Freshwater molluscs as indicators of bioavailability and toxicity of metals in surface-water systems

Freshwater molluscs--snails and bivalves--have been used frequently as bioindicator organisms. With increasing needs for research on contaminant effects in freshwater ecosystems, this kind of biomonitoring is likely to develop further in the future. Molluscs can be used effectively for studies of both organic and inorganic contaminants; this review focuses on studies involving bioaccumulation and toxicity of metals. Two important advantages of snails and bivalves over most other freshwater organisms for biomonitoring research are their large size and limited mobility. In addition, they are abundant in many types of freshwater environments and are relatively easy to collect and identify. At metal concentrations that are within ranges common to natural waters, they are generally effective bioaccumulators of metals. Biomonitoring studies with freshwater molluscs have covered a wide diversity of species, metals, and environments. The principal generalization that can be drawn from this research is that bioaccumulation and toxicity are extremely situation dependent; hence, it is difficult to extrapolate results from any particular study to other situations where the biological species or environmental conditions are different. Even within one species, individual characteristics such as size, life stage, sex, and genotype can have significant effects on responses to contaminants. The bioavailability of the metal is highly variable and depends on pH, presence of organic ligands, water hardness, and numerous other controlling factors. Despite this variability, past studies provide some general principles that can facilitate planning of research with freshwater snails and bivalves as metal bioindicators. These principles may also be useful in understanding and managing freshwater ecosystems. Bioaccumulation of metals in biota is a function of both uptake and depuration. Uptake in molluscs may be through either of two vectors--ingestion of food and other metal-containing substances or through direct adsorption of dissolved constituents. Under some conditions, the bioconcentration factors can be in the range of 10(3) to 10(6), relative to water. Most studies that provide comparisons among taxonomic groups indicate that bioaccumulation in molluscs is greater than that is fish. However, such comparisons should be interpreted with caution because metals tend to be nonuniformly distributed among different organs in both molluscs and fish. Bioaccumulation and acute and chronic toxicity are highly dependent on metal speciation. Mainly because of this influence of metal speciation, toxicity and bioaccumulation do not have a consistent relation to each other. Sensitivity to toxic effects of a metal is likely to be considerably greater in juvenile or larval stages than in adults.