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 fossils”, in the world. Three of four species of horseshoe crabs (Tachypleus tridentatus, T. gigas, and Carcinoscorpius rotundicauda) are distributed along coastal Southeast Asia to Japan (Sekiguchi 1988). Almost all Asian populations of these species are considered to be 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)

The shapes of the prosoma (fore-carapace) are quite different between males and females, and this male character is specific to this species. Males have 6 pairs of marginal spines on both sides of the back carapace (opisthosoma), and females only have 3 pairs of them because males should mount here during their breeding season. Morphology of juveniles is similar to that of females, but they have 6 pairs 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 (tail). Both sexes have chelate claspers, but male's one more thick than females.

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 (genital operculum) probably help our judgement.

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

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

(Photo@Pasuruan, East Java, Indonesia)

Google Sites

Report abuse