How many horseshoes are there?

To accurately assess population trends in any group of animals, an initial or baseline survey 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.

The Atlantic States Marine Fisheries Commission (ASMFC) reports “limited time-series of horseshoe crab population data make it difficult to assess its status. However, the 2013 stock assessment update indicates horseshoe crab abundance has increased in the Southeast (North Carolina through Florida) and remains stable in the Delaware Bay region (New Jersey through coastal Virginia). The New York and New England regions continue to see a decrease in abundance. These continued declines are being investigated by the Stock Assessment Subcommittee.”

The International Union for the Conservation of Nature (IUCN) Species Survival Commission (SSC) Horseshoe Crab Specialist Group, recently determined that based on Red List criteria the American Horseshoe Crab is Vulnerable to local extinction and that the degree and extent of the risk varies among and within genetically-defined regions. The scientific assessment which provided the basis for the Red List assessment has been peer-reviewed and published in Reviews in Fish Biology and Fisheries. The citation is: Smith, D. R., H. J. Brockmann, M. Beekey, T. L. King, M. J. Millard, and J. Zaldívar Rae. 2016. Status Assessment of the American Horseshoe Crab (Limulus polyphemus).

As one can imagine, the challenge to accurately assess the three Asian horseshoe crab species (Tachypleus tridentatus, Tachypleus gigas and Carcinoscorpius rotundicauda) is more complex. However, the IUCN Horseshoe Crab Specialist Group has completed its Red List assessment of the Asian Tachypleus tridentatus species. Its status throughout its spawning range goes from Endangered to On The Verge of Extinction. Although we are not finished with the assessment of the two other Asian horseshoe crab species, their status is beginning to look very much the same. It will take years of dedicated work to complete this assessment, time the Asian horseshoe crab species does not have, which makes it essential that risk-averse management policies are put in place to protect the species while fisheries management data is being compiled. A scientific review of the fisheries and conservation status of the three Asian horseshoe crab species has been published by B. Akbar John, B. R. Nelson, Hassan I. Sheikh, S. G. Cheung, Yusli Wardiatno, Bisnu Prasad Dash, Keiji Tsuchiya, Yumiko Iwasaki, and Siddhartha Pati. 2017. A Review on Fisheries and Conservation Status of Asian Horseshoe Crabs. 

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 ERDG’s “Just flip ‘em!™” program ; beyond this being a ‘humane’ action, the Botton & Loveland (1989) study provides the rationale.”
– Dr. Carl N. Shuster, Jr., 1999

Conch and Eel Fisheries

Since the 1970’s the American horseshoe crab (Limulus polyphemus) have been harvested in substantial numbers for bait in the American eel and conch fisheries. The American eel fishery prefers to use adult females, whereas the conch fishery will use adult horseshoe crabs of either sex. In 1999 the harvest of horseshoe crabs for bait along the US Atlantic Coast was around 3 million annually. That year ERDG initiated 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. Today, bait bags are widely used throughout the bait fishery with an annual coast-wide harvest of  estimated to be 750,000.

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.

ERDG is currently working to engage commercial and recreational bait manufactures in the development of an alternative bait, which if successful, could eliminate the harvesting pressure from this user group altogether.

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

• Alternative Gear and Supplemental Bait Workshop, March 3, 2004 (pdf)
• Decrease in Crabs Raises Concerns
• ERDG completes Phase II of its bait bag initiative
• Horseshoe Crab Bait Bags are adopted by the Virginia Conch Fishery
• ERDG begins Phase I of its Bait Bag Initiative
• Alternative Bait Workshop (minutes)

Biomedical Use

Horseshoe crabs are one of the most researched marine animals, in part to their ease of capture and their primitive biological systems, that reduce the number of variables when testing theories. Dr. Keffer Hartline won the Nobel prize in Medicine in 1967 based upon research performed on the horseshoe eye. But perhaps the most important is the use of a component in their copper-based blue 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 collect adult horseshoe crabs, remove a portion (1/3) of their blood, then release them alive. Approximately 15 percent of the animals do not survive the bleeding procedure. As of 2016 the bleeding mortality associated with LAL production was estimated to be 70,600 animals per year.

As the Limulus polyphemus species is only found in the United States, LAL is manufactured in the United States.  It is, however, sold globally.  Tachypleus tridentatus is a species located in Asia.  TAL derived from this species is therefore manufactured in Asia and is used primarily in China. Although the manufacturing and use of TAL is similar to LAL, the harvesting, bleeding and post-production methods could not be different. Animals for TAL are bled to death, their body parts are sold for human consumption and the shell is dried and sold for chitin, resulting in 100% mortality. The number of animals harvested for TAL is unknown but estimated to be in the hundreds of thousands.

In 2009 ERDG launched an initiative to bring awareness to the growing threat to the three Asian horseshoe crab species (and ultimately the American horseshoe crab) as a result of excessive and lethal harvesting practices by the endotoxin testing industry engaged in the production of Tachypleus amoebocyte lysate (TAL). In 2011 ERDG shared these concerns with the attendees of the International Workshop on the Science and Conservation of Asian Horseshoe Crabs, held in Hong Kong and again in 2015 at the International Workshop on the Science and Conservation of Horseshoe Crabs held in Sasebo City, Japan.

In 2015 ERDG authored a peer-reviewed paper titled “Current Horseshoe Crab Harvesting Practices Cannot Support Global Demand for TAL/LAL: The Pharmaceutical and Medical Device Industries’ Role in the Sustainability of Horseshoe Crabs” which was published by Springer Publishing Company in a book titled “Changing Global Perspectives on Horseshoe Crab Biology, Conservation and Management.”

In 2017 at the International Conference for the Conservation of Asian Horseshoe Crabs (ICCA-HSC) held in Bangkok, Thailand, ERDG presented its concern about the over-harvesting of the Tachypleus species for the production of TAL and the detrimental impact it is having on the Tachypleus population throughout Southeast Asia. In this presentation, ERDG invites the pharmaceutical/biomedical industry to join the horseshoe crab conservation community in the conservation of these remarkable Asian species, by eliminating unsustainable endotoxin testing products from their supply chain. “The Harvest of the Tachypleus Horseshoe Crab Species for the Production of TAL is Unsustainable.”

To raise awareness and provide tools to promote communication regarding how the pharmaceutical/biomedical industry can become a driving force behind the conservation of the world’s four horseshoe crab species, the “Protecting Health” section was developed.

Human Consumption

Two of the three Asian species of horseshoe crabs (Tachypleus tridentatus, Tachypleus gigas) are consumed throughout coastal regions of many Southeast Asian countries, particularly in Vietnam, Thailand and Malaysia. Carcinoscorpius rotundicauda is thought to harbor tetrodotoxin, so is spared a similar fate. It is unknown how many animals find there way into the human food chain, but it is estimated to be in the hundreds of thousands annually.

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 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 Backyard Stewardship™ Community 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 and its habitat. Although there is no existing data that suggests unusual sensitivity by horseshoe crabs to pesticides, herbicides and hydrocarbons from agricultural and stormwater runoff, there is data that shows an adverse impact of heavy metal concentrations on juvenile development. Red tide events may also contribute to mortality in juveniles that inhabit intertidal areas and tidal flats during their early stages of life.

Regardless of the direct impact of pollution on the horseshoe crab population, the indirect effect from a diminished habitat and compromised food resource can be just as lethal, for the long-term survival of the species will depend upon suitable spawning beaches, intertidal nurseries and deep bay waters.

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
• Bait and human consumption harvesters prefer gravid females (those carrying eggs).
• Biomedical producers of TAL prefer females and bleed the animals to death (biomedical producers of LAL bleed and release).
• 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.

Identifying and maintaining optimal sustainable yield for the commercial fishery and biomedical use is critical. Appropriate coast-wide management of the horseshoe crab population throughout the four species spawning range 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.

Eco-tourism is critical to the economies in many areas where horseshoe crabs spawn, which is dependent upon the abundance and health of the ecosystems within the region.

The challenge for the world’s four horseshoe crab species is complex, for not only is its range extensive, it encompasses a host of countries with very different social, economic and environmental priorities.