Tachypleus gigas

Tachypleus gigas
Conservation status
Data Deficient (IUCN 2.3)
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Merostomata
Order:	Xiphosura
Family:	Limulidae
Genus:	Tachypleus
Species:	T. gigas
Binomial name
Tachypleus gigas (Müller, 1785)
Synonyms [1]
Limulus gigas Müller, 1785 Limulus moluccanus Latreille, 1802

Tachypleus gigas is one of the four extant species of horseshoe crab. It is found in shallow water in South East Asia at depths of up to 40 m (130 ft). It grows up to 39 cm (15 in) long and 17.5 cm (6.9 in) wide, and is covered by a sturdy carapace, with a long caudal spine (tail).

Description

Tachypleus gigas has a "sage green" chitinous exoskeleton.^[2] In a sample of five individuals, Lazarus et al. recorded maximum and minimum total lengths of 39 centimetres (15 in) and 27 cm (11 in), respectively.^[1] The prosoma is the widest part of the animal, at 14–17.5 cm (5.5–6.9 in) and 8.2–11.5 cm (3.2–4.5 in) long, while the opisthosoma is 9–11.5 cm (3.5–4.5 in) wide and 6–7.5 cm (2.4–3.0 in) long.^[1] The caudal spine (the long, pointed part of the tail) is 13.5–19 cm (5.3–7.5 in) long, bears a crest dorsally, and is concave ventrally.^[1] The anterior part of the tail is sharply serrated.^[3] Males are smaller than females, have a paler and rougher carapace, and act as hosts to a greater number of epibionts.^[2]

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.^[4]

On the underside of the animal, T. gigas has seven pairs of legs, five of which bear small claws, and five pairs of book gills, which are used for gas exchange.^[5]

Life cycle

The life cycle of T. gigas is relatively long and involves a large number of instars. The eggs are approximately 3.7 millimetres (0.15 in) in diameter.^[6] The freshly hatched larvae, known as trilobite larvae, have no tail, and are 8 millimetres (0.31 in) long.^[7] Males are thought to pass through 12 moults before reaching sexual maturity, while females pass through 13 moults.^[8]

Distribution

Tachypleus gigas has the broadest latitudinal distribution of the three Asian horseshoe crab species, ranging across the western Pacific Ocean from Japan to the Philippines, extending into the Indian Ocean only on the west coast of Sumatra.^[9]

Ecology and conservation

T. gigas inhabits sandy and muddy shores^[3] at depths up to 40 metres (130 ft),^[1] and is the only horseshoe crab to have been observed swimming at the surface of the ocean.^[10] Its diet is chiefly composed of molluscs, detritus and polychaetes, which it seeks on the ocean floor.^[11] House Crows have been observed to turn T. gigas over and eat the soft underside, while gulls only attack individuals that are already stranded upside-down.^[12]

Since horseshoe crabs do not moult after they have reached sexual maturity, they are often colonised by epibionts.^[2] The dominant diatoms are species of the genera Navicula, Nitzschia and Skeletonema.^[2] Among the larger organisms, the sea anemone Metridium, the bryozoan Membranipora, the barnacle Balanus amphitrite and the bivalves Anomia and Crassostrea are the most frequent colonists of T. gigas.^[2] Rarer epibionts include green algae, flatworms, tunicates, isopods, amphipods, gastropods, mussels, pelecypods, annelids and polychaetes.^[2]

T. gigas is listed as Data Deficient on the IUCN Red List.^[13]

Taxonomy

External identifiers for Tachypleus gigas
EOL	393280
NCBI	6852
WoRMS	238271

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.^[1]

T. gigas is estimated to have diverged from the other Asian species of horseshoe crab 52.5 million years ago.^[14] 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.^[15] T. gigas has a chromosome number of 2n = 28, compared to 26 in T. tridentatus, 32 in Carcinoscorpius and 52 in Limulus.^[16]

Common names for the species include Indo-Pacific horseshoe crab,^[12] Indonesian horseshoe crab,^[12] Indian horseshoe crab^[17] and Chinese horseshoe crab.^[18]

See also

Arthropods portal

• Tachypleus tridentatus
• Carcinoscorpius rotundicauda
• Limulus polyphemus

References

1. ^ ^{a b c d e f} S. Lazarus, V. Narayana Pillai, P. Devadoss & G. Mohanraj (1990). "Occurrence of king crab, Tachypleus gigas (Muller), off the northeast coast of India" (PDF). Proceedings of the first workshop on scientific results of FORV Sagar Sampada, 5–7 June 1989, Kochi: 393–395. http://eprints.cmfri.org.in/5222/1/51_Occurrence_of_King_crab_Tachypleus_gigas_%28Muller%29_off_the_northeast_coast_of_India.pdf. 
2. ^ ^{a b c d e f} J. S. Patil & A. C Anil (2000). "Epibiotic community of the horseshoe crab Tachypleus gigas". Marine Biology 136 (4): 699–713. doi:10.1007/s002270050730. http://drs.nio.org/drs/handle/2264/1623. 
3. ^ ^{a b} P. Gopalakrishnakone (1990). "Class Merostomata". A Colour Guide to Dangerous Animals. NUS Press. pp. 114–115. ISBN 9789971691509. http://books.google.co.uk/books?id=HE5j0_P4DXcC&pg=PA114. 
4. ^ Liza Carruthers. "Horseshoe crab". The Internet Encyclopedia of Science. http://www.daviddarling.info/encyclopedia/H/horseshoe_crab.html. Retrieved January 22, 2011. 
5. ^ "COAST / Horseshoe crabs" (PDF). Project Oceanography. University of South Florida. 2001. pp. 81–91. http://www.marine.usf.edu/pjocean/packets/f01/f01u5p3.pdf. 
6. ^ Koichi Sekiguchi and Hiroaki Sugita (1980). "Systematics and hybridization in the four living species of horseshoe crabs". Evolution 34 (4): 712–718. http://www.jstor.org/stable/2408025. 
7. ^ J. K. Mishra (2009). "Larval culture of Tachypleus gigas and its molting behavior under laboratory conditions". In John T. Tanacredi, Mark L. Botton & David R. Smith. Biology and Conservation of Horseshoe Crabs. Springer. pp. 513–519. doi:10.1007/978-0-387-89959-6_32. ISBN 978-0-387-89959-6. 
8. ^ Koichi Sekiguchi, Hidehiro Seshimo & Hiroaki Sugita (1988). "Post-embryonic development of the horseshoe crab". Biological Bulletin 174 (3): 337–345. http://www.biolbull.org/cgi/content/abstract/174/3/337. 
9. ^ Lyall I. Anderson & Carl N. Schuster Jr. (2003). "Throughout geologic time: where have they lived?". In Carl N. Shuster Jr., Robert B. Barlow & H. Jane Brockmann. The American Horseshoe Crab. Harvard University Press. pp. 189–223. ISBN 9780674011595. http://books.google.co.uk/books?id=0OSAKny-6M4C&pg=PA203. 
10. ^ Carl N. Schuster Jr. & Lyall I. Anderson (2003). "A history of skeletal structure: clues to relationships among species". In Carl N. Shuster Jr., Robert B. Barlow & H. Jane Brockmann. The American Horseshoe Crab. Harvard University Press. pp. 154–188. ISBN 9780674011595. http://books.google.co.uk/books?id=0OSAKny-6M4C&pg=PA164. 
11. ^ Anil Chatterji, J. K. Mishra & A. H. Parulekar (1992). "Feeding behaviour and food selection in the horseshoe crab, Tachypleus gigas (Müller)". Hydrobiologia 246 (1): 41–48. doi:10.1007/BF00005621. 
12. ^ ^{a b c} Mark L. Bolton and Carl N. Schuster Jr. with John A. Keinath (2003). "Horseshoe crabs in a food web: who eats whom?". In Carl N. Shuster Jr., Robert B. Barlow & H. Jane Brockmann. The American Horseshoe Crab. Harvard University Press. pp. 133–153. ISBN 9780674011595. http://books.google.co.uk/books?id=0OSAKny-6M4C&pg=PA138. 
13. ^ "Tachypleus gigas". IUCN Red List of Threatened Species. Version 2.3. International Union for Conservation of Nature. 1996. http://www.iucnredlist.org/apps/redlist/details/21308. Retrieved January 18, 2011. 
14. ^ Shun-ichiro Kawabata, Tsukasa Osaki & Sadaaki Iwanaga (2003). "Innate immunity in the horseshoe crab". In R. Alan B. Ezekowitz & Jules Hoffmann. Innate Immunity. Humana Press. pp. 109–125. ISBN 9781588290465. http://books.google.co.uk/books?id=Xo508yZcnacC&pg=PA109. 
15. ^ Xuhua Xia (2000). "Phylogenetic relationship among horseshoe crab species: effect of substitution models on phylogenetic analyses". Systematic Biology 49 (1): 87–100. doi:10.1080/10635150050207401. PMID 12116485. http://www.jstor.org/stable/2585308. 
16. ^ Carl N. Schuster Jr. & Koichi Sekiguchi (2003). "Growing up takes about ten years and eighteen stages". In Carl N. Shuster Jr., Robert B. Barlow & H. Jane Brockmann. The American Horseshoe Crab. Harvard University Press. pp. 103–132. ISBN 9780674011595. http://books.google.co.uk/books?id=0OSAKny-6M4C&pg=PA113. 
17. ^ Mark L. Botton (2001). "The conservation of horseshoe crabs: what can we learn from the Japanese experience?". In John T. Tanacredi. Limulus in the Limelight: a Species 350 Million Years in the Making and in Peril?. Springer. pp. 41–52. ISBN 9780306466816. http://books.google.co.uk/books?id=CJiybSNmo7YC&pg=PA42. 
18. ^ Carl N. Schuster Jr. (2001). "Two perspectives: horseshoe crabs during 420 million years worldwide, and the past 150 years in Delaware Bay". In John T. Tanacredi. Limulus in the Limelight: a Species 350 Million Years in the Making and in Peril?. Springer. pp. 17–40. ISBN 9780306466816. http://books.google.co.uk/books?id=CJiybSNmo7YC&pg=PA25.