Horseshoe crabs are marine arthropods with a hard, helmet-shaped shell and a long, pointed tail, known as a telson. These creatures have remained largely unchanged for hundreds of millions of years, often called “living fossils.” Their lineage dates back approximately 445 million years. Despite their common name, horseshoe crabs are not true crabs; instead, they are more closely related to arachnids like spiders and scorpions.
Their Role in Ecosystems
Horseshoe crabs are important to coastal ecosystems. Their eggs, laid in vast numbers on sandy beaches during spawning season, provide a food source for many species. Migratory shorebirds rely on these eggs to refuel during their long migrations. The Delaware Bay, for example, is an important stopover point where millions of shorebirds, including the federally threatened Red Knot, time their migration to coincide with the horseshoe crab spawning.
Their burrowing activities also benefit other seafloor organisms. As they move through sediment, they aerate and turn over the substrate, improving conditions for other invertebrates. This supports the biodiversity and health of these coastal environments.
Contribution to Human Health
Horseshoe crab blood is invaluable to human health and medicine. Unlike human blood, their blood is blue due to copper-based hemocyanin, which transports oxygen. The most significant medical application comes from specialized cells in their blood that contain Limulus Amebocyte Lysate (LAL).
The LAL test is used globally to detect bacterial endotoxins, harmful substances released by bacteria. These endotoxins can cause severe reactions in humans, including fever, shock, and organ failure, if introduced into the body through injectable drugs, vaccines, or medical devices. When LAL contacts even minute traces of endotoxins, it rapidly coagulates, forming a clot. This provides a highly sensitive and specific method for contamination detection.
The FDA approved the LAL test in 1977 as a standard for ensuring the safety of medical products, replacing the slower rabbit pyrogen test. This test remains a widely accepted method in pharmaceutical and medical device manufacturing for quality control. Beyond endotoxin detection, their compound eyes have also contributed to vision research.
Conservation and Future
Despite their resilience, horseshoe crab populations face significant threats. Habitat loss and degradation, due to coastal development and shoreline alteration, directly impacts their spawning grounds. Overharvesting for bait, particularly for whelk and eel fisheries, also pressures their numbers.
Biomedical bleeding, where crabs are captured and blood is extracted for LAL production before being returned to the sea, also contributes to population declines. Studies indicate that up to 30% of bled crabs may not survive the process, and those that do can experience behavioral changes and reduced activity, potentially impacting their breeding success. The American horseshoe crab (Limulus polyphemus) is listed as vulnerable by the International Union for Conservation of Nature (IUCN), while some Asian species are endangered.
Various conservation efforts are underway to address these challenges. Sustainable harvesting practices aim to minimize the impact of human activities on crab populations. The development of synthetic alternatives to LAL, such as recombinant Factor C (rFC), is also gaining traction. These recombinant assays mimic the clotting cascade of horseshoe crab blood but are produced without animal-derived materials, offering a more sustainable solution for endotoxin testing. Habitat restoration projects and conservation initiatives, including community-based sanctuary programs, are working to protect and restore spawning and nursery areas.