Breeding Activity and Mate Selection in the Horseshoe Crab, Limulus Polyphemus

The breeding behavior of a living fossil, the horseshoe crab (Limulus polyphemus), was studied at Seahorse Key, Florida. This Limulus population differs from others that have been studied in lunar and diel rhythmicity, and in the location of nests. Large numbers of females nest at the tide line each spring and summer during high tides associated with full and new moons. Males arrive on the beach either accompanying the females, clasping their carapaces, or alone, joining other nonclasping males in breeding groups around pairs. Recapture data show that nonclasping males return to the beach with a much higher frequency than do females and clasping males. Moreover, circumstantial evidence suggests that these nonclasping males may also shed sperm into nests, thereby achieving some fertilizations. The strongly male-biased sex ratio on the breeding beach would suggest strong competition for access to females. Despite this expectation, we found no displacements of clasping by nonclasping males, only weak evidence of assortative mating for body size and no differences between clasping and nonclasping males in body or clasper size. Seven nests were excavated and five clutches were counted, ranging from about 2,000 to 30,000 eggs. This suggests substantial differences in fecundity among females. However, we have no clear evidence that males use such differences in choosing mates.