Despite their name, horseshoe crabs are not true crabs. The horseshoe crab, Limulus polyphemus in popular name called Horseshoe Crab, is the only member of the Arthropoda subclass Xiphosura found in the Atlantic. Unlike true crabs, which have two pairs of antennae, a pair of jaws and five pairs of legs, horseshoe crabs lack antennae and jaws and they have seven pairs of legs, including a pair of chelicerae. Chelicerae are appendages similar to those used by spiders and scorpions for grasping and crushing. In addition, horseshoe crabs have book lungs, similar to spiders and different from crabs, which have gills. Thus, horseshoe crabs are more closely related to spiders and scorpions than they are to other crabs. Their carapace is divided into three sections: the anterior portion is the prosoma; the middle section is the opithosoma; and the “tail” is called the telson. Horseshoe crabs have two pairs of eyes located on the prosoma: one anterior set of simple eyes and one set of lateral compound eyes similar to those of insects. In addition, they possess a series of photoreceptors on the opithosoma and telson.
Horseshoe crabs are long-lived animals; after attaining sexual maturity at 9 to 12 years of age, they may live for another 10 years or more. Like other arthropods, horseshoe crabs must molt in order to grow. As the crab ages, more and more time passes between molts, with 16 to 19 molts occurring before a crab becomes mature, stops growing and switches energy expenditure to reproduction. Adult horseshoe crabs feed on a variety of bottom-dwelling organisms including marine worms, shellfish and decaying animal matter. The larvae and juvenile stages are preyed upon by many species of fish and birds and adult horseshoe crabs are known to be a food item for the threatened loggerhead sea turtle, Caretta caretta.
Horseshoe crabs are also harvested for use in biomedicine. A clotting agent in the crab’s blood,
known as Limulus Amoebocyte Lysate (LAL), is used to detect microbial pathogens in medical
intravenous fluids, injectable drugs and supplies (Rudloe, 1983). Biomedical companies purchase large crabs, which are harvested by trawlers or by hand from spawning beaches. The crabs are transported to the LAL production facility, bled, then transported back to the general harvest vicinity and released alive. LAL is currently used worldwide as the standard (FDA required) test for microbial contamination in injectable pharmaceutical products (Walls and Berkson 2000). Horseshoe crabs have also been used in eye research and the development of wound dressings and surgical sutures. In addition, horseshoe crabs are currently the primary bait used in the whelk and eel fisheries along the Atlantic coast.
This species is not currently listed as threatened or endangered; however, horseshoe crabs are an important species, both commercially and ecologically. Ecologically, horseshoe crabs are an
important component of coastal food webs. In particular, horseshoe crab eggs are the primary
source of fat for at least 20 species of migratory shore birds (Harrington 2001). Larval and juvenile crabs are also food for many species of fish and invertebrates, while adult crabs are
favored by loggerhead sea turtles and sharks (Keinath et al 1987). In addition, horseshoe crabs
have been shown to be a controlling factor in benthic species composition through their feeding
activities. There is great concern about the harvest of horseshoe crabs in the mid Atlantic and how it affects the red knot, Calidris canutus, another imperiled species.
Horseshoe crabs are relatively common in trawls in South Carolina. Based on research trawl
collections, we are able to get some ideas of relative abundance. However, there is no estimate
of population size at this time. The range of the horseshoe crab extends from northern Maine to the Yucatan Peninsula. They are particularly abundant in Delaware Bay, the center of their distribution, and in coastal areas between Virginia and New Jersey. Different populations of horseshoe crabs are thought to inhabit every major estuary along the Atlantic coast. Each population can be differentiated from the others based on size of adult crabs, the color of their carapace and pigments present in their eyes. In South Carolina, horseshoe crabs can be found in shallow estuarine areas and offshore habitats near the continental shelf.
Adult horseshoe crabs are benthic animals. Early each spring, as estuarine water temperature
approaches 20°C (68°F), adult horseshoe crabs move inshore to seek suitable spawning habitat
along intertidal beaches of the sea-islands. The characteristics associated with preferential
spawning locations are the presence of large intertidal sand flats near the spawning beach, a
depth to reducing layer greater than 30 cm (12 inches) from the surface and accretional, rather
than erosional, sediments.
Throughout the spring, females with males attached to their carapace follow flooding tides high
onto the beach, where they excavate nests and deposit thousands of eggs. During mating, the male grasps the female’s carapace and fertilizes her eggs as she deposits them in the nest cavity.
Oftentimes, other unattached “satellite” males may also fertilize some of the eggs. Mating and
nesting coincide with high tides. Nests are excavated by the female on the intertidal zone of
sandy beaches and eggs are laid in clusters. Spawning activity is especially heavy during
nighttime spring tides. Females nest several times per season, usually returning to deposit more
eggs on subsequent high tides.
After approximately two weeks, depending on temperature, moisture and oxygen levels, larval horseshoe crabs emerge from the nest. Larval horseshoe crabs are semi planktonic for about three weeks before their transition to a benthic existence. They then settle to the bottom and assume a benthic existence, typically spending their first two years in intertidal sand flat habitats near beaches where they were spawned. Adults return to deeper estuary bays and continental shelf waters after the breeding season.