SPERM COMPETITION IN HORSESHOE CRABS (LIMULUS-POLYPHEMUS)

Male horseshoe crabs have two mating tactics. Some males come ashore a ttached to a female (clasping the posterior margin of the females' car apace with their modified pedipalps) and nest with her on the intertid al portion of the beach during the high tide. Other males come ashore unattached and crowd around nesting couples. Fertilization is external and unattached males that are in contact with a pair, i.e. 'satellite ' males, release sperm, so the assumption has been that they are ferti lizing eggs. We conducted a paternity analysis to determine the propor tion of eggs fertilized by attached and satellite males. Pairs with on e satellite were observed during nesting on beaches in Florida and Del aware and their eggs were collected and reared to the late trilobite o r first instar horseshoe crab stage. DNA was extracted from these offs pring and from each adult (female, attached and satellite male) for us e in paternity analysis. A Limulus-specific hypervariable microsatelli te locus was identified and primers were constructed to amplify this l ocus via the polymerase chain reaction (PCR). Genotypes of putative pa rents and offspring were determined by resolving length variants of th ese PCR products on acrylamide gels. This allowed us to determine pare ntage of the offspring. We demonstrate that satellite males fertilized 40% of the eggs on average, attached males fertilized 51% and 4% of t he eggs that were laid by the female were fathered neither by the atta ched male nor by the satellite (and 5% could not be determined unambig uously). There is high variability in the success of satellite males, ranging from 0 to 88%. Part of this variability can be explained by th e position of the satellite relative to the attached male. We discuss the mechanics of fertilization and the possible advantages for multipl e mating in this species.