Horseshoe crabs have an explosive breeding system not unlike that of some frogs and toads. They synchronize nesting to only a few hours each day at the time of the spring new and full-moon high tides. Males search for females as they come to the breeding beaches, grasp them with specially modified claws and cling to them, sometimes for weeks. Females lay several clutches of eggs in the sand and the male fertilizes them externally, the only extant arthropod with such a reproductive system. Unattached males cluster around the nesting couple, pushing on and occasionally displacing attached males. An experimental manipulation demonstrated that satellite males are capable of fertilizing eggs which suggests that sperm competition is the primary explanation for the presence of unattached males on the beach. Like other explosively breeding species, male Limulus search for females, often grabbing inappropriate objects, and satellite males compete for access to females. There is little assortative mating and attached and unattached males do not differ in size. In extreme explosively breeding species like Limulus, selection favors those males that are best able to locate and remain attached to females, and there is little opportunity for female choice or male-male competition. Horseshoe crabs have an explosive breeding system not unlike that of some frogs and toads. They synchronize nesting to only a few hours each day at the time of the spring new and full-moon high tides. Males search for females as they come to the breeding beaches, grasp them with specially modified claws and cling to them, sometimes for weeks. Females lay several clutches of eggs in the sand and the male fertilizes them externally, the only extant arthropod with such a reproductive system. Unattached males cluster around the nesting couple, pushing on and occasionally displacing attached males. An experimental manipulation demonstrated that satellite males are capable of fertilizing eggs which suggests that sperm competition is the primary explanation for the presence of unattached males on the beach. Like other explosively breeding species, male Limulus search for females, often grabbing inappropriate objects, and satellite males compete for access to females. There is little assortative mating and attached and unattached males do not differ in size. In extreme explosively breeding species like Limulus, selection favors those males that are best able to locate and remain attached to females, and there is little opportunity for female choice or male-male competition.