The horseshoe crab is an ancient marine arthropod, often called a living fossil, that has persisted for over 450 million years. Whether this species is endangered is complex and depends heavily on its geographic location. While the four extant species face population pressures, their conservation status varies significantly across global and regional boundaries due to multiple human-driven threats.
Understanding the Conservation Status
The conservation status of the horseshoe crab is assessed by the International Union for Conservation of Nature (IUCN), which maintains a Red List of Threatened Species. The American horseshoe crab (Limulus polyphemus), found along the Atlantic coast of North America and the Gulf of Mexico, is globally listed as Vulnerable. In contrast, the Asian tri-spine horseshoe crab (Tachypleus tridentatus) has been classified as Endangered since 2019, reflecting a severe global decline.
The status of the American species is highly regional, with populations exhibiting mixed trends based on local management and harvest pressure. Populations in the Mid-Atlantic’s Delaware Bay area are considered stable due to regulatory controls and conservation efforts. However, populations in the Gulf of Maine and New England show elevated risk and continuing declines resulting from over-harvesting. This disparity means that while the species as a whole is not classified as Endangered, several distinct regional populations face a serious threat of local extirpation.
The Dual Pressures: Biomedical Harvesting and Bait Fishing
Two primary human activities exert pressure on horseshoe crab populations: biomedical harvesting and commercial bait fishing. The biomedical industry collects the crabs for their unique, copper-based blue blood, which contains Limulus Amebocyte Lysate (LAL). The LAL test is the global standard for detecting bacterial endotoxins in injectable drugs, vaccines, and medical devices.
Biomedical Harvesting
To produce LAL, over half a million American horseshoe crabs are collected annually, a portion of their blood is extracted, and they are then returned to the water. The estimated mortality rate for the American species following this procedure is cited as 15%, though some studies suggest the actual mortality for females can be nearly 30%. For the Asian species, the mortality associated with bleeding is often 100%, as the animals are sold for consumption or chitin production afterward.
Bait Fishing
Commercial fishing also drives significant population pressure, as horseshoe crabs are highly valued as bait for the American eel and whelk pot fisheries along the Atlantic coast. Fishermen favor the larger females because they are highly effective in attracting channeled whelk and eel. The harvest for bait escalated in the 1990s, peaking at nearly six million pounds in 1997, resulting in the complete removal and death of the harvested animals. The combined impact of mortality from both the biomedical and bait fisheries is a major factor driving regional population declines.
The Vital Ecological Link
The horseshoe crab occupies a foundational position in the coastal ecosystem, most notably through a critical interdependency with migratory shorebirds. Each spring, millions of horseshoe crabs converge on beaches, particularly in the Delaware Bay, to spawn, depositing billions of nutrient-rich eggs in the sand. This event is precisely timed to coincide with the arrival of various shorebird species, which stop at the bay during their annual migration.
The Red Knot (Calidris canutus rufa) is the most heavily impacted species, relying almost exclusively on horseshoe crab eggs during its stopover in the Delaware Bay to refuel. This shorebird travels over 9,000 miles from South America to its Arctic breeding grounds. The eggs provide a high-lipid, easily digestible food source that allows the knots to rapidly double their body weight, which is necessary for them to successfully complete their journey and breed. The steep decline in crab populations and subsequent egg availability has directly correlated with a reduction in the Red Knot’s ability to gain weight, contributing to the species being federally listed as threatened.
Protecting the Living Fossil
Conservation efforts are being implemented across the American horseshoe crab’s range, focusing on habitat protection and harvest regulation. Management strategies include fishing quotas and seasonal closures, which restrict the number of crabs harvested for bait and prohibit collection during the peak spring spawning season. Some regions, like New Jersey, have implemented a moratorium on all crab harvesting to bolster population recovery in the Delaware Bay, the largest spawning area.
Habitat protection is a major focus, as preserving the integrity of coastal spawning grounds is necessary for population recovery. Significant effort has also been directed toward developing and adopting synthetic alternatives to the horseshoe crab-derived LAL test. The most prominent alternative is recombinant Factor C (rFC), a synthesized version of the protein in the crab’s blood that detects endotoxins. Adoption of rFC is increasing globally, with the European Pharmacopoeia recognizing it as a compendial test, offering a path toward reducing the biomedical industry’s reliance on wild-caught horseshoe crabs.