Relationships between freshwater mussel (Mollusca: Unionida) shell traits, life history strategies and stream size using a large dataset

The presence of shell sculpture (e.g. pustules, ribs) or elongated dorsal ends (i.e. wings) may influence the broadscale distribution and abundance of freshwater mussels across habitat types (e.g. small streams, large rivers, and lakes). Shell sculpture and shape can improve anchoring and prevent dislodgement in stable sediments of large rivers but may also inhibit burrowing. In lakes and reservoirs, unsculptured mussels, especially those with thin shells, may be at an advantage because they will not sink into fine substrates like thicker-shelled, sculptured individuals. Small streams may also harbour more unsculptured mussels because they can re-burrow more quickly if they are dislodged during high-energy events. We used a large database of timed searches for freshwater mussels at 4368 sites throughout Minnesota. We examined whether the distribution and abundance of species with different shell traits varied among habitat types: lake, reservoir and stream size. We found more sculptured mussels and mussels with thicker anterior shells in large streams, and more unsculptured, thinner-shelled individuals in lakes, reservoirs and small streams. Our analyses also demonstrated that shell sculpture varied with species’ life-history strategies, with sculptured species more likely to be long-lived equilibrium strategists and shorter-lived opportunistic species largely unsculptured. A multiple correspondence analysis showed strong covariation between habitat type, shell characteristics and life-history strategies. Although at the statewide level, larger streams had more individuals with shell sculpture, and small streams, lakes and reservoirs had more unsculptured individuals, this pattern varied among watersheds in the state, likely due to biogeographic differences in species distributions and anthropogenic stressors. This study describes a framework for predicting mussel assemblage composition based on stream size and biogeography, which provides information that can be used to assess the effect of anthropogenic stressors that cause deviation from natural patterns.