Ecological Importance of Horseshoe Crabs

Horseshoe crabs play an important ecological role in the food web. A decline in the number of horseshoe crabs will impact other species, particularly shorebirds and sea turtles, a federally-listed threatened species that uses the Chesapeake Bay as a summer nursery area. Shorebirds primarily feed on horseshoe crab eggs exposed on the surface, but sufficient surface eggs are available only if horseshoe crabs are spawning at high densities. Therefore, adequate spawning densities must be maintained to ensure availability of horseshoe crab eggs for shorebirds. Sea turtles feed on adult horseshoe crabs, but their diet depends on relative abundance of the prey species.

For more information on the horseshoe crab and its ecological niche, please see Natural History.

Assessing the Population:

How many horseshoes are there?

To accurately assess population trends in any group of animals, an initial or baseline census must be taken and then repeated annually, using the same methods and counting locations. Because government and environmental groups have employed inconsistent methods over the years, no one can say with any certainty how many crabs inhabit the Atlantic coast, or whether their population has gone up or down significantly in the past 10-20 years.

One conservative estimate places the number at 2.3 to 4.5 million horseshoes on the Atlantic Coast between New Jersey and Virginia; other studies believe the number to be higher. There is no accurate scientific data on population decline in recent years, though anecdotal evidence seems to indicate smaller numbers during spawning season on many beaches.
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Mortality:Threats to the Horseshoe Population

Natural Mortality

Natural causes of death include beach strandings, predation, and other factors such as disease. Beach strandings during spawning cause an estimated 10% of adult deaths, either from prolonged exposure to heat and oxygen or from seabirds, especially gulls, who eat the overturned crabs.

"The common occurrence of stranded horseshoe crabs during breeding season spurred the "Just flip ‘em®" program on Delaware beaches; beyond this being a ‘humane’ action, the Botton & Loveland (1989) study provides the rationale."
- Dr. Carl N. Shuster, Jr., 1999

Conch and Eel Fisheries

More recently, horseshoe crabs have been taken in substantial numbers to provide bait for fisheries, including (primarily) the American eel and conch fisheries. Horseshoe crabs, particularly females, are sectioned and placed in American eel pots as bait. The conch fishery uses horseshoe crabs of either sex. The 1996 fishing mortality accounted for at least 2 million individuals throughout the Atlantic Coast. Additional, specific information on Harvesting and Harvest Regulations by State is available in our Detailed Information section.

ERDG was responsible for initiating the first study to test the effectiveness of bait bags in reducing demand on horseshoe crabs as bait in the U.S. conch fishery. Working with the Virginia Institute of Marine Science (VIMS), it was shown that bait needs could be reduced by half if placed in a bait bag. To promote this initiative, ERDG manufactured and distributed, free of cost, over 15,000 bait bags to conch fishermen along the Atlantic Coast of the United States. In addition, ERDG is actively facilitating dialog between various interest groups towards the development of alternative bait.

In 2004, ERDG organized an alternative bait and gear workshop, which was held in Baltimore, Maryland and sponsored by the Atlantic States Marine Fisheries Commission (ASMFC). The purpose of the two-day meeting was to bring together watermen, fisheries managers, researchers, distributors, and Limulus amebocyte lysate (LAL) manufactures from up and down the Atlantic Coast to share ideas, designs, and strategies that would reduce the need for horseshoe crabs in the conch and eel fishery.

For information about Alternative Gear and Supplemental Bait, see the following articles in our News section:

BioMedical Industry

In the biomedical industry, horseshoe crabs have been used in eye research, the manufacture of surgical sutures, and the development of wound dressings for burn victims. But perhaps most important is the use of a component of the horseshoe crab’s blood called Limulus Amebocyte Lysate (LAL), which is indispensable for the detection of bacterial endotoxins in drugs and intravenous devices. To manufacture LAL, the companies catch adult horseshoe crabs, collect a portion (1/3) of their blood, and then release them alive. (For more information abut this, see the Medical Uses section.) Although this industry bleeds individuals and then releases the animals, two studies estimate 10 to 15 percent of animals do not survive the bleeding procedure, which accounts for the mortality of 20,000 to 37,500 horseshoe crabs per year.

Shoreline Development and Habitat Loss

Beach areas, nearshore shallow waters, intertidal flats, and deep bay waters are all essential to the success of the horseshoe crab as a species. Shoreline development and subsequent habitat degradation is likely an important threat to horseshoe crabs. Groins and bulkheads may adversely impact horseshoe crab spawning habitat. Bulkheads can block access to intertidal spawning beaches, while groins and seawalls intensify local shoreline erosion and prevent natural beach migration.

Horseshoe Crab Sanctuaries

The future survival of the world’s four remaining horseshoe crab species will ultimately depend upon the preservation of its spawning habitat — a challenging prospect in light of the ever-increasing human density along the same inland beaches horseshoe crabs have relied upon for thousands of years. To respond to this challenge, ERDG launched its community based "Horseshoe Crab Sanctuary Program" in 1999, designed to encourage coastal communities to declare their shared habitat a horseshoe crab conservation area or sanctuary.


Pollution has the potential to adversely impact the horseshoe crab population or its habitat, though there is no existing data that suggests unusual sensitivity by horseshoe crabs to pesticides and herbicides. However, mosquito control agencies in New Jersey and Delaware have recently expanded their use of the mosquito larvicide methoprene, an insect growth regulator (IGR) that mimics juvenile growth hormones. IGR insecticides have been found to adversely affect crustaceans when the animals attempt to molt, though it is unclear if the low concentrations of IGRs applied would have a measurable impact.

Red tide events may result in significant mortality, especially to juveniles inhabiting intertidal areas and tidal flats.
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Biological And Environmental Impacts

If harvesting is not carefully managed, the risk of adversely affecting the horseshoe crab population becomes a certainty. Several factors that contribute to this risk include

  • Horseshoe crabs mature slowly, requiring nine to eleven years to attain sexual maturity

  • Some bait harvesters prefer gravid females (those carrying eggs).

  • Horseshoe crabs congregate inshore seasonally to spawn, which makes them especially vulnerable to exploitation.

  • Changes in abundance (increases or decreases) are not readily recognizable because they occur over a period of years.

  • Population data indicate that after harvesting ceases, horseshoe crabs do not rebound for approximately one decade, which corresponds to the time required for horseshoe crabs to reach sexual maturity.

Socioeconomic Impacts

Identifying and maintaining optimal sustainable yield for the commercial fishery is critical. Appropriate coast-wide management of the horseshoe crab population would ensure the long-term viability of the population for continued harvest and would provide necessary quantities of adults and eggs for fish and wildlife resources.

Horseshoe crabs are the primary bait for the American eel and conch fisheries in many mid-Atlantic States. In 1996, the commercial harvest of horseshoe crabs was estimated to be a $1.5 million industry.

Horseshoe crabs are vital to medical research and the pharmaceutical products industry. The worldwide market for LAL is currently estimated to be approximately $50 million per year. The biomedical industry pays approximately $375,000 per year for horseshoe crabs based on an estimate of 250,000 horseshoe crabs harvested at an average price of $1.50 per crab.

Eco-tourism is critical to the economies of many states, including New Jersey and Delaware, and it depends on the abundance and health of the ecosystems within the region. The 1996 regional economic impact resulting from expenditures by wildlife watchers in New Jersey and Delaware was the creation of 15,127 jobs and the generation of a total household income of $399 million.
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