Geographic variation in sperm and ejaculate quantity and quality of horseshoe crabs

Abstract

Populations of a single species may vary substantially in sexually selected traits. However, the majority of studies investigating such variation across populations have focused on traits important for pre-copulatory reproductive success, such as male ornamentation or song. Relatively fewer studies have investigated whether traits important in post-copulatory sexual selection vary across populations. In this study, we examine how sperm and ejaculate quantity and quality in the American horseshoe crab, Limulus polyphemus differ at six sites, from Maine to Florida. Horseshoe crabs from these sites differ in many respects, including average body size, age, and the risk and intensity of sperm competition as measured by the operational sex ratio. We find that sperm and ejaculate quantity and quality significantly differ across this range and that this variation persists after taking body size and age into account. Sperm traits also do not follow strictly linear latitudinal clines. Finally, we find that the site-specific operational sex ratio correlates with changes in sperm and ejaculate quantity. This latter result may indicate that the same processes that lead to variation in post-copulatory sexually selected traits across related species may also promote trait divergence across populations of the same species. This study describes variation in sperm and ejaculate traits from horseshoe crabs collected at six sites from Maine to Florida. While geographic variation has been described for many traits, relatively few studies have demonstrated that sperm and ejaculate traits follow similar patterns. The variation we found does not follow simple latitudinal patterns, suggesting that other forces may be driving sperm and ejaculate trait divergence across sites. We also have data that suggest sexual selection may play a role in driving sperm trait variation, but we caution that additional sites need to be sampled.