Horseshoe crabs in Asia

-species identification-

Laurie, K., Chen, C.-P., Cheung, S.G., Do, V., Hsieh, H., John, A., Mohamad, F., Seino, S., Nishida, S., Shin, P. & Yang, M. 2019. Tachypleus tridentatus. The IUCN Red List of Threatened Species 2019: e.T21309A133299524.

There are only four species of horseshoe crabs, known as “a living fossil”, in the world. Three of four species of horseshoe crabs (Tachypleus tridentatus, T. gigas and Carcinoscorpius rotundicauda) are distributed coastal Southeast Asia to Japan (Sekiguchi 1988). The almost of all Asian populations of these species are considered that they are facing extinction due to loss of tidal flats and spawning beaches, and the deterioration of coastal waters (Sekiguchi 1988; Itow 1993), and are in urgent need of conservation measures and protection.

Distribution of horseshoe crabs in Asia (modified from Sekiguchi 1999).

What are differences among them?

(Identification Keys of these species)

Tachypleus tridentatus (tri-spine horseshoe crab, Tt )

(Photo@Hakata Bay, Fukuoka, Japan)

Front view of male (left) and female (right) of Tt. (Photo@Morie Bay, Oita, Japan)

Shapes of prosoma (fore-carapace) are quite different between male and female, and this male character is specific in this species. Male have 6 pairs of marginal spines at both side of back carapace, and female only have 3 pairs of them because male should mount here during their breeding season. Morphology of juvenile is similar with female but they have 6 pair of marginal spines for both Tt and Tg.

Why is it tridentatus (tri-spine)?

(Photo@Balikpapan, East Kalimantan, Indonesia)

Sexual dimorphism of legs for Tt.

Female have chelate clasper (like a scissors), but first and second walking leg of male were changed hemichelate clasper (like a hook). Male use these hooks to catch female during their breeding.

Tachypleus gigas (southern horseshoe crab, Tg )

Male (left) and female (right) of Tg. (Photo@Pasuruan, East Java, Indonesia)

Sexual dimorphism of legs for Tg; male (left) and female (right).(Photo@Pasuruan, East Java, Indonesia)

Female have chelate clasper (like a scissors), but first and second walking leg of male were changed hemichelate clasper (like a hook). Male use these hooks to catch female during their breeding.

Long spine on the 5th walking leg at Tg. (Tt. also have this spine)

Carcinoscorpius rotundicauda (round-tailed/mangrove horseshoe crab, Cr )

Male (left) and female (right) of Cr. (Photo@Pasuruan, East Java, Indonesia)

Sexual dimorphisms of legs for Cr ; male (left) and female (right). (Photo@Pasuruan, East Java, Indonesia)

This species have round shape of telson (tale). Both sexes of them have chelate clasper but male's one more thick than female.

Identification of sex (in case of C. r.)

Although it is difficult to identify female or juvenile of this species, shape/condition of genital pole probably help our judgement.

Size and color differences and variations of adult individuals between Tg and Cr.

Tt become larger size (or almost same) than Tg.

(Photo@Pasuruan, East Java, Indonesia)

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