Horseshoe crab (Limulus polyphemus)

Also known as: king crab
KingdomAnimalia
PhylumArthropoda
ClassMerostomata
OrderXiphosura
FamilyLimulidae
GenusLimulus (1)
SizeWidth of male prosoma: 137 - 213 mm (2)
Width of female prosoma: 243 - 351 mm (2)

The horseshoe crab is classified as Near Threatened (NT) on the IUCN Red List (1).

The horseshoe crab (Limulus polyphemus) is a ‘living fossil’: forms almost identical to this species were present during the Triassic period 230 million years ago, and similar species were present in the Devonian, a staggering 400 million years ago (3). Despite their common name, horseshoe crabs are not crabs but are related to arachnids (spiders, scorpions, ticks and mites) (4), and are the closest living relatives of the now extinct trilobites (3). Horseshoe crabs have three main parts to the body: the head region, known as the ‘prosoma’, the abdominal region or ‘opisthosoma’ and the spine-like tail or ‘telson’ (2). It is the tail that earns this order its name Xiphosura, which derives from the Greek for ‘sword tail’ (4). The sexes are similar in appearance, but female horseshoe crabs are much larger than males (2). The carapace is shaped like a horseshoe, and is greenish grey to dark brown in colour. A wide range of marine species become attached to the carapace, including algae, flat worms, molluscs, barnacles and bryozoans, and horseshoe crabs have been described as ‘living museums’ due to the number of organisms that they can support (3). On the underside of the prosoma there are six paired appendages, the first of which (the chelicera) are used to pass food into the mouth. The second pair, the pedipalps, are used as walking legs; in males they are tipped with ‘claspers’ which are used during mating to hold onto the female’s carapace. The remaining four pairs of appendages are the ‘pusher legs’, also used in locomotion. The opisthosoma bears a further six pairs of appendages; the first pair houses the genital pores, while the remaining five pairs are modified into flattened plates, known as book gills, that are used in ‘breathing’ (4). The horseshoe crab has a compound eye on each side of the prosoma, five eyes on the top of the carapace, and two eyes on the underside, close to the mouth, making a total of nine eyes. In addition, the tail bears a series of light-sensing organs along its length (2). A further unique and intriguing feature of this ancient species is that it has blue copper-based blood (2).

The horseshoe crab is found along the east coast of North America from Maine through south Florida and the Gulf of Mexico to the Yucatan Peninsula. Along this range there are distinct populations, with the largest population in Delaware Bay (4).

The horseshoe crab dwells on the bottom of muddy and sandy bays and estuaries. In the Gulf of Mexico, individuals have been found down to depths of 30 metres, with concentrations at five to six metres (3). They require sloping sandy beaches on which to lay their eggs (4).

Horseshoe crabs are typically active at night, with activity peaking around the time of the full moon. They dig for food, such as worms, algae and molluscs in the sediment (3).

During the spring and summer, adult horseshoe crabs migrate in huge numbers towards sandy beaches and congregate in the shallow water (4). Breeding is associated with the lunar and tidal cycles, with most adults arriving at the full or new moon and within a couple of hours of high tide. The direction of the waves guides the females towards the beach (3). Male horseshoe crabs patrol along the bottom of the beach in the shallow water, waiting to intercept beach-bound females (4). Pairs make their way to the high tide mark and the male fertilises the eggs as they are laid into a 15 centimetre deep nest in the sand (3). From 2,000 to 20,000 eggs may be produced in a single clutch (4). Very often there may be more than one male accompanying each female; in some cases there have been as many as 14 males to one female. As the tide begins to retreat, the horseshoe crabs make their way back to the sea (3).

The sticky eggs hatch after around five weeks (3), but this is dependent on temperature (4). The larvae, which are known as ‘trilobite’ larvae, may remain buried in the sand in aggregations for a number of weeks (4) before emerging at high tide. After they enter the water, they undergo a ‘swimming frenzy’ of constant, vigorous activity. Six to eight days after emerging, they moult into the first juvenile stage, which is very similar in appearance to the adult stage. At this point they cease swimming and start to live on the bottom (3). Horseshoe crabs are slow-growing. Males reach sexual maturity at 9 to 11 years of age and females between 10 to 12 years. Although it is difficult to assess age in this species, the average life-span is thought to be 20 to 40 years (4).

The horseshoe crab is an essential part of the ecosystem in which it occurs. Their eggs provide a valuable source of food for many species including wading birds, sea turtles, alligators and fish. Furthermore, the action of the horseshoe crab as it ploughs the sea bed in search of food aerates the substrate, resulting in a higher level of species richness (2).

The population of horseshoe crabs has declined dramatically (4). In the past they were killed in very large numbers by clam diggers, as the species preys on clams. They were also used for animal food (3). In the 1920s and 30s between four and five million individuals were harvested each year (3). At present they are harvested in very large numbers for use as eel and conch bait; in 1996 alone, at least two million horseshoe crabs were killed for this reason (2). Horseshoe crabs have also been used extensively in the biomedical industry for the manufacture of surgical sutures, making dressings for burn victims, and in eye research (2). Furthermore, the copper-based blue blood of this species clots when it comes into contact with toxins released from bacteria. This clotting property, called Limulus Amebocyte Lysate (LAL) is harnessed by pharmaceutical companies needing to test the safety of drugs and other fluids that are to be used on humans (4). In order to make LAL, the companies harvest live horseshoe crabs from breeding beaches and remove a third of their blood before releasing them back into the sea. Studies have shown that 10 to 15 percent of the individuals bled in this way die as a result, accounting for the loss of 20,000 to 37,500 horseshoe crabs each year (2). The world market for LAL is a $50 million per year industry, and this species is essential in its production (2). Other threats facing this ancient species include habitat loss and shoreline development, as well as pollution (2). The horseshoe crab is exceptionally vulnerable as it matures very slowly, gathers in large numbers making them ‘easy pickings’ and by the fact that changes in abundance are not easy to detect. Furthermore, once the population has been reduced it can take as long as ten years for it to recover after harvesting, which roughly corresponds to the length of time it takes for individuals to reach maturity (2). Natural strandings are also a source of considerable mortality (2).

It is a sad fact that this ancient living fossil, which has been carrying out its unique life cycle for millions of years, is now threatened by human activities. It is clear that the massive level of harvesting of this species must be carefully controlled if the horseshoe crab is to survive, and finding a sustainable level of exploitation is essential (2). It must be carefully managed both as a valuable biological resource, and in its own right, as an amazing remnant of an ancient lineage that pre-dates the dinosaurs. Current actions to conserve the horseshoe crab include tagging and radio-tracking schemes that aim to shed light on the migratory patterns and spawning behaviour of this species. Hopes are that the more we learn about the horseshoe crab, the more likely it is that increasingly effective conservation actions can be devised (2). The Delaware-based Ecological Research and Development Group (ERDG) have been working towards the conservation of the horseshoe crab for a number of years. It places a strong emphasis on educating people about this species and encouraging locals to get involved in conservation action. In 2000 the residents of Broadkill Beach, Delaware designated the three-mile stretch of coast as a horseshoe crab sanctuary, which bans harvesting on the beach. Local people also venture out to return stranded crabs to the water (2). In 2002 the ERDG helped a second Delaware shorefront community, Kitts Hummock, to set up a sanctuary. These are encouraging signs and indicate that decisive conservation action can take place without government intervention (2).

For further information on the horseshoe crab:

This information is awaiting authentication by a species expert, and will be updated as soon as possible. If you are able to help please contact:
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  1. IUCN Red List (April, 2011)
    http://www.iucnredlist.org/
  2. Ecological Research and Development Group: The Horseshoe Crab (March, 2004)
    http://www.horseshoecrab.org/
  3. Well, S.M., Pyle, R.M. and Collins, N.M. (1983) The IUCN Invertebrate Red Data Book. IUCN, Gland, Switzerland.
  4. Smithsonian Marine Station - Limulus polyphemus (Horseshoe crab) (March, 2004)
    http://www.sms.si.edu/irlspec/Limulu_polyph.htm