Appraisal of prospective bivalve immunomarkers

Worldwide concern over threats to natural resources and public health has led to increased efforts to monitor and assess environmental conditions. This has stimulated the need for development and application of select biological and ecological measurements, or indicators, that are responsive to environmental stress. Measures of bivalve mollusc defence activities, such as haemocyte density, phagocytic activity, locomotion and production of cytotoxic molecules; and haemolymph constituents, such as agglutinins and lysozyme, have potential as indicators and appear to be responsive to xenobiotic chemical insults in the aquatic environment. However, basic research on the relevance of these measurements in inferring resistance to disease or enhanced survival is currently insufficient, reducing their value as potential biomarkers to address environmental objectives. In addition, variation in defence activities caused by seasonal temperature and reproductive cycling, salinity changes, nutritional status, diseases and parasites, and genetic stocks is high and may limit applicability of bivalve defence-related measurements as indicators. This review examines these sources of variability and their possible implications for interpreting changes in bivalve defence activity as an indicator of stress. Examples of contaminant-induced changes in bivalve defence functions are described.