page_title: Tachypleus gigas
section_title: Description
hierarchical_section_title: Tachypleus gigas / Description
caption_attribution_description: Limulus moluccanus (Clusius) = Tachypleus gigas (Müller, 1785), adult male from above (left) / below (right)
context_page_description: 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 species of horseshoe crab. It is found in coastal water in South and Southeast Asia at depths to 40 m. It grows up to about 50 cm long, including the tail, and is covered by a sturdy carapace that is up to about 26.5 cm wide.
context_section_description: T. gigas has a sage-green chitinous exoskeleton. Like other horseshoe crabs, the carapace of T. gigas consists of a larger frontal one (the prosoma) and a smaller, spine-edged rear one (the opisthosoma). There are six pairs of prosomal appendages/legs, consisting of a small frontal pair in front of the mouth and five larger walking/pushing legs on either side of the mouth. The book gills are located on the underside of the opisthosoma. They have a long spiny tail known as the telson. The tail bears a crest dorsally and is concave ventrally, giving it an essentially triangular cross section.
  Despite the scientific name T. gigas, the close relative Tachypleus tridentatus reaches a larger size. Both are considerably larger than Carcinoscorpius rotundicauda. Like the other species, females of T. gigas grow larger than males. On average in Sarawak, Malaysia, females are about 42 cm (17 in) long, including a tail that is about 20 cm (7.9 in), and their carapace (prosoma) is about 22 cm (8.7 in) wide. In comparison, the average for males is about 34 cm (13 in) long, including a tail that is about 17.5 cm (6.9 in), and their carapace is about 17.5 cm (6.9 in) wide. There are some geographic variations in the average size, but most are similar to, or somewhat smaller, than the ones from Sarawak. An outlier are individuals from West Bengal in India where the average carapace width only is about 17 cm (6.7 in) and 14 cm (5.5 in) in females and males respectively. The largest females of the species reach a total length of more than 50 cm (20 in) and can weigh more than 1.8 kg (4.0 lb). In addition to their smaller size, males have a paler and rougher carapace, act as hosts to a greater number of epibionts, have six (instead of three) long spines on either side of the rear carapace, and their two front pairs of walking legs, prosomal appendages two and three, have hooks (they are scissor-like in females). Juveniles (both sexes) also have six long spines on either side of the rear carapace, similar to adult males.
  The carapace which shields the prosoma also bears two pairs of eyes – a pair of simple eyes at the front, and a pair of compound eyes positioned laterally. In common with other horseshoe crabs, T. gigas also has ventral eyes near the mouthparts, and photoreceptors in the caudal spine.
caption: Illustrations of male from above and below. Notice that the illustration from below incorrectly shows all walking legs as scissor-like, as in females (in males, the two frontal pairs of walking legs have hooks)
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