Horseshoe crab
Animal
Horseshoe crabs are marine arthropods of the family Limulidae and order Xiphosura or Xiphosurida.

Horseshoe crabs are marine arthropods of the family Limulidae and order Xiphosura or Xiphosurida, that 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, a decline in the population has 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 (Mya), horseshoe crabs are considered living fossils.[3]

Contents

1 Classification
2 Anatomy and behavior
3 Breeding
4 Blood
5 Fishery
6 References
7 Further reading
8 External links

Classification

Horseshoe crabs resemble crustaceans, but belong to a separate subphylum, Chelicerata, and are closely related to arachnids.[4] 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 contain all four living species of horseshoe crabs:[1]

Carcinoscorpius rotundicauda, the mangrove horseshoe crab, found in Southeast Asia
Limulus polyphemus, the Atlantic horseshoe crab, found along the American Atlantic coast and in the Gulf of Mexico
Tachypleus gigas, found in Southeast and East Asia
Tachypleus tridentatus, found in Southeast and East Asia

Anatomy and behavior
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 1000 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. Despite having a relatively poor eyesight, the animals have the largest rods and cones of any known animal, about 100 times the size of humans'.[5][6] 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 grinding 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. The long, straight, rigid tail can be used to flip the animal over if turned upside down, so a horseshoe crab with a broken tail is susceptible to desiccation or predation.
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, HostOurCoast.com
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. 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.[7] 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.[8]
Horseshoe crabs normally swim upside down, inclined at about 30° to the horizontal and moving at about 10-15 cm/s.[9][10][11]
Horseshoe crabs have two primary compound eyes and seven secondary simple eyes. Two of the secondary eyes are on the underside.[12][13]
Painting by Heinrich Harder c. 1916
Breeding
Horseshoe crabs mating

During the breeding season, horseshoe crabs migrate to shallow coastal waters. A male selects a female and clings to her back. The female digs a hole in the sand and lays her eggs while the male fertilizes 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.[14] Raising horseshoe crabs in captivity has proven to be difficult. Some evidence indicates 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.[15]
Blood

Unlike vertebrates, horseshoe crabs do not have hemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue. Their blood contains amebocytes, which play a role similar to 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. The blood of horseshoe crabs is harvested for this purpose.[16]

Harvesting horseshoe crab blood 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.[17] Estimates of mortality rates following blood harvesting vary from 3-15%[18] to 10-30%.[19]
Fishery

Horseshoe crabs are used as bait to fish for eels (mostly in the United States) and whelk. However, fishing with horseshoe crab is temporarily forbidden in New Jersey (moratorium on harvesting) and restricted to only males in Delaware. A permanent moratorium is in effect in South Carolina.[20] The eggs are eaten in parts of Southeast Asia and China.[21]

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.[22] An effort is ongoing to develop adaptive-management plans to regulate horseshoe crab harvests in the bay in a way that protects migrating shorebirds.[23]
References

Kōichi Sekiguchi (1988). Biology of Horseshoe Crabs. Science House. ISBN 978-4-915572-25-8.
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.
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.
Garwood, Russell J.; Dunlop, Jason A. (2014). "Three-dimensional reconstruction and the phylogeny of extinct chelicerate orders". PeerJ 2: e641. doi:10.7717/peerj.641. Retrieved June 15, 2015.
Anatomy: Vision - The Horseshoe Crab
Horseshoe Crab, Limulus polyphemus ~ MarineBio.org
The biology of cartilage. I. Invertebrate cartilages: Limulus gill cartilage
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.
Manton SM (1977) The Arthropoda: habits, functional morphology, and evolution Page 57, Clarendon Press.
Shuster CN, Barlow RB and Brockmann HJ (Eds.) (2003) The American Horseshoe Crab Pages 163–164, Harvard University Press. ISBN 9780674011595.
Vosatka ED (1970) "Observations on the swimming, righting, and burrowing movements of young horseshoe crabs, Limulus polyphemus" Ohio Journal of Science, 70 (5): 276–283. pdf
Battelle, B.A. (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.
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.
23rd September 2014. "The Rabbit and the Horse Shoe Crab". Retrieved 9 March 2015.
David Funkhouser (April 15, 2011). "Crab love nest". Scientific American 304 (4): 29. doi:10.1038/scientificamerican0411-29.
http://www.horseshoecrab.org/med/med.html
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.
"Crash: A Tale of Two Species – The Benefits of Blue Blood", PBS
The Blood Harvest The Atlantic, 2014.
"Horseshoe crab". SC DNR species gallery. Retrieved 6 June 2011.
大西一實. "Vol.56 食うか食われるか?". あくあは〜つ通信. Retrieved 2008-04-18.
"Red knots get to feast on horseshoe crab eggs". Environment News Service. March 26, 2008. Retrieved January 19, 2011.
October 26, 2011. "Critter Class Hodge Podge (Horseshoe crabs and Wooly Bears)" (PDF). The Wildlife Center. Retrieved 9 March 2015.