Thermal Tolerances of Freshwater Mussels and Their Host Fishes: Species Interactions In a Changing Climate

Rising environmental temperatures cause significant shifts in the composition and distribution of species within communities.  In freshwater systems, the larval life stage of Unionida mussels develops as an obligate parasite on host fish.  Because of the relationship between freshwater mussels and their host fish, mussels may be potentially limited by the thermal tolerances of their hosts.  We developed thermal sensitivity data for eight species of freshwater mussels and then compiled thermal data for their host fishes from the literature to determine if the species interactions are at risk from rising environmental temperatures.  Mussels were both more and less thermally sensitive than specific host fish species (2.9 ºC mean absolute difference between mussel and host; range = 0 – 6.8 ºC).  In 62% of mussel-host fish comparisons, freshwater mussels were more thermally tolerant than their hosts (3.4 ºC mean difference; range = 0.2 – 6.8 ºC).  Further analysis revealed that variation in mussel thermal tolerance could not be attributed to mussel acclimation temperature, species, life stage, or mean host fish thermal tolerance, suggesting that mussel thermal tolerance is controlled by multiple interacting and complex factors.  Our findings highlight the importance of considering global change effects in a community context.