Effect of light, food and silt on the toxicity of copper to juvenile Lampsilis straminea claibornensis mussels

There is no standard acute toxicity test method for unionid mussels. Test methods used to evaluate the toxicity of contaminants to juvenile mussels have been adapted from techniques used with fish and zooplankton. However, mussels occupy benthic habitats of streams and lakes where light is diminished compared to that measured nearer the surface, and where they are exposed to more particulate matter than are pelagic species. A recently published 9-day test method for juvenile unionids requires the use of 24 hour darkness, 800 mg/L silt and a daily dose of an algal mixture. Although the assumption is that these conditions better mimic real-world exposures, and therefore, provide more accurate results than would water-only tests using a 16:8 photoperiod and no food, there are no supporting data. We evaluated the impact of each of these components on the sensitivity of juvenile mussels in a series of toxicity tests with copper sulfate. The presence or absence of light or food made no difference in mussel sensitivity to copper. Little or no difference was detected between LC50s (available copper) calculated for tests that included silt compared to those that did not. Therefore, there appears to be no requirement for darkness, feeding or silt in acute juvenile unionid toxicity tests.