The horseshoe crab is a strange, ancient creature that has been around since
before the dinosaurs. It is not a crab or even a crustacean. It is more closely related to
spiders and scorpions of the Class Arachnida.
There are only four species of living horseshoes crabs in the world .Limulus polyphemus is found on the Atlantic coast. In Southeast Asia there are three: the Mangrove horseshoe crab (Carcinoscorpius rotundicauda) which is also the smallest horseshoe, and the Coastal horseshoe crab (Tachypleus gigas) and in Japan, China and southern Sabah are found the Chinese horseshoe (Tachypleus tridentatus).
Tachypleus gigas, commonly known as the Indo-Pacific horseshoe crab, Indonesian horseshoe crab, Indian horseshoe crab, or southern horseshoe crab, is one of the four extant (living) species of horseshoe crab. It is found in coastal water in South and Southeast Asia at depths to 40 m (130 ft). It grows up to about 50 cm (20 in) long, including the tail, and is covered by a sturdy carapace that is up to about 26.5 cm (10.4 in) wide.