Sexual Competition Among Male Blue Crab, Callinectes sapidus.

Experiments and field data on blue crab, Callinectes sapidus, from mid-Chesapeake Bay between 1991 and 1994 were used to test whether large males have advantages over small males in accessing females and in sperm competition. In the field, large males were paired more often, especially with large, more fecund females. However, the variance in the relationship between male and female size in mating pairs was high, suggesting that mating with large females may not be the primary determinant of male reproductive success. Large males had proportionately longer chelipeds, which may provide an advantage in aggressive interactions for females or in struggles to control females. Previous work indicates that sperm competition may occur in blue crabs and that ejaculate size may influence a male's ability to compete during sperm competition. Large males stored more seminal fluid and spermatophores and passed a larger volume of ejaculate to each mate than did small males. Ejaculate volume averaged 47% of a male's stored supply. However, ejaculate volume increased with the duration of copulation but decreased with successive matings, such that males needed about 15 days between matings to pass similar-sized ejaculates to successive mates. Pre-copulatory mate guarding may serve as a time to replenish ejaculate contents, and thus its duration also influences a male's performance in sperm competition.