A comparison of the copper sensitivity of two economically important saltwater mussel species and a review of previously reported copper toxicity data for mussels: important implications for determining future ambient copper saltwater criteria in the USA.

Saltwater bivalves of the genus Mytilus are among the most copper sensitive taxa listed in both the current and recently proposed U.S. EPA ambient saltwater copper criteria documents. The copper saltwater quality criteria are somewhat unique in that the criteria were set specifically to protect Mytilus. However, there is considerable uncertainty in the reported taxonomy of Mytilus species in the criteria database and it has recently been demonstrated the copper toxicity to M. galloprovincialis is dependent on the organic matter content of the test water. A review of the toxicity and biogeography literature was conducted to rationalize the existing criteria database. Elimination of some data is suggested due to the uncertainty of test organism genotype. Moreover, due to the lack of reported dissolved organic matter content of the test waters in tests included in the criteria database, it is impossible to determine if the difference in species mean acute values reported in the criteria documents for Mytilus was due to differences in water chemistry or differences in species sensitivity. Experiments were designed and conducted with M. galloprovincialis and M. edulis (genetically confirmed) to determine if copper toxicity is a function of organic matter content for these two species and if there is a significant difference in species copper sensitivity. Results showed that copper toxicity is a function of organic matter concentration for both species and copper sensitivity of each species was statistically similar. Results support the normalization of the saltwater copper criteria database with respect to dissolved organic matter when developing ambient saltwater copper criteria. The USEPA toxicity database would benefit from future testing of M. trossulus and M. californianus.