This key is kindly provided by Dr. Shin Nishida, Department of Biodiversity Sciences, Graduate School of Social and Cultural Studies, Kyushu University, Japan.
Tachypleus tridentatus (Tri-spined horseshoe crab)
(Photo@Hakata Bay, Fukuoka, Japan)
The shapes of the prosoma (fore-carapace) are quite different between male and female, and this male character is specific in this species. Males have 6 pairs of marginal spines at both side of back carapace, while female only have 3 pairs of them. This is because males mount here during breeding. The morphology of juveniles is similar in females, but with 6 pairs of marginal spines in both T. tridentatus and T. gigas.
Front view of male (left) and female (right) of T. tridentatus.
(Photo@Morie Bay, Oita, Japan)
Why is it it named T. tridentatus (tri-spined)?
(Photo@Balikpapan, East Kalimantan, Indonesia)
Sexual dimorphism of legs for T. tridentatus.
Females have chelate claspers (like a scissors), but the first and second walking legs of males are hemichelate clasper (like a hook). Males use these hooks to catch females during breeding.
Tachypleus gigas (Southern horseshoe crab)
Male (left) and female (right) of Tachypleus gigas.
(Photo@Pasuruan, East Java, Indonesia)
Sexual dimorphism of legs for Tachypleus gigas; male (left) and female (right).