The role of life history and behavior in explaining unionid mussel distributions

Abstract

In unionid mussels, it has been proposed that opportunistic life histories are adaptive in flashy tributaries, while equilibrium life histories and thicker, sculptured shells are adaptive in higher order streams. We sampled the distributions of 9 common mussels along a gradient in stream size in southeastern Louisiana, USA, to test for life history and behavior traits as explanations of longitudinal pattern of species distributions. We found two species distribution groups, cosmopolitan species, and higher order stream specialists. Shell sectioning revealed that cosmopolitan species matured earlier, had shorter life cycles, possessed lighter shells, and grew faster. We also studied mussel movement and sediment erosion around shells in the laboratory, and our results indicated that cosmopolitan species had faster rates of movement, and were better at tracking receding water levels in the laboratory, all traits that could be adaptive in flashier headwater streams, but also suffered more scouring in a laboratory flume than species with heavier shells and sculpture. Our results verify that mussel life histories and behavior help in understanding mussel distributions in rivers, and also suggest that higher order stream species may be more at risk to disturbances like spates.