Ecology and genetics of freshwater mussels in the Neosho and Cottonwood Rivers.

Abstract

A multitude of environmental factors influence the community structure of freshwater mussels. To determine which variables are most important in structuring mussel communities, and what species-environment relationships exist, I quantitatively sampled mussels at eight locations on the Neosho and Cottonwood rivers, and measured 20 physical and water chemistry variables. Using mussel abundance and biomass as dependent variables, Canonical correspondence analysis (CCA) showed that chemical (total hardness, alkalinity, pH, and total acidity) and stream size (stream width, water depth, and current velocity) variables constituted the main environmental gradient, while substratum formed a secondary gradient. Both CCA ordinations showed consistent species-environment relationships. Quadrula pustulosa, Quadrula guadrula, and Obliguaria reflexa were positioned nearer to the center of the ordination diagrams, suggesting these species may have little environmental preference. Amblema plicata, Leptodea fragilis, Potamilus purpuratus, Tritogonia verrucosa, and Truncilla donaciformis were positioned more toward the periphery of the ordination diagrams, which suggests more specific environmental preferences. water chemistry and stream size variables also accounted for the majority of significant correlations to the abundance and biomass of mussel species. Leptodea fragilis and Truncilla donaciformis were positively correlated with basic conditions, while A. plicata and Q. reflexa were negatively correlated with basic conditions. Amblema plicata and Elliptio dilatata correlated positively with stream width and water depth. These data should form a basis for studies designed to determine causal factors in the population and community dynamics of freshwater mussels. Fish are thought to be the main dispersal mechanism of unionid mussels, and thus should influence the amount of gene flow. To test predictions of genetic structure based on dispersal ability, I compared the genetic variability of Q. pustulosa, a mussel species that is known to parasitize at least six species of fish, and Q. reflexa, a mussel species that is thought to complete development without parasitism. I collected individuals from six locations on the Neosho and Cottonwood rivers, and measured genetic variability at 15 presumptive loci using starch gel electrophoresis. Although few significant differences were observed, Q. pustulosa generally exhibited higher polymorphism, allelism, and heterozygosity, while Q. reflexa had higher inbreeding among populations. PopUlations of Q. reflexa that were closer in proximity were more genetically similar, and genetic distance between Q. reflexa populations decreased farther downstream. There was no apparent geographic logic in similarity among populations of Q. pustulosa. These data give support to the hypothesis that fish affect the genetic structure of unionid mussels. As we increase our knowledge concerning the host species of freshwater mussels, more comparisons between species should be made to gain a better understanding of unionid evolution.