AMPLEXUS AND MATING-BEHAVIOR IN THE JAPANESE HORSESHOE-CRAB, TACHYPLEUS-TRIDENTATUS

Amplexus and mating behavior in the horseshoe crab, Tachypleus trident atus, were studied at Imari Bay and Kitsuki Bay, Kyushu, Japan, during Summer, 1994. The pairwise size distribution of mated pairs (n = 28) showed a lack of size-assortative mating. Long-term amplexus is primar ily maintained by the male's pair of posterior claspers, which is sign ificantly larger than the anterior claspers. The posterior claspers al ways attach directly to the female's opisthosoma, just lateral to the terminal spines, but the anterior claspers attach in various anteriorw ard positions on the lateral edges of the female's opisthosoma. We con clude that the mating system of T: tridentatus is fundamentally simila r to the American horseshoe crab (Limulus polyphemus), despite the >10 0 million years of isolation between the two groups. T.tridentatus mor phologies, however, show more adaptations to long-term amplexus than t hose of L. polyphemus.