No effect of sperm interactions or egg homogenate on sperm velocity in the blue mussel,Mytilus edulis(Bivalvia: Mytilidae)

Abstract

We investigated the possible effects of sperm interactions and homogenized eggs on sperm velocity in blue mussels ( Mytilus edulis L., 1758) using computer-assisted sperm analysis. To test whether sperm competition results in an increase in sperm velocity, using seven pairs of males, we compared the mean curvilinear and average path velocities of sperm from two males measured separately with the corresponding values from a mixture of sperm from the same two males. To test whether the presence of eggs results in an increase in sperm velocity, we compared curvilinear and average path velocities from 11 individual males with the corresponding measures from the same 11 sperm samples mixed with aliquots of homogenized eggs. Neither experimental treatment resulted in an increase in sperm velocity. We interpret these results as consistent with the hypothesis that mussel sperm have been selected to immediately begin swimming at an optimal initial velocity that is adaptive for the particular environment in which they are located. Critical factors affecting the evolution of sperm velocity for broadcast spawning, external fertilizers such as M. edulis likely include population density and intraspecific spawning synchronicity. As has been suggested by others, the importance of sperm limitation (i.e., having much less than 100% of eggs being fertilized in the wild) may be as important an evolutionary driving force in broadcast spawning invertebrates as sperm competition is in internally or directly fertilized species.