Understanding the temperature tolerances of organisms is critical because thermal regimes of freshwater ecosystems are changing globally due to climate change, river regulation, and land development, which may create conflicts between the needs of humans and aquatic ecosystems. Native freshwater mussels are especially sensitive to increasing water temperatures because of their physiology and unique life-history. Detailed knowledge on lethal temperatures for mussels has been limited to less than 5% of the species known to occur in North America, and little is known about thermal tolerances of mussel species from rivers within the southwestern United States. To determine the effects of elevated water temperature on mussels from the southwestern United States, I tested the upper thermal tolerances of larvae (glochidia) for the following species across 4 basins in Texas (Neches, Guadalupe, San Antonio, and Colorado): Amblema plicata, Cyclonaias petrina, Fusconaia mitchelli, Lampsilis bracteata, Lampsilis hydiana, Lampsilis satura, Lampsilis teres, Leptodea fragilis, and Obovaria arkansasensis. I then tested the upper thermal tolerances of adults of 3 of these species (Amblema plicata, Cyclonaias petrina, and Fusconaia mitchelli) from the Guadalupe River. I evaluated upper lethal limits of freshwater mussels acclimated to 27 ?C across a range of experimental temperatures (30-39 ?C) in standard acute laboratory tests. The results of the adult trials were then related to in situ water temperature and flows using a uniform continuous above-threshold (UCAT) analysis which evaluates the duration and frequency of continuous events above a specified temperature threshold. Median lethal temperature (LT50) in 24-h tests among glochidia averaged 32.4 ?C and ranged from 26.9 to 36.4 ?C. The mean LT50 in acute 96-h adult tests averaged 36.4 ?C and ranged from 33.7 to 37.5 ?C, while the chronic 10-d adult tests averaged 35.9 ?C and ranged from 32.4 to 37.5 ?C. Thermal tolerances of F. mitchelli were significantly lower than both A. plicata and C. petrina, and the UCAT analysis showed that LT05 (temperature affecting 5% of the population) thresholds were exceeded for F. mitchelli in the Guadalupe River at both acute (96-h) and chronic (10-d) values. Findings from my study indicate freshwater mussels from the arid and semi-arid regions of the Southwest are already at risk from rising environmental temperatures and altered hydrologic flows.
