Exposure to anoxia of the clam Chamelea gallina I. Effects on immune responses.

The effects of anoxia on the immune responses of a commercially important bivalve species, Chamelea gallina, were studied in two different experiments (24-h and 48-h exposure to deoxygenated seawater). After anoxic stress, the capability of clams to recover was also evaluated by maintaining animals for 24 h in aerated seawater. Total haemocyte count (THC), phagocytosis and lysozyme activity (in both haemocyte lysate and cell-free haemolymph) were chosen as cellular biomarkers of exposure to anoxic conditions. Anoxia significantly reduced THC with respect to controls in both experiments. However, clams which recovered from the 24-h anoxia test showed THC values similar to those of controls, whereas in the 48-h test recovery clam THC was similar to that of stressed clams. Significantly decreased phagocytic activity with respect to controls was also observed in both experiments. In the first experiment, haemocytes from recovery clams showed slightly higher phagocytic activity with respect to controls, whereas after 48 h of exposure haemocytes from recovery animals were unable to recover their phagocytic activity. After both 24-h and 48-h anoxia, lysozyme activity was significantly reduced in both haemocyte lysate and cell-free haemolymph. In recovery clams from 24-h anoxia, the lysozyme activity of lysate was significantly lower than in control and stressed clams, whereas in haemolymph it was significantly higher. Conversely, in recovery clams from 48-h anoxia, the enzyme activity of both media decreased significantly with respect to controls. Interestingly, an unexpected additional stress during the 48-h anoxia test was recorded: recovery clams spawned during the recovery phase. To validate the hypothesis that spawning acts as an additional type of stress, the survival-in-air test was also performed. Significant decreases in resistance to air exposure were observed after 48-h anoxia in both stressed and recovery animals. Results show that anoxia strongly affects the functional responses of haemocytes, reducing immunosurveillance in stressed clams. The present study also highlights that additional stress, such as spawning, may further decrease immunosurveillance and reduce the resistance of clams to exposure to air.