The horseshoe crab is a living fossil—sometimes called a stabilomorph, a body design that evolution got so right, it hasn’t had to change.

They’ve been roaming the coastal shelves and brackish bays of Earth for more than 400 million years in nearly the identical form as today, having survived all five great extinction events we’ve discussed on EarthDate—including the asteroid that wiped out the dinosaurs.

Why are they so enduring? A few reasons:

They’re generalists that can live in shallow or deep, warm or cold water, and eat pretty much anything they find on the sea floor.

They breed very successfully, grow fairly quickly, and have long lives—typically 20 to 40 years.

And they’ve developed a unique and powerful immune system to keep them healthy.

Horseshoe crabs live in bacteria-rich environments yet have a primitive open circulation system that can be prone to infection.

So their blood—actually a blood-like substance called hemolymph—developed special cells that release granules when they detect even the tiniest concentration of pathogens or the toxins they can produce.

The granules cause the blood to coagulate into a gel around the intruders, trapping them and sealing them off from the rest of the crab’s system. It’s so effective, it can close off the wound from a lost limb, sealing out bacteria.

This capability has made the lowly horseshoe crab absolutely invaluable to human medicine, which we’ll discuss in another EarthDate.