Contaminants of Emerging Concern May Pose Prezygotic Barriers to Freshwater Mussel Recruitment

Increased detection of contaminants of emerging concern (CECs) throughout the Laurentian Great Lakes Basin has raised concern about the impact CECs may have on aquatic life. In particular, the impacts of CECs on the highly imperiled freshwater mussels (family: Unionidae) have not yet been assessed. Contaminants may alter reproductive success during either the pre- or postzygotic stages of reproduction and lead to population declines. This study sought to address possible disruption of prezygotic processes while acknowledging the complexity of CEC mixtures in nature. River waters from sites with dominant urban and agricultural land use in the Milwaukee River Basin and Lower Maumee River Basin respectively were used as treatments during 21-day exposures of adult Lampsilis cardium, Lampsilis siliquoidea, and Eurynia dilatata. At the conclusion of exposures, the development of gonad tissue was histologically examined to address deviations from expected seasonal development as a barrier to recruitment. No abnormalities in development were found compared to control waters. Sperm densities were also examined; and, though no significant differences were found among treatments, lampsiline species sperm density tended to increase along the gradient of CEC mixture complexity while E. dilatata sperm density correlated positively with a lower concentration of pharmaceuticals, especially antibiotics, despite increased pesticide detection. Given the complexity of natural occurring CEC mixtures and variable unionid response, future work may utilize these data to elucidate combinations of CECs most likely to create impacts on gametogenesis which could lead to further population declines.