A REVIEW OF THE EFFECTS OF HEAVY-METALS ON FRESH-WATER MUSSELS

The widespread recent decline in the species diversity and population density of freshwater mussels in North America may be partly related t o chronic, low-level exposure to toxic metals. As benthic filter-feedi ng organisms, freshwater mussels are exposed to metals that are dissol ved in water, associated with suspended particles and deposited in bot tom sediments. Thus, freshwater mussels can bioaccumulate certain meta ls to concentrations that greatly exceed those dissolved in water. In adult mussels, the most common site of metal uptake is the gill, follo wed by the mantle and the kidney. The toxic effects of metals on fresh water mussels have been examined in a few acute toxicity tests, but th e sublethal effects of long-term exposure to low environmental concent rations are little understood. Sublethal exposure to metals can alter growth, filtration efficiency, enzyme activity and behaviour. Subletha l effects are frequently observed at concentrations that are only half the lethal concentrations. However, few toxicity tests have used envi ronmentally realistic exposure concentrations. Total concentrations of Cd, Cu, Hg and Zn in many oxic surface waters are in the ngl(-1) rang e, yet many toxicity studies have exposed mussels to concentrations in the mu gl(-1) or even the mgl(-1) range. An understanding of the proc esses by which metals affect freshwater mussels would provide insights on the ecotoxicological significance of metal contamination to natura l mussel populations and aid in the development of water-quality crite ria that adequately protect mussels.