Number of horseshoe crabs that were bled in America in 2020. The crabs are strapped to steel countertops in labs and, while still alive, drained of about a third of their blue-coloured blood. Then they are returned to the ocean. This liquid is vital for America’s biomedical industry. Its blood is the only known natural source of limulus amebocyte lysate (lal), an extract that detects endotoxin, a nasty and sometimes fatal chemical produced by certain bacteria. Drug firms use it to ensure the safety of medicines and implanted devices, including antibiotics, anti-cancer drugs, heart stents, insulin and vaccines. The immune cells in the crab’s blood clot around toxic bacteria, giving a visual signal of unwanted contamination.