Horseshoe crabs (Limulus polyphemus) are benthic or bottom-dwelling organisms found in both estuarine and continental shelf habitats. Horseshoe crabs are arthropods, part of the largest group of all living animals that includes insects, spiders, scorpions and crabs. Horseshoe crabs are not true crabs, and actually are closer in form to spiders and scorpions, because they lack antennae and mandibles. Unlike true crabs, horseshoe crabs have book gills (a breathing organ in many arachnids containing folds of membrane like leaves of a book), chelicera (an anterior pair of appendages in arachnids, often specialized as fangs) and five pairs of legs. The external appearance of the horseshoe crab has remained relatively unchanged over the last 360 million years.

The horseshoe crab population ranges from the Yucatan peninsula to northern Maine, but they are most commonly found in the mid-Atlantic region between Virginia and New Jersey. Each major estuary along the coast is believed to have a discrete horseshoe crab population, distinguished from one another by individuals’ adult size, carapace color and eye pigmentation.

In the Chesapeake Bay, horseshoe crabs are present year-round near the mouth and have been documented in the Eastern Bay, Rappahannock, Miles, Chester and Choptank rivers. Spawning has been observed around the mouth of the Patuxent River and probably occurs in other areas throughout the Bay.

Life Cycle

• In late spring, migrating adults move inshore from deep bay and coastal waters to spawn. Large numbers of horseshoe crabs move onto sandy Chesapeake Bay beaches to mate and lay eggs in the latter part of May.
• Although horseshoe crabs have highly visible mating activities, prime spawning habitat (protected beach areas within bays and coves) is scattered throughout the Bay and has not been well documented. The peak in spawning activity usually coincides with the full moon and evening spring tides.
• Females lay eggs–approximately 88,000 each–in clusters or nest sites along the beach, usually between the tide marks. Egg development usually takes a month or more, depending on temperature, moisture and oxygen. The larvae are mobile after they hatch and spend about a week swimming until they settle to the bottom, where they molt. Juvenile horseshoe crabs generally spend their first two summers on the intertidal flats.
• Young horseshoe crabs molt or shed their chitinous exoskeleton several times during the first 2 to 3 years. As the crab grows larger, the period between molts lengthens. After at least 16 to 17 molts over a period of 9 to 12 years, the horseshoe crabs reach sexual maturity. During their final molt, males develop specialized clasping claws for holding the female during reproduction. After sexual maturity is reached, the horseshoe crab can live an additional 2 to 6 years.

Habitat

Horseshoe crabs can tolerate a wide range of temperatures and have distinct physiological processes that enable them to survive low oxygen environments. Adult horseshoe crabs have been found burrowed into anoxic muds and intertidal flats at low tide, but spawning adults will avoid anaerobic sediments in beach areas. They can move out of the water during spawning and survive extended periods of time out of the water if their book gills are kept moist. Since horseshoe crabs undertake inshore and offshore migrations they are particularly affected by environmental degradations in both estuarine and oceanic habitats. Activities that alter the protected sandy beaches that horseshoe crabs use for spawning, such as creating bulkheads, placing riprap, replenishing beaches, creating landfills and revetments activities, ultimately will have a negative impact on the horseshoe crab population.

• Adult horseshoe crabs feed primarily on marine worms and shellfish, including razor clams and soft-shelled clams.
• Because they lack jaws, horseshoe crabs use the spiny bases of their legs to crush and grind their food and to push it into their mouths.
• Horseshoe crabs play an important ecological role in the food web. Adults are an important component in the diet of juvenile loggerhead turtles, a threatened species that uses the Chesapeake Bay as a summer nursery area. Horseshoe crab eggs also provide seasonal food to several fin fish species. And several shorebird species eat the eggs to replenish their fat supply on their migrations to Canadian breeding grounds. It’s estimated that migratory shorebirds consume nearly 320 tons of eggs annually; a decrease in the horseshoe crab population would rob many of these birds of the necessary food resources to complete their annual trips.

The Fishery

Horseshoe crabs were once primarily harvested for use in poultry and livestock food and fertilizers. Currently horseshoe crabs–most often females with eggs–are commercially harvested for use as eel, conch, and catfish bait along the Atlantic coast. Commercial harvest from the northeastern Atlantic coast has ranged between 10,000 pounds and 2 million pounds over the last 30 years. Since 1988 commercial landings have averaged 950,000 pounds. The commercial statistics are based on a horseshoe crab’s average weight of 4 pounds. Reported dockside value from the northeastern Atlantic coast fishery statistics probably underestimates the catch of horseshoe crabs because the sale of crabs for bait is often arranged between private individuals rather than through centralized dealers.

In recent years horseshoe crabs also have been important in medical research. Scientists have used the crabs in ophthalmological research, for surgical sutures, in the development of wound dressings and in the detection of bacteria in drugs. The discovery of limulus amoebocyte lysate (LAL), a clotting agent found in horseshoe crab blood, has made it possible to detect human pathogens such as spinal meningitis and gonorrhea in patients and in drugs. Any drug produced by a pharmaceutical company must pass a test using LAL, which manufacturing companies obtain by bleeding large horseshoe crabs. Two hundred thousand crabs are estimated to be used annually in medical research; however these must be returned to the environment. Although the adults are released alive, they have approximately a 10 percent greater mortality than horseshoe crabs that have not been captured and bled. Horseshoe crabs caught for medical use are an additional source of mortality statistics and are not included in the commercial catch statistics.

Maryland has been responsible for harvesting between 23 percent and 78 percent of the total commercial catch of horseshoe crabs from the northeastern Atlantic coast since 1980.

• Between 1978 and 1992 the commercial harvest of horseshoe crabs has ranged between 50,000 pounds (1984) and 746,000 pounds (1989) with a 10 year average of 357,000 pounds.
• Reported dockside value has increased from $1,000 in 1977 to $121,000 in 1993. In the last 10 years, over half of the commercial catch was reported between August and October when adult horseshoe crabs migrate offshore.
• Thirty-three percent of the catch was reported between April and May as horseshoe crabs move into beach areas to spawn.
• Historically, horseshoe crabs have been caught in clam dredges, otter trawls and hard clam rakes.

In Virginia, the commercial harvest of horseshoe crabs averaged 190,000 pounds between 1980 and 1988. Since the ban on trawling within state waters was implemented in 1989, horseshoe crab landings have decreased significantly with landings averaging 22,000 pounds. Reported dockside value has ranged between $200 (1976) and $26,000 (1987).

In Virginia and Maryland, no specific laws or regulations pertain to horseshoe crabs. There is a ban on trawling within Virginia state waters, within Chesapeake Bay and coastal bays and up to one mile off the Maryland Atlantic coast. Special scientific collection permits have been issued to trawlers in Virginia to catch horseshoe crabs for medical purposes. Virginia has limited their conch pot fishery to 20 permits, which limits the number of horseshoe crabs that can be used for baiting pots. In April 1998, Maryland restricted coastal ocean harvest by 70 percent and the Atlantic States Marine Fisheries Commission has proposed a 25 percent reduction in the horseshoe crab catch in every Atlantic state.

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Last modified: 12/2/04