Freshwater mussels (order Unionoida) are the most imperiled group of fauna in North America. The factors driving this decline chiefly affect juvenile recruitment and survival, rather than adult mortality. However, our present knowledge generally consists of inferences from studies of adult distributions. Juvenile mussels are rarely collected in the field, leaving our understanding of this critical period in the mussel life cycle incomplete. Popular hypotheses, most notably that shear stress during floods scours small mussels from the substrate and largely confines mussel beds to sheltered areas, are supported by circumstantial evidence and have rarely been tested directly. The role of presettlement processes such as fish behavior is poorly understood. I collected adult and juvenile mussels in the Sabine River north of Tyler, Texas. I hand-collected adults from randomly placed 0.25-m2 quadrats, then collected 10 cm of sediment from the same area, sieved it, and inspected all but the finest portions for bivalves. I collected 468 adult and 137 juvenile mussels representing 14 species. Texas pigtoes outnumbered all other species as both adults and juveniles, while pistolgrips were highly abundant as adults but extremely rare as juveniles. Fragile papershells were common as juveniles but rarely located as adults. Adult Texas pigtoes, adult Western pimplebacks, juvenile deertoes, and juvenile fragile papershells were most common in the riffle portion of the study area, while adult deertoes and Southern mapleleafs were more abundant and more consistently present in the run portion. Juvenile Texas pigtoes were somewhat denser in the riffle, but not as dramatically as adults; this is consistent with the hypothesis that post-settlement mortality plays a key role in determining mussel distributions. Field measurements of shear stress were not significantly correlated with abundance or presence of most species; Southern mapleleafs, which were more consistently present at low shear, were the sole exception. I estimate that the young-of-year Texas pigtoes collected excysted around the end of June or later. This coincides with a substantial flood, which may have killed juveniles that excysted earlier in the season. Deertoes and fragile papershells excysted from May to early June and did not continue to excyst later in the season. I suspect that host fish behavior plays a major role in controlling the distribution of these two species, both of which rely exclusively on freshwater drum in the Sabine River. Overall, I did not collect enough juveniles of most species to draw strong conclusions. Additionally, a single season of data is not necessarily representative of long-term trends. Juvenile mussel sampling is useful for studying post-excystment settling patterns and first-year mortality, but the large amount of labor required for a small amount of data limits its effectiveness as a primary tool.

Date of publication

Fall 8-20-2013

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