Reproductive competition and sexual selection in horseshoe crabs

Abstract

The four species of horseshoe crabs share many similarities but differ in the intensity of reproductive competition. Although all horseshoe crabs nest synchronously, only Limulus polyphemus, the American horseshoe crab, has intense male-male competition (high operational sex ratios and multi-male groups around nesting pairs) and very high female nesting densities. These differences in reproductive competition are reflected in differences between American and Asian species (Tachypleus gigas, T. tridentatus, or Carcinoscorpius rotundicauda) in their reproductive structures and in egg and sperm size. However, the American and Asian species do not differ in the degree of sexual dimorphism in body size. The intensity of reproductive competition is correlated with nesting density in L. polyphemus, but this is not the case in the Asian species, which rarely have multi-male groups or highly male-biased operational sex ratios. © Springer Science+Business Media, LLC 2009. All rights reserved.