Horseshoe crab

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Temporal range: Late Ordovician-present
Limulus polyphemus.jpg
Limulus polyphemus
Scientific classification e
Kingdom: Animalia
Phylum: Arthropoda
Subphylum: Chelicerata
Class: Merostomata
Order: Xiphosura
Suborder: Xiphosurida
Family: Limulidae
Leach, 1819 [1]

Horseshoe crabs are marine arthropods of the family Limulidae, suborder Xiphosurida, and order Xiphosura.

Horseshoe crabs live primarily in and around shallow ocean waters on soft sandy or muddy bottoms. They occasionally come onto shore to mate. They are commonly used as bait and in fertilizer. In recent years, population declines have occurred as a consequence of coastal habitat destruction in Japan and overharvesting along the east coast of North America. Tetrodotoxin may be present in the roe of species inhabiting the waters of Thailand.[2]

Because of their origin 450 million years ago, horseshoe crabs are considered living fossils.[3]


Horseshoe crabs superficially resemble crustaceans but belong to a separate subphylum of the arthropods, Chelicerata, and are closely related to arachnids.[4] Horeshoe crabs are closely related to the extinct eurypterids (sea scorpions), which include some of the largest arthropods to have ever existed, and the two may be sister groups.[4][5] (Other studies have placed eurypterids closer to the arachnids in a group called Metastomata.)[6] The earliest horseshoe crab fossils are found in strata from the late Ordovician period, roughly 450 million years ago.

The Limulidae are the only recent family of the order Xiphosura, and contains all four living species of horseshoe crabs:[1]

Anatomy and behavior[edit]

Underside of a horseshoe crab showing the legs and book gills

The entire body of the horseshoe crab is protected by a hard carapace. It has two compound lateral eyes, each composed of about 1,000 ommatidia, plus a pair of median eyes that are able to detect both visible light and ultraviolet light, a single endoparietal eye, and a pair of rudimentary lateral eyes on the top. The latter become functional just before the embryo hatches. Also, a pair of ventral eyes is located near the mouth, as well as a cluster of photoreceptors on the telson. The horseshoe crab has five additional eyes on top of its shell. Despite having relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans',[7][8] and their eyes are a million times more sensitive to light at night than during the day.[9] The mouth is located in the center of the legs, whose bases are referred to as gnathobases and have the same function as jaws and help grind up food. The horseshoe crab has five pairs of legs for walking, swimming, and moving food into the mouth, each with a claw at the tip, except for the last pair.

External video
Limulus polyphemus horseshue crab on coast.jpg
Rendezvous with a Horseshoe Crab, August 2011, 4:34, NewsWorks
The Horseshoe Crab Spawn, June 2010, 5:08,
Horseshoe Crabs Mate in Massive Beach "Orgy", June 2014, 3:29, National Geographic

Behind its legs, the horseshoe crab has book gills, which exchange respiratory gases, and are also occasionally used for swimming.[10] As in other arthropods, a true endoskeleton is absent, but the body does have an endoskeletal structure made up of cartilaginous plates that support the book gills. They are more often found on the ocean floor searching for worms and molluscs, which are their main food. They may also feed on crustaceans and even small fish.[citation needed]

Females are larger than males; C. rotundicauda is the size of a human hand, while L. polyphemus can be up to 60 cm (24 in) long (including tail). The juveniles grow about 33% larger with every molt until reaching adult size.[11]

Horseshoe crabs normally swim upside down, inclined at about 30° to the horizontal and moving at about 10-15 cm/s.[12][13][14]
Horseshoe crabs have two primary compound eyes and seven secondary simple eyes. Two of the secondary eyes are on the underside.[15][16]
Painting by Heinrich Harder, c. 1916


Horseshoe crabs mating
Horseshoe crab eggs

During the breeding season, horseshoe crabs migrate to shallow coastal waters. A male selects a female and clings to her back. Often, several males surround the female and all fertilize together, which makes it easy to spot and count females as they are the large center carapace surrounded by 3-5 smaller ones. The female digs a hole in the sand and lays her eggs while the male(s) fertilize them. The female can lay between 60,000 and 120,000 eggs in batches of a few thousand at a time. The eggs take about two weeks to hatch; shore birds eat many of them before they hatch. The larvae molt six times during the first year.[17] Raising horseshoe crabs in captivity has proven to be difficult. Some evidence indicates that mating takes place only in the presence of the sand or mud in which the horseshoe crab's eggs were hatched. It is not known with certainty what is in the sand that the crab can sense, nor how they sense it.[18]


Harvest for blood[edit]

Horseshoe crabs use hemocyanin to carry oxygen through their blood. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play a similar role to the white blood cells of vertebrates in defending the organism against pathogens. Amebocytes from the blood of L. polyphemus are used to make Limulus amebocyte lysate, which is used for the detection of bacterial endotoxins in medical applications.[19] This means there is a high demand for harvesting the blood, which involves collecting and bleeding the animals, and then releasing them back into the sea. Most of the animals survive the process; mortality is correlated with both the amount of blood extracted from an individual animal, and the stress experienced during handling and transportation.[20] Estimates of mortality rates following blood harvesting vary from 3-15%[21] to 10-30%.[22] Approximately 500,000 crabs are harvested annually.[23]

The mortality rate of bled horseshoe crabs could be as high as 29%.[24][25] Bleeding may also prevent female horseshoe crabs from being able to spawn or decrease the number of eggs they are able to lay. Up to 30% of an individual's blood is removed, according to the biomedical industry, and the horseshoe crabs spend between one and three days away from the ocean before being returned. Some scientists are skeptical that certain companies return their horseshoe crabs to the ocean at all, instead suspecting them of selling the horseshoe crabs as fishing bait.[26]


Horseshoe crabs are used as bait to fish for eels (mostly in the United States) and whelk, or conch. However, fishing with horseshoe crab is temporarily forbidden in New Jersey (moratorium on harvesting) and restricted to males in Delaware. A permanent moratorium is in effect in South Carolina.[27] The eggs are eaten in parts of Southeast Asia and China.[28]

A low horseshoe crab population in the Delaware Bay is hypothesized to endanger the future of the red knot. Red knots, long-distance migratory shorebirds, feed on the protein-rich eggs during their stopovers on the beaches of New Jersey and Delaware.[29] An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.[30]

Shoreline development[edit]

Development along shorelines is dangerous to horseshoe crab spawning, limiting available space and degrading habitat. Bulkheads can block access to intertidal spawning regions as well.[31]


  1. ^ a b Kōichi Sekiguchi (1988). Biology of Horseshoe Crabs. Science House. ISBN 978-4-915572-25-8. 
  2. ^ Attaya Kungsuwan; Yuji Nagashima; Tamao Noguchi; et al. (1987). "Tetrodotoxin in the Horseshoe Crab Carcinoscorpius rotundicauda Inhabiting Thailand" (PDF). Nippon Suisan Gakkaishi. 53 (2): 261–266. doi:10.2331/suisan.53.261. 
  3. ^ David Sadava; H. Craig Heller; David M. Hillis; May Berenbaum (2009). Life: the Science of Biology (9th ed.). W. H. Freeman. p. 683. ISBN 978-1-4292-1962-4. 
  4. ^ a b Garwood, Russell J.; Dunlop, Jason A. (13 November 2014). "Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders". PeerJ. 2: e641. doi:10.7717/peerj.641. PMC 4232842Freely accessible. PMID 25405073. Retrieved 2015-06-15. 
  5. ^ Clarke JM, Ruedemann R. The Eurypterida of New York. 
  6. ^ Weygoldt P, Paulus HF (1979). "Untersuchungen zur Morphologie, Taxonomie und Phylogenie der Chelicerata". Zeitschrift für zoologische Systematik und Evolutionsforschung. 17 (2): 85–116, 177–200. doi:10.1111/j.1439-0469.1979.tb00694.x. 
  7. ^ Anatomy: Vision – The Horseshoe Crab
  8. ^ "Horseshoe Crabs, Limulus polyphemus". 
  9. ^ Palumbi, Stephen R.; Palumbi, Anthony R. (23 February 2014). "The Extreme Life of the Sea". Princeton University Press – via Google Books. 
  10. ^ "The biology of cartilage. I. Invertebrate cartilages:Limulus gill cartilage". ResearchGate. 
  11. ^ Lesley Cartwright-Taylor; Julian Lee; Chia Chi Hsu (2009). "Population structure and breeding pattern of the mangrove horseshoe crab Carcinoscorpius rotundicauda in Singapore" (PDF). Aquatic Biology. 8 (1): 61–69. doi:10.3354/ab00206. 
  12. ^ Manton SM (1977) The Arthropoda: habits, functional morphology, and evolution Page 57, Clarendon Press.
  13. ^ Shuster CN, Barlow RB and Brockmann HJ (Eds.) (2003) The American Horseshoe Crab Pages 163–164, Harvard University Press. ISBN 9780674011595.
  14. ^ Vosatka ED (1970). "Observations on the Swimming, Righting, and Burrowing Movements of Young Horseshoe Crabs, Limulus Polyphemus" (PDF). The Ohio Journal of Science. 70 (5): 276–283. Archived (PDF) from the original on November 25, 2017. 
  15. ^ Battelle, B.A. (December 2006), "The eyes of Limulus polyphemus (Xiphosura, Chelicerata) and their afferent and efferent projections.", Arthropod Structure & Development, 35 (4): 261–74, doi:10.1016/j.asd.2006.07.002, PMID 18089075. 
  16. ^ Barlow RB (2009) "Vision in horseshoe crabs" Pages 223–235 in JT Tanacredi, ML Botton and D Smith, Biology and Conservation of Horseshoe Crabs, Springer. ISBN 9780387899589.
  17. ^ "The Rabbit and the Horse Shoe Crab". September 23, 2014. Retrieved 2016-12-20. 
  18. ^ David Funkhouser (April 15, 2011). "Crab love nest". Scientific American. 304 (4): 29. doi:10.1038/scientificamerican0411-29. 
  19. ^ "Data". / 
  20. ^ Lenka Hurton (2003). Reducing post-bleeding mortality of horseshoe crabs (Limulus polyphemus) used in the biomedical industry (PDF) (M.Sc. thesis). Virginia Polytechnic Institute and State University. 
  21. ^ "Crash: A Tale of Two Species – The Benefits of Blue Blood – Nature – PBS". 10 June 2008. 
  22. ^ The Blood Harvest The Atlantic, 2014.
  23. ^ Chesler, Caren. "The Blood of the Crab". Popular Mechanics (13 April 2017). Retrieved 16 April 2017. 
  24. ^ Carmichael, R.H.; Shin, M.L.; Cheung, S.G. (Eds.) (2015). Changing Global Perspectives on Horseshoe Crab Biology, Conservation and Management. Springer International Publishing. 
  25. ^ Chesler, Caren. "Medical Labs May Be Killing Horseshoe Crabs". Scientific American. Scientific American. Retrieved 10 May 2018. 
  26. ^ Chesler, Caren (June 9, 2016). "Medical Labs May Be Killing Horseshoe Crabs". Scientific American. Retrieved 2017-11-03. 
  27. ^ "Horseshoe crab". SC DNR species gallery. Archived from the original on March 31, 2016. Retrieved 2011-06-06. 
  28. ^ 大西一實. "Vol.56 食うか食われるか?". あくあは〜つ通信. Archived from the original on 2003-08-13. Retrieved 2008-04-18. 
  29. ^ "Red knots get to feast on horseshoe crab eggs". Environment News Service. March 26, 2008. Retrieved 2011-01-19. 
  30. ^ "Critter Class Hodge Podge (Horseshoe crabs and Wooly Bears)" (PDF). The Wildlife Center. October 26, 2011. Retrieved 2015-03-09. 
  31. ^ "Conservation". ERDG. Retrieved 2016-05-19. 

Further reading[edit]

External links[edit]