Differential response mechanisms to acute and long-term simulations of global warming for two closely related species of unionid freshwater mussel, Villosa lienosa and Villosa nebulosa

Freshwater mussels are keystone species in aquatic environments, providing ecological services that protect aquatic resources. Their global decline resulting from habitat alterations such as global warming is thus alarming. Since agencies such as the Alabama Aquatic Biodiversity Center are expending great effort in restoring imperiled species it would be beneficial to understand how thermally tolerant and thermally sensitive species (Villosa lienosa and Villosa nebulosa, (respectively) respond to environmentally relevant thermal stress. This dissertation presents the findings from a series of experiments aimed at developing resources and response models capable of answering questions related to thermal stress in freshwater mussels. We created a high quality transcriptomics database for both species that includes both ambient and heat stressed expressed transcripts. Using this publicly available database we assessed the acute heat shock response of both species and found that regulatory mechanisms at the transcriptomic level differ between the two species. We then conducted two chronic thermal simulations, 4 months and 11 months, respectively, in natural conditions at environmentally relevant temperatures predicted by the Intergovernmental Panel on Climate Change for the year 2100. Along with molecular biomarkers, these studies also included physiological biomarkers (condition index and growth rates) and biochemical markers (glycogen and triglyceride content, and lipid peroxidation levels as determined by MDA concentrations). During the summer months, V. nebulosa responded to warming at lower temperatures than V. lienosa and experienced significant mortality. Further, V. nebulosa appeared to undergo metabolic depression after a period of higher metabolic rates that could not be maintained with available resources. Villosa lienosa, on the other hand was able to cope with the thermal stress via upregulation of molecular chaperones and did not display symptoms of increased metabolic demand. As a whole, this dissertation supports the need for natural field experiments on longer temporal scales using environmentally relevant temperatures. It is clear that predicted global warming will impact imperiled populations of freshwater mussels. Future research needs to further validate sensitive biomarkers of mussel physiological response for use in conservation and management strategies.