The role of age on sperm traits in the American horseshoe crab, Limulus polyphemus

Sperm competition is an important component of postcopulatory sexual selection. Despite the fact that sperm traits may be important in determining fitness and thus may be under directional selection, they are often highly variable in mating systems with intense sperm competition. One possible explanation for this variation is that sperm traits vary with age. Age affects the expression of many life-history traits and sexual selection signals, but its influence on sperm traits is not well understood. In this study, we examined the correlation between individual age and sperm traits in a natural population of the American horseshoe crab. We compared five sperm traits (ejaculate size, concentration, total sperm ejaculated, velocity and viability) between males of three age categories. Young males ejaculated more total sperm and had significantly more concentrated sperm than old males. Males of different ages did not differ in sperm velocity or viability. Our results suggest that age influences traits associated with sperm quantity more than it does traits associated with sperm quality. Our results also suggest that individual age may be an important, but often overlooked, factor in studies examining sperm traits in natural populations.