Dreissenid mussels Dreissena bugensis (quagga mussel) and Dreissena polymorpha (zebra mussel) are prolific invasive species to the freshwaters of the United States and Western Europe. In the Great Lakes, D. polymorpha has initially dominated the system since its invasion in the mid-1980s; however, recently D. bugensis has displaced D. polymorpha as the dominant species. Dreissena bugensis has several competitive advantages over D. polymorpha, including greater tolerances to deeper and colder waters and lower respiration rates. Nevertheless, physiological differences between the species remain largely unknown. The oxidative stress response is a mechanism used by all organisms to mitigate environmental stress by reducing oxygen radicals in the body, and comparing this mechanism between similar species can be useful for understanding how different species compete in aquatic environments. We compared oxidative stress biomarkers (lipid peroxidation [LPO] and catalase [CAT] activity) in mussels after exposure to 4 stressors (i.e., high densities, temperature, hypoxia, and polychlorinated biphenyls [PCBs]) independently and in combinations of 2 stressors. Overall, D. bugensis had lower LPO and CAT activity than D. polymorpha when exposed to single stressors; however, in multiple stressor treatments D. bugensis had increased LPO, especially with high temperatures and PCBs. The lower lipid damage in D. bugensis compared with D. polymorpha under single stressor conditions may come at the cost of the ability to respond to multiple stressors. Environ Toxicol Chem 2018;37:1643-1654. © 2018 SETAC.

There is increasing recognition of the importance of rapid adaptation in the dynamics of populations and communities. While the effects of rapid adaptation on the stability of populations have been shown in experimental systems, demonstration of their impacts in natural populations are rare. We examined the relationship between clonal dynamics and population stability of natural Daphnia pulex populations experiencing seasonal environmental variation. We show that the degree of asynchrony in a population's clonal dynamics is tightly linked to its population-level stability. Populations whose clonal abundances were more asynchronous were more stable temporally. Variation in asynchrony was related to variability in primary productivity, and experiments using clones from the study populations revealed significant genotype by environment interactions in response to food level. This suggests that clonal turnover was not due to neutral dynamics alone but may be linked to variation in functional traits associated with resource acquisition and conversion.

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