Examination of large exuviae with mating scars: Do female American horseshoe crabs, Limulus polyphemus, molt after sexual maturity?

Abstract

Determining when a female horseshoe crab reaches sexual maturity, and presumably experiences a terminal molt, is of increasing interest to interpret the dynamics of American horseshoe crab populations for management. Males acquire easily recognizable and biologically determinate sexually dimorphic characteristics at maturity, while the most distinctive traits acquired by females are carapace scars that result from amplexus with another crab, presumably during mating. We found well defi ned mating scars on the axial ridge of the opisthosoma and clasper depressions on the opisthosomal projections of 35 large-sized (prosomal width 176–257 mm) horseshoe crab molts collected from Delaware Bay. The greatest intensity of scarring per unit area was found on molts >230 mm, a size range that overlaps with known sizes of females at spawning beaches, but is larger than sizes typically reported for subadults. These data represent the fi rst published description and analysis of female horseshoe crab molts with mating scars typical of prolonged amplexus and suffi cient to have occurred during spawning. We lack data to defi nitively prove that the molts examined came from females that engaged in reproductive activity beyond extended amplexus, but prolonged amplexus without reproduction is generally disad-vantageous. Molting after fi rst reproduction, however, may enhance reproductive success through shell repair and increased fecundity. This study describes a quantitative approach that can serve as the basis for additional targeted studies to quantify mating scars on large-sized molts and characterize mating scar formation on live spawning crabs that will be useful to further evaluate molt timing relative to reproduction.