Horseshoe crab blood is an E. coli detective.
Scientists use the precious substance—specifically, the crab blood's clotting agent—to make a concoction called Limulus Amoebocyte Lysate (LAL). LAL is used to detect Gram-negative bacteria like Escherichia coli ("E. coli"), which can wreak havoc on humans.
(...and then the story gets complicated...)
The article and details are at the link...