Biological and ecological mechanisms for overcoming sperm limitation in invasive dreissenid mussels

Many broadcast-spawning benthic invertebrates are subject to sperm limitation yet achieve high population densities, as for example dreissenid mussels (Dreissena polymorpha and Dreisssena bugensis) that were introduced into the Laurentian Great Lakes. The question remains whether biological or ecological/physical mechanisms reduce sperm limitation. Gamete dilution/longevity experiments were undertaken to determine whether dreissenid mussels are subject to sperm limitation, and computational fluid dynamic modeling was used to determine the potential influence of bottom roughness on sperm dilution in nature. Results indicated that dreissenid mussels may be sperm limited, but the extent to which sperm dilution affects them is lower than what was reported for other broadcast spawning invertebrates. Importantly, model mussel clusters influenced external fertilization by retaining sperm in downstream eddies but allowing downstream transport from one cluster to another. This, in addition to high sperm potency at low sperm concentrations, may help to explain the success of dreissenid mussels as invasive species.