A Comparison of Ecological Factors Affecting Two Species of Lotic Molluscs.

This study examined habitat-mediated differences in the population structure of two species of stream molluscs, the Louisiana pearl-shell mussel, Margaritifera hembeli, a protected species, and Elimia semicarinata, a pleurocerid gastropod. In 1991 I located Margaritifera hembeli in a drainage system where it was previously unknown, and describe here the distribution of M. hembeli in the Bayou Rigolette drainage in Grant Parish, Louisiana. The mussel was limited to small second- and third-order lotic systems. Of several physicochemical and habitat parameters, specific conductance, Ca$\sp{+2}$ concentration, sediment particle size, sediment compaction and channel width were most important in explaining abundance. Substratum stability was also important, indicating mussels may be susceptible to spate events. Mussel densities and size distributions varied among stream sites, with evidence of recent recruitment at only half the sites. Growth rates differed by a factor of three among study sites, leading to minimum estimates of longevity between 25 to 80 years. Growth-rate differences caused allometric and shell-morphology differences among populations as well. For the gastropod Elimia semicarinata, a riverine prosobranch from north-central Kentucky, population density and size structure were also dependent upon mircrohabitat differences. Densities were always greater in vegetation beds than in non-vegetated areas, irrespective of flow regime, and snails were smaller as well, suggesting density dependence. Snails were also smaller in lotic than in lentic unvegetated areas, suggesting greater bioenergetic costs to snail growth in areas of high flow. These differences were maintained even though colonization experiments suggested high migration rates among habitats. Experiments with flow-reducing baffles suggested snails could orient to flow refugia. Finally, a grazer-manipulation experiment suggested that Elimia semicarinata does not reduce periphytion biomass, even in lotic habitats that have reduced abundance of periphyton. Overall, these results suggest that lotic snails and bivalves have complicated population structures which are dependent upon biotic (density) and abiotic (current velocity, substratum stability) factors, and management strategies must take these habitat-specific differences into account