Fact
Horseshoe crab blood is used to test nearly every vaccine
It is bright blue, and it clots on contact with bacterial contamination.
Limulus amebocyte lysate, derived from horseshoe crab blood, detects endotoxins at extraordinarily low concentrations and became the industry standard for testing injectable drugs. Crabs are bled and returned to the sea, though mortality is significant. Synthetic alternatives now exist and are gradually being adopted.