An experimental approach to altering mating tactics in male horseshoe crabs (Limulus polyphemus)

Abstract

Alternative reproductive tactics are often correlated with phenotype, density, environment, or social context. Male horseshoe crabs (Limulus polyphemus) have two mating tactics that are associated with phenotype. Males in good condition arrive at the nesting beach and spawn while attached to females, whereas those in poorer condition come ashore unattached and crowd around the nesting couples as satellites, fertilizing eggs through sperm competition. The correlation between mating tactic and phenotype may be due to males choosing tactics based on condition, or it may be that males that have not found a female choose to come ashore as satellites. To distinguish between these two possibilities, I conducted an experiment on male horseshoe crabs in the field at Seahorse Key on the northern Gulf coast of Florida. I prevented males from attaching to females by placing small plastic bags over the claws they use to attach. The results showed that males in poor condition came ashore as satellites, whereas males in good condition remained at sea. This means that mating tactics are cued by information about the male's condition and not about whether he found a female. The evolution of phenotype-correlated mating tactics can be represented by a model in which the fitness of each tactic changes with condition and fitness curves cross. I hypothesize that male horseshoe crabs in good condition have higher fitness when attached and that males in poorer condition to better when unattached.