Effects of elevated water temperature on physiological responses in adult freshwater mussels

Freshwater mussels (order Unionoida) face multiple environmental stressors, which pose serious conservation challenges to this diverse assemblage of aquatic invertebrates. Of these stressors, elevated water temperature from global climate change and other anthropogenic sources may be the most ubiquitous and could be placing many mussel populations dangerously close to their thermal maxima. We tested the hypothesis that elevated water temperatures (20, 25, 30 and 35 °C) adversely affected physiological responses in adults of four North American species of mussels (Amblema plicata, Elliptio complanata, Fusconaia flava and Lampsilis cardium) in 21-d laboratory tests. Oxygen consumption rates were directly affected by temperature in E. complanata and L. cardium, and indirectly affected by temperature in A. plicata and F. flava. Rates of O₂ consumption were generally positively correlated with water temperature. Ammonium excretion rates varied significantly with temperature in E. complanata and generally increased with temperature. The amount of O₂ consumed relative to nitrogen excreted (O : N ratio), varied significantly with temperature in A. plicata, E. complanata and F. flava. The tissue condition index varied among temperatures and species. These data suggest that elevated temperatures can alter metabolic rates in native mussels and may decrease the amount of energy that is available for key biological processes, such as survival, growth and reproduction.