Horseshoe crabs are ancient marine arthropods facing threats. Often called “living fossils” due to their evolutionary longevity, they play a significant role in coastal ecosystems. Their eggs provide a crucial food source for migratory shorebirds, including the federally threatened red knot, which times its migration to coincide with the crabs’ spawning season. Their decline impacts not only the species but also the many organisms that rely on them.
Exploitation for Biomedical and Fishing Industries
The unique properties of horseshoe crab blood have made them indispensable to the biomedical industry. Their blood contains Limulus Amebocyte Lysate (LAL), used to detect bacterial endotoxins in medical devices, vaccines, and injectable drugs. This test is vital for pharmaceutical safety.
LAL is obtained by collecting adult crabs, transporting them to facilities, and bleeding them. About one-third of their blue, copper-based blood is extracted before they are returned to the water. While efforts are made to ensure survival, studies indicate a mortality rate of 15% to 30% for bled crabs, with estimates suggesting around 112,000 died in 2021. Increasing LAL demand pressures populations.
Horseshoe crabs are also harvested for the fishing bait industry. They are a primary bait source for American eel and conch fisheries in many mid-Atlantic states. Commercial bait harvest accounted for at least 2 million individuals in 1996. The practice involves sectioning crabs, particularly females, for use in eel and conch traps. This extensive harvesting, combined with biomedical demand, significantly impacts populations.
Vulnerability Due to Life Cycle and Habitat
Horseshoe crabs are susceptible to population declines due to their life cycle. They are a long-lived species, maturing at 9 to 11 years of age and living up to 20 years. This slow maturation means populations recover slowly from reductions. Females typically reach sexual maturity about a year later than males.
Their reproductive cycle also presents vulnerabilities. Horseshoe crabs migrate to specific sandy beaches during late spring and early summer, often coinciding with full/new moons and high tides, to spawn. Females lay tens of thousands of eggs in sand nests, with one female laying up to 100,000 eggs per season. This reliance on specific coastal habitats means disturbances can severely impact reproductive success.
Horseshoe crabs are relatively slow-moving and possess limited natural defenses against human harvesting, making them easy targets. Their ancient lineage, spanning over 450 million years, highlights their resilience. However, this history does not equip them for the rapid pace of modern human-induced pressures, implying slower adaptation to swift environmental changes.
Coastal Development and Environmental Changes
Coastal development contributes to population decline through habitat loss. Construction, urbanization, and structures like bulkheads destroy or alter sandy beach spawning habitats. Bulkheads can block access to intertidal spawning beaches, while groins and seawalls intensify shoreline erosion and prevent natural beach migration. This widespread alteration reduces areas where crabs can reproduce successfully.
Pollution also threatens horseshoe crabs, affecting their health, food sources, and eggs. Chemical runoff degrades water quality in spawning estuaries. While eggs and larvae show some tolerance to contaminants like heavy metals and oil, severe pollution can cause developmental issues. Plastic pollution is a growing concern; microplastics can be ingested, obstructing digestive systems, reducing growth and mobility, and increasing mortality, especially in juveniles.
Climate change adds pressure to horseshoe crab populations. Rising sea levels erode spawning beaches, and with coastal development, reduce suitable habitat. Changes in ocean temperature and salinity affect egg and larval survival, potentially reducing recruitment and productivity. Extreme weather events, like hurricanes, can destroy optimal habitat, as seen with Hurricane Sandy in Delaware Bay. These shifts further challenge their long-term survival.