Limulus blood contains a single cell type, the amoebocyte, which reacts to gram-negative bacterial endotoxin by triggering a coagulation cascade that gels around the contaminant. In the 1960s that reaction was developed into the Limulus amoebocyte lysate assay, which detects endotoxin at extraordinarily low concentrations.
The assay became the industry standard. Injectable drugs, vaccines and implantable devices are tested with it because endotoxin in a bloodstream causes fever and shock and survives ordinary sterilisation. Crabs are collected, bled of roughly a third of their volume and returned to the water. Reported post-release mortality varies substantially across studies, with figures commonly falling somewhere between a few per cent and around 30 per cent.
The blue colour and the clotting are unrelated properties that happen to belong to the same animal. A synthetic alternative, recombinant factor C, has been developed and is gaining regulatory acceptance, but the transition is incomplete. Sublethal effects of bleeding on spawning behaviour are an active area of study.