Ecological Relevance In Freshwater Mussel Thermal Tolerance Tests: Implications for Climate Change

Because the global climate is warming, and available lethal temperature (LT) data on early life stages of freshwater mussels suggest they may be living near their upper thermal tolerances in some systems, we expanded mussel LT research to include ecological factors that affect mussels in natural systems, such as sediment and flow regime.  We developed a method for assessing thermal sensitivity of juvenile freshwater mussels in sediment, thus incorporating their benthic ecology into the tests.  Using these sediment testing protocols, we evaluated the relative sensitivity of four species of mussels (juvenile life stage) to a range of temperatures common during summer in streams with low flow and drought conditions, using two temperature acclimation and exposure regimes.  We also conducted water-only LT tests with mussel larvae (glochidia) and juveniles of four previously untested mussel species.  The median lethal temperatures (LT50s) for all water-only and sediment tests ranged from 33.3 to 37.2 °C, indicating a narrow range of upper thermal sensitivity, regardless of life stage, test type, species, or conservation status.  Moreover, LT50s from sediment tests did not differ from water-only tests, suggesting that any thermal refuge in a natural stream would emanate from other ecological or physical habitat interactions.