Body size,morphological constraints,and mated pair formation in four populations of horseshoe crabs (Limulus polyphemus) along a geographic cline

The relationship of body size to mating success was studied in four populations of horseshoe crabs, Limulus polyphemus L., along the east coast of the U.S.A. in the spring and summer from 1986 to 1989. Crabs of both sexes from Great Bay, New Hampshire, were significantly smaller than crabs from three middle Atlantic coast populations: Sandy Hook Bay and Delaware Bay, New Jersey, and Chincoteague Bay, Virginia. The formation of mated pairs was independent of body size in each population; there were no significant size differences between mated and single individuals, and size assortative mating did not occur. A comparision of male clasper dimensions with the corresponding point of attachment on the female indicates that there are no morphological constraints limiting amplexus between any male and any female within a population. The ratios of male to female prosoma width within amplexed mated pairs averaged from 0.78 to 0.80 in each population, despite the large difference in absolute size between southern and nothern populations. This may suggest a role for natural selection in regulating the relative sizes of each sex.