Comparative physiological, biochemical and molecular thermal stress response profiles for two unionid freshwater mussel species.

Freshwater mussels, aquatic keystone species, are in global decline. Long life spans, sedentary lifestyles, and unique reproductive strategies involving obligate parasitic stages make unionid freshwater mussels particularly sensitive to environmental perturbations resulting from global climate change. A greater understanding of the mechanisms by which closely related species differ in their response to thermal challenge is critical for successful conservation and management practices. As such, both an acute heat shock and a chronic warming simulation were conducted in order to evaluate responses between hypothesized thermally tolerant (Villosa lienosa) and thermally sensitive (Villosa nebulosa) freshwater mussels in response to predicted thermal warming. Multiple biological responses were quantified, including mortality, condition index, growth rates, glycogen and triglyceride content, and candidate gene expression. During acute heat shock, both species upregulated HSP90 and HSP70, although V. lienosa showed consistently greater transcript levels during upregulation. This pattern was consistent during the chronic warming simulation, with V. nebulosa showing greater induction of HSP60 Chronic warming stimulated increases in condition index for V. nebulosa; however, declines in growth rates during a recovery period were observed with no concurrent change in tissue glycogen levels. This contrasts with V. lienosa, where tissue glycogen significantly increased during chronic warming, although no response was observed for condition index or growth rates. These biological differences might indicate disparate thermal stress response mechanisms correlated with metabolic demands and resource utilization, and could thus be a factor influencing current ranges of these two species and their ability to cope with future persistent warming in their native habitats.