Unchanged for more than 200 million years, the crab's blood gets its blue color from copper in its system and its special properties that make the blood invaluable to modern medicine.
"People began to notice that if [the crab] got wounded and it got infected, their blood would gunk up and coagulate," said Eric Hallerman, director of the Horseshoe Crab Research Center at Virginia Polytechnic Institute and State University.
And that's exactly the way it's used today. A protein in the blood called Limulus Amebocyte Lysate (LAL) reacts to all kinds of microorganisms and can easily detect dangerous endotoxins that cause fever and can be fatal. Any contamination and scientists will see the blood react.
"Over millions of years the crab has been exposed to an awful lot of microbes," Hallerman said, "making them immune to a wider range of threats than any other animal."
When collecting the blood -- valued at $15,000 per pint -- the crabs are picked up by trollers who deliver the animals to a lab where they are bled. The crabs are then released back into the wild.
Hallerman's organization works with fisheries all over North America to make sure the crab populations remain high and that measures are taken to protect them.
"Humans looked to nature and nature gave us answers," Hallerman said. "Over 200 million years, the crab survived the asteroid that killed the dinosaurs, and survived human assaults on habitat. I know they'll be here long after we are gone."