Mechanical disturbance affects haemocyte functionality in the Venus clam Chamelea gallina

The clam Chamelea gallina is fairly common along the western coastline of the Adriatic Sea, where it is subjected to intense fishing. To evaluate possible causes of stress in clam populations, the effects of acute mechanical disturbance on haemocyte functionality were analysed in both laboratory and field experiments. Among the various sources of stress that clams undergo during commercial fishing by hydraulic dredges, water pressure and mechanical sorting were considered, and three increasing levels of stress were applied. A reduction in immunosurveillance was highlighted, concentrations of circulating haemocytes, phagocytic and acid phosphatase activity indices decreased with increasing mechanical stress. The opposite trend shown by the beta-glucuronidase activity index is discussed. The response of clam haemocytes, detected on seasonal bases in two sites, often exhibited peculiar patterns, mostly depending on the reproductive cycle, and were probably influenced by different features of the sea bottom. Although recovery after stress was observed in laboratory experiments, some considerations are made on detrimental effects experienced in the field by under-sized clams, which are first fished and then thrown back into the sea.