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Tachypleus gigas, commonly known as the Indo-Pacific horseshoe crab, Indonesian horseshoe crab, (2026). 9780674011595, Harvard University Press. ISBN 9780674011595 Indian horseshoe crab, or southern horseshoe crab, is one of the four Extant taxon (living) species of horseshoe crab. It is found in coastal water in South Asia and Southeast Asia at depths to .
Tachypleus gigas has a sage-green 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 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.
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.
In addition to their smaller size, males have a paler and rougher carapace, act as hosts to a greater number of , 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.
Tachypleus gigas inhabits seagrass meadows, sandy and muddy shores at depths to ; it is the only horseshoe crab to have been observed swimming at the surface of the ocean.
Since horseshoe crabs do not moult after they have reached sexual maturity, they are often colonised by . The dominant are species of the genera Navicula, Nitzschia, and Skeletonema. Among the larger organisms, the sea anemone Metridium, the Membranipora, the barnacle Balanus amphitrite, and the Bivalvia Anomia and Crassostrea are the most frequent colonists of T. gigas. Rarer epibionts include green algae, , , Isopoda, Amphipoda, , , , , and .
Tachypleus gigas is estimated to have speciation from the other Asian species of horseshoe crab . While it is clear that the American horseshoe crab Limulus polyphemus is distinct from the remaining extant species of horseshoe crab, relationships within the Asian horseshoe crabs remains uncertain. T. gigas has a chromosome number of 2 n = 28, compared to 26 in T. tridentatus, 32 in Carcinoscorpius, and 52 in Limulus.
Sexual difference
Like the other species, females of T. gigas grow larger than males. On average in Sarawak, Malaysia, females are about long, including a tail that is about , and their carapace (prosoma) is about wide. In comparison, the average for males is about long, including a tail that is about , and their carapace is about 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 and in females and males respectively. The largest females of the species reach a total length of more than and can weigh more than .
Distribution and habitat
Tachypleus gigas is one of three living species of horseshoe crabs in Asia, the others being Tachypleus tridentatus and Carcinoscorpius rotundicauda. The fourth living species, Limulus polyphemus, is found in the Americas. T. gigas is found in tropical South and Southeast Asia, ranging from the Bay of Bengal to the South China Sea, with records from India, Malaysia, Singapore, Indonesia, Thailand, Vietnam and the Philippines. Although records are lacking, it likely also occurs in Myanmar.
(2026). 9780674011595, Harvard University Press. ISBN 9780674011595 It occurs in both Marine life and brackish waters in salinities down to 15 PSU, but their eggs only hatch above 20 PSU.
Breeding
The lifecycle of T. gigas is relatively long and involves a large number of . The eggs are about in diameter. The freshly hatched , known as trilobite larvae, have no tail, and are long. Males are thought to pass through 12 ecdysis before reaching sexual maturity, while females pass through 13 moults.
Ecology
The diet of T. gigas is chiefly composed of Mollusca, detritus, and , which it seeks on the ocean floor. have been observed to turn T. gigas over and eat the soft underside, while only attack individuals that are already stranded upside-down.
Conservation
Tachypleus gigas is listed as Data Deficient on the IUCN Red List.
Taxonomy
Tachypleus gigas was first described by Otto Friedrich Müller in 1785. It was originally placed in the genus Limulus, but was transferred to the genus Tachypleus by Reginald Innes Pocock in 1902.
(2026). 9780674011595, Harvard University Press. ISBN 9780674011595
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