During the warmer months, especially at night during the full moon, horseshoe crabs emerge from the sea to spawn. Waiting for them are teams of lab workers who capture the horseshoe crabs by the hundreds of thousands, take them to labs, harvest their cerulean blood, then return them to the sea.
Oddly enough, we capture horseshoe crabs on the beach because that’s the only place we know we can find them. A female horseshoe crab lays as many as 20 batches of up to 4,000 eggs on her annual visit to the beach. When the eggs hatch, the juvenile horseshoe crabs often stay near shore, periodically shedding their shells as they grow. Once they leave these shallow waters, they don’t return until they reach sexual maturity ten years later. Despite our best efforts, we don’t know where they spend those years.
Though we’ve spotted the occasional horseshoe crab as deep as 200 meters below the ocean’s surface, we only see large groups of adults when they come ashore to spawn. Horseshoe crab blood contains cells called amebocytes that protect them from infection by viruses, fungi, and bacteria. Amebocytes form gels around these invaders to prevent them from spreading infections. This isn’t unusual.
All animals have protective immune systems. But horseshoe crab amebocytes are exceptionally sensitive to bacterial endotoxins. Endotoxins are molecules from the cell walls of certain bacteria, including E. coli. Large amounts of them are released when bacterial cells die, and they can make us sick if they enter the blood stream. Many of the medicines and medical devices we rely on can become contaminated, so we have to test them before they touch our blood. We do have tests called Gram stains that detect bacteria, but they can’t recognize endotoxins which can be there even when bacteria aren’t present.
So scientists use an extract called LAL produced from harvested horseshoe crab blood to test for endotoxins. They add LAL to a medicine sample, and if gels form, bacterial endotoxins are present. Today, the LAL test is used so widely that millions of people who’ve never seen a horseshoe crab have been protected by their blood. If you’ve ever had an injection, that probably includes you.
How did horseshoe crabs end up with such special blood? Like other invertebrates, the horseshoe crab has an open circulatory system. This means their blood isn’t contained in blood vessels, like ours. Instead, horseshoe crab blood flows freely through the body cavity and comes in direct contact with tissues. If bacteria enters their blood, it can quickly spread over a large area.
Pair this vulnerability with the horseshoe crab’s bacteria-filled ocean and shoreline habitats, and it’s easy to see why they need such a sensitive immune response. Horseshoe crabs survived mass extinction events that wiped out over 90% of life on Earth and killed off the dinosaurs, but they’re not invincible. And the biggest disruptions they’ve faced in millions of years come from us.
Studies have shown that up to 15% of horseshoe crabs die in the process of having their blood harvested. And recent research suggests this number may be even higher. Researchers have also observed fewer females returning to spawn at some of the most harvested areas.
Our impact on horseshoe crabs extends beyond the biomedical industry, too. Coastal development destroys spawning sites, and horseshoe crabs are also killed for fishing bait. There’s ample evidence that their populations are shrinking. Some researchers have started working to synthesize horseshoe crab blood in the lab. For now, we’re unlikely to stop our beach trips, but hopefully, a synthetic alternative will someday eliminate our reliance on the blood of these ancient creatures.