Atlantic Horseshoe Crab (Limulus polyphemus)

Stephanie F. Loria

This month we honor an organism which will soon begin its breeding season in NYC - the Atlantic horseshoe crab, Limulus polyphemus. Despite their name and superficial resemblance, horseshoe crabs are not crabs. They actually belong to their own class Xiphosura in Chelicerata, an arthropod group that also includes the classes Arachnida (spiders, scorpions, ticks, etc), Eurypterida (the extinct sea scorpions and also MSNH's logo taxon), and Pycnogonida (sea spiders). The placement of Xiphosura within Chelicerata has been debated and recent research has even placed Xiphosura within Arachnida (Sharma et al. 2014). Worldwide only four extant species of horseshoe crabs exist and all species except L. polyphemus are found in the Indo-Pacific Ocean (Xia 2000). Extinct horseshoe crab species have also been described and the oldest fossil, found in Canada, dates to the Upper Ordovician, 445 million years ago (Rudkin et al. 2008)! Despite their remarkably old age, horseshoe crabs have changed little morphologically since their first appearance and are therefore often referred to as 'living fossils' in the scientific literature (Avise et al. 1994). 

The breeding season of L. polyphemus runs from March to July with peak season occurring in May and June (Rudloe 1980; Rutecki et al. 2004). During the breeding season, male and female L. polyphemus arrive on the shores of eastern North America in droves with most breeding happening at high tide on new and full moon nights (Rudloe 1980). Males typically mount females using special claspers and eggs are fertilized externally (Brockmann 1990). However, eggs may also be fertilized by satellite males which are not attached to females and surround the mating couple (Sasson et al. 2015). Eggs develop in the sand, hatching 3 to 4 weeks later and larvae disperse into the ocean (Bakker et al. 2016; Botton and Loveland, 2003; Rudloe, 1979). Horseshoe crabs live longer than dogs typically reaching 19 years of age (Rutecki et al. 2004).

During the breeding season, red knots (Calidris canutus rufa) feast on horseshoe crab eggs, an important food source for these birds (Niles et al. 2009). Horseshoe crabs are also harvested by humans for biomedical use as their blood contains amoebocyte lysate (ACL), a compound that can be used to detect bacterial endotoxins (Rutecki et al. 2004). Although biomedically harvested individuals are typically released once blood has been taken, mortality does occur among released individuals (Rutecki et al. 2004). Horseshoe crabs are also harvested for fishing bait and overharvesting from fishing and the biomedical industry and shoreline destruction has led to population declines (Land et al. 2015). In order to help track the health of horseshoe crab populations, nonprofit organizations such as NYC Audubon, and researchers survey horseshoe crabs populations each year during the breeding season. The MSNH teams up annually with NYC Audubon in the survey so that we can interact and help protect this fascinating and ancient species.

Horseshoe crabs ( L. polyphemus ) at Jamaica Bay. Courtesy of Maurice Chen.

Horseshoe crabs (L. polyphemus) at Jamaica Bay. Courtesy of Maurice Chen.


Avise, J. C., W. S. Nelson, and H. Sugita. 1994. A speciational history of" living fossils": molecular evolutionary patterns in horseshoe crabs. Evolution: 1986-2001.

Bakker, A. K., J. Dutton, M.  Sclafani and N. Santangelo. 2016. Environmental exposure of Atlantic horseshoe crab (Limulus polyphemus) early life stages to essential trace elements. Science of The Total Environment 572: 804-812.

Botton, M. L. and R. E. Loveland. 2003. Abundance and dispersal potential of horseshoe crab (Limulus polyphemus) larvae in the Delaware estuary. Estuaries 26: 1472-1479.

Brockmann, H. J. 1990. Mating behavior of horseshoe crabs, Limulus polyphemusBehaviour 114: 206-220.

Landi, A. A., J. C. Vokoun, P. Howell, and P. Auster. 2015. Predicting use of habitat patches by spawning horseshoe crabs (Limulus polyphemus) along a complex coastline with field surveys and geospatial analyses. Aquatic Conservation: Marine and Freshwater Ecosystems. 25: 380-395.

Niles, L. J ., J. Bart, H. P. Sitters, A. D. Dey, K. E. Clark, P. W. Atkinson, A. J. Baker, K. A. Bennett, K. S. Kalasz, N. A. Clark, and J. Clark. 2009. Effects of horseshoe crab harvest in Delaware Bay on Red Knots: are harvest restrictions working? Bioscience59: 153-164.

Rudkin, D. M., G. A.  Young, and G. S. Nowlan. 2008. The oldest horseshoe crab: a new Xiphosurid from Late Ordovician Konservat‐Lagerstätten Deposits, Manitoba, Canada. Palaeontology 51: 1-9.

Rudloe, A. 1979. Locomotor and light responses of larvae of the horseshoe crab, Limulus polyphemus (L.). The Biological Bulletin 157: 494-505.

Rudloe, A. 1980. The breeding behavior and patterns of movement of horseshoe crabs, Limulus polyphemus, in the vicinity of breeding beaches in Apalachee Bay, Florida. Estuaries and Coasts 3: 177-183.

Rutecki, D., R. H. Carmichael, and I. Valiela. 2004. Magnitude of harvest of Atlantic horseshoe crabs, Limulus polyphemus, in Pleasant Bay, Massachusetts. Estuaries and Coasts 27: 179-187.

Sasson, D. A., S. L. Johnson, and H. J. Brockmann. 2015. Reproductive tactics and mating contexts affect sperm traits in horseshoe crabs (Limulus polyphemus). Behavioral ecology and sociobiology 69: 1769-1778.

Sharma, P. P., S. T. Kaluziak, A. R. Pérez-Porro, V. L. González, G. Hormiga, W. C. Wheeler, and G. Giribet. 2014. Phylogenomic interrogation of Arachnida reveals systemic conflicts in phylogenetic signal. Molecular Biology and Evolution, p.msu235.

Xia, X. 2000. Phylogenetic relationship among horseshoe crab species: effect of substitution models on phylogenetic analyses. Systematic Biology 49: 87-100.