Latitudinal variation in freshwater mussel potential maximum length in Eastern North America

As body size often predicts energetic requirements and fecundity, understanding the drivers behind size variation is important. Neo-Bergmann's rule states that larger individuals are found at higher latitudes and this size variation is attributable to temperature gradients. In ectotherms, this macroecological pattern has mixed support within the literature-both the direction and mechanism of size correlation with latitude varies. We asked if two species of freshwater mussels with different thermal niche preferences, Amblema plicata and Lampsilis cardium, follow neo-Bergmann's rule and what mechanisms might drive that latitudinal variation. Lampsilis cardium is a thermally sensitive species intolerant of high temperatures, and A. plicata is more tolerant of a wide range of temperatures. We predicted that the thermally sensitive species at southern latitudes would have stunted growth in the summer and that this stunting would produce a steeper relationship with latitude than in the thermally tolerant species. We collected and thin-sectioned 113 A. plicata shells from 23 sites and 85 Lampsilis cardium shells from 12 sites across a latitudinal gradient in the eastern U.S.A. We used back-calculated size-at-age data to determine logistic growth parameters for each taxon across this gradient. We used Bayesian model selection to evaluate how environmental information correlated with potential maximum size. We found that both mussel species reached a larger potential maximum size at higher latitudes, with the thermally sensitive species having the larger increase in potential maximum size across the gradient when compared to the thermally tolerant species. Average annual water temperature explained mussel potential maximum size better than annual catchment precipitation, annual minimum flow ratio, and species identity. Both increased average water temperature and annual catchment precipitation, two predicted outcomes of climate change, were associated with smaller potential maximum size. Reductions in mussel maximum size due to climate change induced shifts in temperature regimes and precipitation patterns could lead to reduced reproductive output of this threatened guild and subsequent changes in ecosystem function and services, such as decreased biofiltration. As climate change alters precipitation patterns and stream productivity, and increases water temperature, understanding size variation and its cause in aquatic organisms will be important for managing these vulnerable populations.