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).
(Photo@Pasuruan, East Java, Indonesia)
Females have chelate claspers (like a scissors), but the first and second walking leg of males are hemichelate claspers (like a hook). Males use these hooks to catch females during breeding.
Long spine on the 5th walking leg in T. gigas (Please note, T. tridentatus also has this spine).
Carcinoscorpius rotundicauda (Round-tailed or Mangrove horseshoe crab)
Male (left) and female (right) of Carcinoscorpius rotundicauda. The telson (i.e. tale) of this species has a round shape.
(Photo@Pasuruan, East Java, Indonesia).
Sexual dimorphism of legs for Carcinoscorpius rotundicauda; male (left) and female (right)
Both individuals have a chelate clasper but the male's clasper (left) is more thick than the female's (right).
(Photo@Pasuruan, East Java, Indonesia)
Identification of sex for Carcinoscorpius rotundicauda.
Although it is difficult to identify a female or juvenile of this species, the shape and condition of the genital pole may help in your judgement.
Size and color differences and variations of adult individuals between T. gigas and C. rotundicauda.
T. tridentatus grow similar or larger in size than T. gigas.
(Photo@Pasuruan, East Java, Indonesia)
(C) Shin Nishida 2012