Sperm competition in horseshoe crabs (Limulus polyphemus)

Abstract

Male horseshoe crabs have two mating tactics. Some males come ashore attached to a female (clasping the posterior margin of the females' carapace with their modified pedipalps) and nest with her on the intertidal 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 fertilizing eggs. We conducted a paternity analysis to determine the proportion of eggs fertilized by attached and satellite males. Pairs with one satellite were observed during nesting on beaches in Florida and Delaware and their eggs were collected and reared to the late trilobite or first instar horseshoe crab stage. DNA was extracted from these offspring and from each adult (female, attached and satellite male) for use in paternity analysis. A Limulus-specific hypervariable microsatellite locus was identified and primers were constructed to amplify this locus via the polymerase chain reaction (PCR). Genotypes of putative parents and offspring were determined by resolving length variants of these PCR products on acrylamide gels. This allowed us to determine parentage of the offspring. We demonstrate that satellite males fertilized 40% of the eggs on average, attached males fertilized 51% and 4% of the eggs that were laid by the female were fathered neither by the attached male nor by the satellite (and 5% could not be determined unambiguously). There is high variability in the success of satellite males, ranging from 0 to 88%. Part of this variability can be explained by the position of the satellite relative to the attached male. We discuss the mechanics of fertilization and the possible advantages for multiple mating in this species.

Electroretinographic Measures of Vision in Horseshoe Crabs with Uniform versus Variegated Carapaces

“…In one respect, this might be a superfi cially attractive conclusion because the mat ing difference is also quite small: Paternity analysis [3] showed that the overall ratio of the descendents of attached males to those of satellite males is only about 5:4 (although the ratio varied considerably with the exact loca tion of the satellite male). And mating posi tion is not perfectly correlated with carapace appearance [4], Perhaps, then, one small but reliable difference (in vision) might cause another small but reliable difference (in mat ing).…”

“…Coupled with the mod est paternity effect [3], these data focus inter est on the long-term survival rates of uniform and variegated individuals. From an evolu tionary point of view, if variegated animals lived only slightly longer than uniform, the positive effects of such increased longevity might compensate for their small paternity burden.…”

“…Now, Brockmann [2] has characterized the various ways in which males compete for this attach ment. And paternity analyses [3] indicate that males who attain a secure midline attachment do indeed give their sperm a better chance of fertilizing the female's eggs, although their competitive advantage appears to be some what modest.…”

Multiple paternity and extra-group fertilizations in a natural population of California grunion (Leuresthes tenuis), a beach-spawning marine fish

“…Using microsatellite markers as applied to dams and their oVspring, plus attendant males, Brockmann et al (1994) documented multiple paternity within many Limulus nests, and also determined that only a small fraction of the fertilization events was attributable to non-attendant males. Fig.…”

“…Fig. 3 Correlation between a female fork length and estimated number of sires (r 2 = 0.006, n = 17), and b female weight and estimated number of sires (r 2 = 0.004, n = 17) Brockmann et al (1994) posited that the success of attendant males may help to make beach spawning advantageous despite the risks. In our current study, the mean number of grunion oVspring genotyped per nest (40.1) was well below the total progeny count, which can approach 3,000 per nest (Thompson and Thompson 1919).…”