''Tachypleus tridentatus'', commonly known as the Chinese horseshoe crab, Japanese horseshoe crab, or tri-spine horseshoe crab, is a species of

horseshoe crab Horseshoe crabs are arthropods of the family Limulidae and the only surviving xiphosurans. Despite their name, they are not true crabs or even crustaceans; they are chelicerates, more closely related to arachnids like spiders, ticks, and scor ...

found in Southeast and East Asia, with records from China, Indonesia, Japan, South Korea, Malaysia, the Philippines, Taiwan, and Vietnam. It is found in coastal marine 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 estuari ...

waters, and tolerates colder temperatures than the other Asian horseshoe crabs (''

Tachypleus gigas ''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 S ...

'' and ''

Carcinoscorpius rotundicauda The mangrove horseshoe crab (''Carcinoscorpius rotundicauda''), also known as the round-tailed horseshoe crab, is a species of horseshoe crab, a chelicerate arthropod found in tropical marine and brackish waters of India, Bangladesh, and Southeast ...

''), although juveniles still need water warmer than to

moult In biology, moulting (British English), or molting (American English), also known as sloughing, shedding, or in many invertebrates, ecdysis, is a process by which an animal casts off parts of its body to serve some beneficial purpose, either at ...

Description

Horseshoe crabs are not crabs at all, but are most closely related to

spider Spiders (order (biology), order Araneae) are air-breathing arthropods that have eight limbs, chelicerae with fangs generally able to inject venom, and spinnerets that extrude spider silk, silk. They are the largest order of arachnids and ran ...

s and

scorpion Scorpions are predatory arachnids of the Order (biology), order Scorpiones. They have eight legs and are easily recognized by a pair of Chela (organ), grasping pincers and a narrow, segmented tail, often carried in a characteristic forward cur ...

s, and may even be arachnids themselves. The

cephalothorax 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 ...

is protected by this single large, horseshoe-shaped plate, and neither it nor the abdomen is visibly segmented. The tail bears a long spike, known as the

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

. Like other horseshoe crabs, the

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 ...

of ''T. tridentatus'' 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 ...

). 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 structured presentation of recorded information, primarily verbal and graphical, through a medium. Originally physical, electronic books and audiobooks are now existent. Physical books are objects that contain printed material, mo ...

s are located on the underside of the opisthosoma. Both the common name tri-spine horseshoe crab and the scientific name ''tridentatus'' refer to the three small spiny processes on the rear part of the opisthosoma (one spine in the middle above the tail and one on either side), while other species only have a single spine (in the middle). Limule(dD).jpg , Limule(dD)Ventral.jpg, LimuleFace.jpg , The tri-spine horseshoe crab is the largest of the living horseshoe crab species. Like the other species, females grow larger than males. The largest females of the tri-spine horseshoe crab can be as much as long, including their tail. On average in

Sabah Sabah () is a States and federal territories of Malaysia, state of Malaysia located in northern Borneo, in the region of East Malaysia. Sabah has land borders with the Malaysian state of Sarawak to the southwest and Indonesia's North Kalima ...

, 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 significant geographic variations in the size, but this does not follow a clear north–south or east–west pattern. The largest are from the

Kota Kinabalu Kota Kinabalu (; formerly known as Jesselton), colloquially referred to as KK, is the state capital of Sabah, Malaysia. It is also the capital of the Kota Kinabalu District as well as the West Coast Division of Sabah. The city is located on the ...

region in Malaysia where the average carapace width is about and in females and males respectively. The smallest are from

Zhoushan Zhoushan is an urbanized archipelago with the administrative status of a prefecture-level city in the eastern Chinese province of Zhejiang. It consists of an archipelago of islands at the southern mouth of Hangzhou Bay off the mainland c ...

region in China where the average carapace width is about and in females and males respectively. Intermediate average sizes, ranging from in females and in males, have been reported from Imari in Japan,

Xiamen Xiamen,), also known as Amoy ( ; from the Zhangzhou Hokkien pronunciation, zh, c=, s=, t=, p=, poj=Ē͘-mûi, historically romanized as Amoy, is a sub-provincial city in southeastern Fujian, People's Republic of China, beside the Taiwan Stra ...

in China, the Philippines, and

Manado Manado (, ) is the capital City status in Indonesia, city of the Indonesian Provinces of Indonesia, province of North Sulawesi. It is the second largest city in Sulawesi after Makassar, with the 2020 census giving a population of 451,916,Badan ...

Tarakan Tarakan is an island and co-extensively the sole city within the Indonesian province of North Kalimantan. The island city is the largest urban area in North Kalimantan population-wise and is located in northern Borneo, midway along the coast of ...

Padang Padang () is the Capital city, capital and largest city of the Indonesian Provinces of Indonesia, province of West Sumatra. It had a population of 833,562 at the 2010 CensusBiro Pusat Statistik, Jakarta, 2011. and 909,040 at the 2020 Census;Bad ...

and

Sibolga Sibolga (formerly sometimes Siboga) is a city and a port located in the natural harbour of Sibolga Bay on the west coast of North Sumatra province, in Indonesia. The city is located on the western side of North Sumatra facing the Indian Ocean, ...

in Indonesia. Females typically reach maturity at a carapace width of about and males at . Females weigh and males . In addition to the difference in size, males' two front pairs of walking legs, prosomal appendages two and three, have hooks (they are scissor-like in females) and they have six (three in females) long spines on either side of the rear carapace. Juveniles (both sexes) also have six long spines on either side, similar to adult males. From the eggs hatch to adulthood takes 4 years and involves 15

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

phases (14

s).

Ecology

Like other species of horseshoe crabs, ''T. tridentatus'' is an omnivore and feeds on

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

worms The World Register of Marine Species (WoRMS) is a taxonomic database that aims to provide an authoritative and comprehensive catalogue and list of names of marine organisms. Content The content of the registry is edited and maintained by scien ...

, other

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from the Ancient Greek word (), meaning "the depths". ...

invertebrates and

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

. Large batches of eggs are laid in holes dug in sandy beaches in special nursery areas off the coast. On hatching, the larvae remain in the nest over winter, feeding on the yolks of their eggs for several months, remaining in this nursery area during the next spring and summer. As juveniles they remain buried in the sediment during high tide, emerging at low tide to feed on the exposed surface. They have limited opportunities to disperse widely. Adults move offshore for the winter, hibernating buried in the seabed at depths of around , coming inshore again when the water warms up the following year. As a

poikilotherm A poikilotherm () is an animal (Greek ''poikilos'' – 'various', 'spotted', and ''therme'' – 'heat') whose internal temperature varies considerably. Poikilotherms have to survive and adapt to environmental stress. One of the most important s ...

, this horseshoe crab is much affected by rising seawater temperatures, reacting by burying itself deeply in the sediment and sometimes going into

diapause In animal dormancy, diapause is the delay in development in response to regular and recurring periods of adverse environmental conditions.Tauber, M.J., Tauber, C.A., Masaki, S. (1986) ''Seasonal Adaptations of Insects''. Oxford University Press It ...

Status

The tri-spine horseshoe crab is at risk from over-fishing, pollution and the loss of their breeding grounds. Populations have declined for decades, and the species was granted protection in Japan in 1928. In China it has become less common and, from being once abundant in Taiwan, it is now seldom seen inshore there. As of 2019, the

International Union for Conservation of Nature The International Union for Conservation of Nature (IUCN) is an international organization working in the field of nature conservation and sustainable use of natural resources. Founded in 1948, IUCN has become the global authority on the stat ...

classifies the species as "

Endangered An endangered species is a species that is very likely to become extinct in the near future, either worldwide or in a particular political jurisdiction. Endangered species may be at risk due to factors such as habitat loss, poaching, inv ...

" based on recent population and habitat declines.

References

External links

* {{Taxonbar, from=Q1328469 Xiphosura Animals described in 1819 Arthropods of China Arthropods of Indonesia Chelicerates of Japan Invertebrates of Malaysia Arthropods of the Philippines Invertebrates of Korea Arthropods of Taiwan Arthropods of Vietnam Taxonomy articles created by Polbot Endangered biota of Asia Taxa named by William Elford Leach