (POD), myeloperoxidase (MPO) and alkaline phosphatase (ALP)) were measured in haemocytes every 12 h for seven days. The results showed that at 15 °C, compared to the control, the 0.5 mg/ml treatment resulted in significant induction of PO activity, the 1.0 mg/ml treatment stimulated PO, ACP, SOD, POD and MPO activities, and the 2.0 mg/ml treatment stimulated PO, ACP, SOD, POD and ALP activities (p < 0.05). At 25 °C, significant activation of PO activity was observed with the 0.5 mg/ml treatment, while PO, ACP and SOD activities were induced by the 1.0 mg/ml treatment and ACP activity by the 2.0 mg/ml treatment (p < 0.05); however, no significant induction of POD or MPO activity was observed. Surprisingly, ALP activity was undetectable at 25 °C throughout the entire experimental period. These results indicate that the 1.0 mg/ml β-glucan treatment consistently had the highest stimulatory effect among the three concentrations. PO, ACP and SOD were more sensitive than POD, MPO and ALP to β-glucan or temperature stress. The extent and duration of activation as well as the time of peak emergence for all six enzymes appeared higher, longer and earlier at 15 °C than 25 °C in all β-glucan treatments, suggesting that high temperature may weaken or delay immune reactions to exogenous stress.