''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 Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

(living) species of

horseshoe crab Horseshoe crabs are marine and brackish water arthropods of the family Limulidae and the only living members of the order Xiphosura. Despite their name, they are not true crabs or crustaceans: they are chelicerates, most closely related to ar ...

. It is found in coastal water in

South South is one of the cardinal directions or compass points. The direction is the opposite of north and is perpendicular to both east and west. Etymology The word ''south'' comes from Old English ''sūþ'', from earlier Proto-Germanic ''*sunþa ...

and

Southeast Asia Southeast Asia, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical south-eastern region of Asia, consisting of the regions that are situated south of mainland ...

at depths to . It grows up to about long, including the tail, and is covered by a sturdy

carapace A carapace is a dorsal (upper) section of the exoskeleton or shell in a number of animal groups, including arthropods, such as crustaceans and arachnids, as well as vertebrates, such as turtles and tortoises. In turtles and tortoises, the unde ...

that is up to about wide.

Description

''Tachypleus gigas'' has a sage-green

chitin Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...

ous

exoskeleton An exoskeleton (from Greek ''éxō'' "outer" and ''skeletós'' "skeleton") is an external skeleton that supports and protects an animal's body, in contrast to an internal skeleton ( endoskeleton) in for example, a human. In usage, some of the ...

. Like other horseshoe crabs, the

of ''T. gigas'' consists of a larger frontal one (the

prosoma The cephalothorax, also called prosoma in some groups, is a tagma of various arthropods, comprising the head and the thorax fused together, as distinct from the abdomen behind. (The terms ''prosoma'' and ''opisthosoma'' are equivalent to ''cepha ...

) and a smaller, spine-edged rear one (the

opisthosoma The opisthosoma is the posterior part of the body in some arthropods, behind the prosoma (cephalothorax). It is a distinctive feature of the subphylum Chelicerata (arachnids, horseshoe crabs and others). Although it is similar in most respects to ...

). 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 gill A book is a medium for recording information in the form of writing or images, typically composed of many pages (made of papyrus, parchment, vellum, or paper) bound together and protected by a cover. The technical term for this physic ...

s are located on the underside of the opisthosoma. They have a long spiny tail known as the

telson The telson () is the posterior-most division of the body of an arthropod. Depending on the definition, the telson is either considered to be the final segment of the arthropod body, or an additional division that is not a true segment on acco ...

. 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 ''Tachypleus tridentatus'', commonly known as the Chinese horseshoe crab, Japanese horseshoe crab, or tri-spine horseshoe crab, is a species of horseshoe crab found in Southeast and East Asia, with records from China, Indonesia, Japan, South Kor ...

'' reaches a larger size. Both are considerably larger than ''

Carcinoscorpius rotundicauda The mangrove horseshoe crab (''Carcinoscorpius rotundicauda''), also known as the round-tailed horseshoe crab, is a chelicerate arthropod found in tropical marine and brackish waters in India, Bangladesh, Indonesia, Malaysia, Brunei, Singapore, T ...

''. Like the other species, females of ''T. gigas'' grow larger than males. On average in

Sarawak Sarawak (; ) is a state of Malaysia. The largest among the 13 states, with an area almost equal to that of Peninsular Malaysia, Sarawak is located in northwest Borneo Island, and is bordered by the Malaysian state of Sabah to the northeast, ...

, Malaysia, females are about long, including a tail that is about , and their

(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 West Bengal (, Bengali: ''Poshchim Bongo'', , abbr. WB) is a state in the eastern portion of India. It is situated along the Bay of Bengal, along with a population of over 91 million inhabitants within an area of . West Bengal is the fou ...

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 . In addition to their smaller size, males have a paler and rougher carapace, act as hosts to a greater number of

epibiont An epibiont (from the Ancient Greek meaning "living on top of") is an organism that lives on the surface of another living organism, called the basibiont ("living underneath"). The interaction between the two organisms is called epibiosis. An ep ...

s, 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 A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...

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.

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 The Bay of Bengal is the northeastern part of the Indian Ocean, bounded on the west and northwest by India, on the north by Bangladesh, and on the east by Myanmar and the Andaman and Nicobar Islands of India. Its southern limit is a line bet ...

to the

South China Sea The South China Sea is a marginal sea of the Western Pacific Ocean. It is bounded in the north by the shores of South China (hence the name), in the west by the Indochinese Peninsula, in the east by the islands of Taiwan and northwestern Ph ...

, with records from India, Malaysia, Singapore, Indonesia, Thailand, Vietnam and the Philippines. Although records are lacking, it likely also occurs in Myanmar. ''Tachypleus gigas'' inhabits sandy and muddy shores at depths to , and is the only horseshoe crab to have been observed swimming at the surface of the ocean. It occurs in both

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military ...

and

brackish Brackish water, sometimes termed brack water, is water occurring in a natural environment that has more salinity than freshwater, but not as much as seawater. It may result from mixing seawater (salt water) and fresh water together, as in estua ...

waters in

salinities Salinity () is the saltiness or amount of salt dissolved in a body of water, called saline water (see also soil salinity). It is usually measured in g/L or g/kg (grams of salt per liter/kilogram of water; the latter is dimensionless and equal ...

down to 15 PSU, but their eggs only hatch above 20 PSU.

Behavior, ecology and conservation

The

lifecycle Life cycle, life-cycle, or lifecycle may refer to: Science and academia *Biological life cycle, the sequence of life stages that an organism undergoes from birth to reproduction ending with the production of the offspring *Life-cycle hypothesis, ...

of ''T. gigas'' is relatively long and involves a large number of

instar An instar (, from the Latin '' īnstar'', "form", "likeness") is a developmental stage of arthropods, such as insects, between each moult (''ecdysis''), until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or ...

s. The

eggs Humans and human ancestors have scavenged and eaten animal eggs for millions of years. Humans in Southeast Asia had domesticated chickens and harvested their eggs for food by 1,500 BCE. The most widely consumed eggs are those of fowl, especial ...

are about in diameter. The freshly hatched

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. ...

e, known as trilobite larvae, have no tail, and are long. Males are thought to pass through 12 moults before reaching

sexual maturity Sexual maturity is the capability of an organism to reproduce. In humans it might be considered synonymous with adulthood, but here puberty is the name for the process of biological sexual maturation, while adulthood is based on cultural definit ...

, while females pass through 13 moults. The diet of ''T. gigas'' is chiefly composed of

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts comm ...

, and

polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...

s, which it seeks on the ocean floor.

House crow The house crow (''Corvus splendens''), also known as the Indian, greynecked, Ceylon or Colombo crow, is a common bird of the crow family that is of Asian origin but now found in many parts of the world, where they arrived assisted by shipping. ...

s have been observed to turn ''T. gigas'' over and eat the soft underside, while

gull Gulls, or colloquially seagulls, are seabirds of the family Laridae in the suborder Lari. They are most closely related to the terns and skimmers and only distantly related to auks, and even more distantly to waders. Until the 21st century ...

s only attack individuals that are already stranded upside-down. Since horseshoe crabs do not moult after they have reached sexual maturity, they are often colonised by

s. The dominant

diatom A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...

s are species of the genera ''

Navicula ''Navicula'' is a genus of boat-shaped diatom algae, comprising over 1,200 species. ''Navicula'' is Latin for "small ship", and also a term in English for a boat-shaped incense-holder. Diatoms — eukaryotic, primarily aquatic, single-celle ...

'', '' Nitzschia'', and '' Skeletonema''. Among the larger organisms, the

sea anemone Sea anemones are a group of predatory marine invertebrates of the order Actiniaria. Because of their colourful appearance, they are named after the '' Anemone'', a terrestrial flowering plant. Sea anemones are classified in the phylum Cnidaria, ...

Metridium Members of the genus ''Metridium'', also known as plumose anemones, are sea anemones found mostly in the cooler waters of the northern Pacific and Atlantic oceans. They are characterized by their numerous threadlike tentacle In zoology, a ...

'', the

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

n '' Membranipora'', the

barnacle A barnacle is a type of arthropod constituting the subclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosiv ...

'' Balanus amphitrite'', and the

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, bival ...

'' Anomia'' and ''

Crassostrea ''Crassostrea'' is a genus of true oysters (family Ostreidae) containing some of the most important oysters used for food. Some species in the genus have been moved to the genus '' Magallana''. Species Extant species Extant species include:< ...

'' are the most frequent colonists of ''T. gigas''. Rarer epibionts include

green algae The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/ Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga ...

flatworm The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegmen ...

tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one t ...

isopods Isopoda is an order of crustaceans that includes woodlice and their relatives. Isopods live in the sea, in fresh water, or on land. All have rigid, segmented exoskeletons, two pairs of antennae, seven pairs of jointed limbs on the thorax, and ...

amphipods Amphipoda is an order of malacostracan crustaceans with no carapace and generally with laterally compressed bodies. Amphipods range in size from and are mostly detritivores or scavengers. There are more than 9,900 amphipod species so far describ ...

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which ...

pelecypod Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biva ...

annelid The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecol ...

s, and

s. ''Tachypleus gigas'' is listed as

Data Deficient A data deficient (DD) species is one which has been categorized by the International Union for Conservation of Nature (IUCN) as offering insufficient information for a proper assessment of conservation status to be made. This does not necessaril ...

on the

IUCN Red List The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biolo ...

Taxonomy

''Tachypleus gigas'' was first described by

Otto Friedrich Müller Otto Friedrich Müller, also known as Otto Friedrich Mueller (2 November 1730 – 26 December 1784) was a Danish naturalist and scientific illustrator. Biography Müller was born in Copenhagen. He was educated for the church, became tutor to a ...

in 1785. It was originally placed in the genus ''Limulus'', but was transferred to the genus ''

Tachypleus ''Tachypleus'' is a genus of south, southeast and east Asian horseshoe crabs in the family Limulidae. Species There are two extant (living) species: * '' Tachypleus gigas'' ( Müller, 1785) * '' Tachypleus tridentatus'' ( Leach, 1819) And two ...

'' by

Reginald Innes Pocock Reginald Innes Pocock Fellow of the Royal Society, F.R.S. (4 March 1863 – 9 August 1947) was a British zoologist. Pocock was born in Clifton, Bristol, the fourth son of Rev. Nicholas Pocock (historian), Nicholas Pocock and Edith Prichard. He ...

in 1902. ''Tachypleus gigas'' is estimated to have diverged 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 Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectivel ...

of 2''n'' = 28, compared to 26 in ''T. tridentatus'', 32 in ''Carcinoscorpius'', and 52 in ''Limulus''.

References

{{Taxonbar, from=Q1313858 Xiphosura Animals described in 1785