Natural enemies of horseshoe crabs include humans, crocodiles, monkeys and turtles. Humans pose a particular threat to horseshoe crabs because their blue blood is widely used in medicine, causing their numbers to plummet.

The horseshoe crab is one of the few animals in the world with blue blood. Blue blood has a significant effect on many diseases today. Effect. Limulus reagent made from Limulus blood is an effective treatment drug in the current medical field against diseases caused by Gram-negative bacteria, such as meningitis, cholera, plague, whooping cough, etc.