Effects of Water Flow, Distance, and Male Density on the Fertilization Success of Freshwater Mussels

Abstract

Previous studies suggest that fertilization of female freshwater mussels is dependent on the distance between mates and may fail at low mussel density. This has critical implications for conservation of sparse populations. This study assesses the fertilization success of two common species, Lampsilis straminea (Conrad, 1834) and Fusconaia ebena (Lea, 1831). We assessed fertilization success by using constructed stream channels and manipulating the distance from mates, presence of water flow, number of males, and number of females. Fertilization success was determined in three ways: proportion of gravid females per pen, fertilization efficiency, and glochidia per female. In L. straminea, water flow and distance from males did not have a significant effect on any measurements of fertilization success. When upstream females were present, fertilization success of distant (25m) females increased, compared to low fertilization success when upstream females were absent. Fertilization efficiency seemed to be “all or nothing” in both species with females being either barren or exhibiting >80% fertilization of brooded eggs. In F. ebena, results were unclear but did show fertilization failure at long distances (25m) with no flow. Overall, fertilization success of freshwater mussels seemed to be independent of distance from males up to 25 m, but dependent on some threshold of female density for optimal fertilization to occur at the farthest distance tested. Results suggest that freshwater mussels have a very efficient fertilization process and can exhibit high fertilization success at low population densities when populations are dominated by females.