Sensitivity of Early Life Stages of Freshwater Mussels to a Range of Common and Extreme Water Temperatures

Abstract

Freshwater mussels fulfill an essential role in benthic aquatic communities, but are also one of the most rapidly declining faunal groups in North America. Rising water temperatures, caused by global climate change, industrial discharges, drought, or land development, can further challenge impaired unionid communities. The aim of this study was to determine the upper thermal tolerances of the early life stages, glochidia and juveniles, of freshwater mussels. Glochidia of eight species of mussels were tested: Lampsilis siliquoidea, Potamilus alatus, Ligumia recta, Ellipsaria lineolata, Lasmigona complanata, Megalonaias nervosa, Alasmidonta varicosa, and Villosa delumbis. Seven of these species were also used in juvenile tests. Survival trends were monitored as mussels held at three acclimation temperatures, 17°C, 22°C, and 27°C, were exposed to a range of common and extreme water temperatures (20 - 42°C) in standard acute laboratory tests. The average median lethal temperature (ET50) in 24-h tests with glochidia was 33.7°C, ranging from 29.1 to 37.5°C. The mean ET50 in 96-h juvenile tests was 34.8°C, and ranged from 32.9 to 36.7°C. As an indicator of sublethal thermal stress, heart rate patterns for seven species of juvenile freshwater mussels were assessed visually through direct observation. Species differences were observed; L. recta and V. delumbis displayed significant changes in heart rate associated with increasing temperature at all three acclimation temperatures. Thermal stress is almost certainly not the only stressor affecting freshwater mussels. Metals, such as copper, are a common source of toxicant exposure in aquatic environments. The toxic effects of copper on the early life stages of freshwater mussels have been well-studied, and freshwater mussels are more sensitive to copper than most aquatic organisms. The effect of a sublethal copper concentration on the upper thermal tolerance of three species, Lampsilis siliquoidea, Potamilus alatus, and Ligumia recta, of juvenile freshwater mussels was determined. ET50s were calculated in the absence and presence of copper, and they ranged from 32.9ºC to 36.7ºC with a mean of 34.8ºC. Based on 95% confidence interval overlap, there were no differences among ET50s caused by acclimation temperature, species, or presence of copper. However, survival trends showed evidence of interactive effects between copper and temperature for all three species, suggesting this is an area that warrants further study. In freshwater systems, the larval life stage, glochidia, of Unionoida mussel species must develop as obligate parasites on host fish gills or fins before transforming into the juvenile life stage and dropping to the sediment to perpetuate their life cycle. Because of the relationship between mussels and their often specific host fish species, freshwater mussels are not only potentially affected by their own variable thermal tolerance limits, but also by the thermal tolerances of their fish hosts. Thermal tolerance data for eight species of freshwater mussels and their host fish was compiled and compared in order to determine whether the community structure of these systems is at risk from rising environmental temperatures; relationships were complicated with mussels being both more and less thermally sensitive than certain host fish species. Freshwater mussels are a valuable part of benthic aquatic ecosystems, and this study has shown that thermal stress can negatively impact these animals; therefore it is necessary to keep rivers and streams thermally acceptable to protect these threatened fauna.