Feeding ecology and lethal thermal tolerances among selected North American freshwater mussels (Order: Unionida)

Abstract

Sixty-five percent of North American unionid mussel species are considered imperiled. Knowledge gaps persist in unionid feeding ecology and lethal thermal tolerance limits which could aid in the conservation and management of these imperiled species. To address these knowledge gaps two studies were conducted to quantify stable carbon (δ13C) and stable nitrogen (δ15N) isotopic signatures for mussels in lotic systems (five mussel species across three seasons and four rivers in central Texas, USA) and a lentic system (three mussel species in Gantt Lake, Alabama, USA). In Texas rivers, overall, C derived from coarse particulate organic matter (CPOM; bulk detrital leaf packs) contributed, on average, 51% to the mussel diet and C derived from suspended particulate organic matter (SPOM) contributed, on average, 41% to the mussel diet and in Gantt Lake limnetic benthic fine particulate organic matter (FPOM) contributed, on average, 99% to the mussel diet. Mussels in both the lotic and lentic systems showed evidence of undergoing self-catabolism and relying on internal nutrient stores as a mechanism for coping with stress (poor food quality; and temperature and emersion for lotic and lentic systems, respectively). Together these data suggest the C in mussel tissues in both system types were primarily of benthic origins, but lotic mussels also received secondary contributions from suspended materials. A third study performed a quantitative fatty acid analysis on five species of mussels from four Texas lotic systems to investigate the relative contribution of putative food resources. Mussels across seasons and drainages had 55% of their fatty acid composition in common, but we observed variation in fatty acid profiles and ecologically relevant groupings of fatty acids across drainages. Algal-derived, source-specific fatty acids comprised 7–10% of fatty acid profiles, while bacterial-derived fatty acids comprised <1% of profiles, indicating that algae are the primary source of dietary fatty acids across drainages sampled. Finally, a systematic literature review of North American unionids was conducted identifying lethal tolerance estimates for only 28 of 302 species in the order Unionida. The mean acute median lethal temperatures were 32.8°C for glochidia (19 species), 35.0°C for juveniles (13 species), and 36.3°C for adults (4 species).