Horseshoe crabs

Despite their common name, horseshoe crabs are not true crabs. They are marine arthropods, and are more closely related to arachnids such as spiders and scorpions than to crustacean crabs. The Atlantic horseshoe crab (Limulus polyphemus) inhabits the western Atlantic coast. Fossils show that horseshoe-crab relatives have a very ancient history, although the living animals are not unchanged survivors of the distant past.

Atlantic horseshoe crabs come ashore to spawn, often on sandy beaches during high tides. Females deposit eggs in the sand, where they develop before larvae emerge and move into shallow coastal waters. Delaware Bay is a major spawning area. The eggs are an important seasonal food for migratory shorebirds, including red knots, which pause there while undertaking long-distance migrations.

Horseshoe-crab blood contains amebocytes whose clotting response is used to make Limulus amebocyte lysate (LAL). LAL tests are used to detect bacterial endotoxins that can contaminate injectable drugs, vaccines and medical devices. This is a specific contamination test, not a general test for every kind of pathogen or sterility failure. Biomedical collection involves taking blood and returning animals to the water, but bleeding can cause mortality and other effects after release. Research and regulatory discussions therefore consider alternative testing methods as well as careful harvest and monitoring.

Horseshoe crabs are also harvested for bait, including for fisheries targeting eel and whelk. Their life cycle connects coastal habitats to the food supply of shorebirds, so changes in crab abundance or egg availability can have wider ecological effects. In the United States, the Atlantic States Marine Fisheries Commission coordinates interstate management, using harvest rules and adaptive approaches that take account of horseshoe crab and shorebird information in Delaware Bay.

There are four living species: the Atlantic horseshoe crab and three Asian species. Their conservation assessments are not interchangeable: the IUCN SSC Horseshoe Crab Specialist Group identifies the Atlantic species as Vulnerable, the tri-spine Asian horseshoe crab as Endangered, and the coastal and mangrove Asian species as Data Deficient. These categories reflect different available assessments and do not imply that all populations face the same conditions. IUCN conservation work highlights harvest pressure, habitat loss and gaps in population data as important concerns, especially for Asian species.

Monitoring populations is challenging because adults spend much of their lives in coastal waters, while spawning and juvenile surveys sample only particular places and times. The IUCN SSC Horseshoe Crab Specialist Group has described efforts to standardize and expand monitoring of Asian populations, including a network of observation stations in China and Hong Kong. In the United States, the Atlantic States Marine Fisheries Commission sets region-specific management measures; its current plan for Delaware Bay-origin animals illustrates how decisions may distinguish sexes and fishing seasons. Regulations and status assessments can change, so current local rules should be checked with the relevant authorities.