Evaluating lethal thermal tolerances and vulnerability of three east Texas freshwater mussels

River water temperature is a key factor in determining habitat suitability for aquatic organisms. Climate change and human-driven impacts to freshwater ecosystems can increase water temperatures above species-specific ranges, posing considerable threats to freshwater ecosystem composition, function, and the services they provide to people. Unionid mussels are globally imperiled, and many species are living near or at their upper thermal limit, which means future increases in water temperature will likely have negative impacts to their long-term persistence. Despite this threat, there remains a knowledge gap on sublethal and lethal temperature thresholds for most mussel species. To better understand how rising water temperatures will affect the survival of freshwater mussel populations, we tested the upper thermal limits (i.e., lethal tolerance estimates resulting in 5% and 50% mortality) of 3 mussel species for which thermal tolerance information is lacking: Cyclonaias pustulosa (Lea, 1831), Fusconaia askewi (Marsh, 1896), and Pleurobema riddellii (Lea, 1862). Mussels were collected from east Texas in the southwestern United States. Individuals were acclimated to 27°C then tested at 4 experimental temperatures (30, 33, 36, 39°C) and an unacclimated control (20°C) for 10 d (240 h). Lethal tolerance estimates resulting in 5% mortality ranged from 31.69 to 37.22°C, and lethal tolerances resulting in 50% mortality ranged from 34.29 to 38.43°C. Relating field water temperature to our laboratory-derived thresholds identified a small number of exceedances, which increased under drought-like conditions (i.e., reduced stream discharge). Climate projections for east Texas show that droughts will become more commonplace in the future, which means that although thermal stress may not be an issue now, it may become a threat in the future. Overlapping thermal tolerance estimates with in situ water temperature and discharge data provides a useful approach for evaluating thermal risk to mussels and guiding efforts focused on protecting and restoring instream flows.