Reproductive tactics and mating contexts affect sperm traits in horseshoe crabs (Limulus polyphemus)

Abstract

In systems with male alternative reproductive tac- tics,discretebehavioralpatternsoftenleadtounequallevelsof sperm competition across tactics. In many of these systems, the disadvantaged tactic shows adaptations to sperm traits that increase the competitive ability of its sperm. However, many studieshavefoundthatnotall spermtraitsdifferacrosstactics, and in some systems no sperm trait adaptations are found. Why some systems and some sperm traits display tactic- specific differences and others do not is not always clear. Here, we investigate three sperm traits—ejaculate size, sperm concentration, and sperm velocity—in the American horse- shoe crab, Limulus polyphemus, a system where males have condition-dependent reproductive tactics and females influ- ence the amount of sperm competition males face during spawning. Contrary to our predictions that the disadvantaged tactic would show sperm trait adaptations, we found that horseshoe crabs of the tactic that faces the least amount of sperm competition had more concentrated sperm than the males of the other tactics.We propose a number of hypotheses to explain this result and suggest that future studies of alter- native reproductive tactics might benefit from considering the role of females in influencing tactic-specific sperm trait adaptations.