Every spring large numbers of horseshoe crabs migrate to beaches along the Atlantic coast. Females, with males attached, bury in the sand, laying eggs which the male fertilizes externally. Unattached males aggregate around nesting pairs and evidence suggests that they also fertilize eggs. The proposed field study will characterize the unusual reproductive behavior of this species. The proportion of offspring fathered by attached and unattached males will be determined using DNA fingerprinting. Descriptive observations, measurements of marked populations and experimental manipulations will quantify nesting and mating behavior, and differences between populations. Limulus is unique. The Xiphosurida are the only extant arthropods with the primitive pattern of external fertilization and aquatic, free-swimming sperm. A study of their breeding will provide insight into the ancestors of modern arthropods. Furthermore, although Limulus is one of the best-studied species on the planet, little is known about their reproduction. This field study will provide a valuable context for, and will take advantage of, research on other aspects of their biology. Finally, because of their synchronized, semi-lunar breeding and external fertilization, Limulus mating shows profound similarities to explosively breeding amphibians. Therefore, the study of Limulus may provide an independent test of theories on the evolution of mating systems.
