It looks like a creature from an Alien movie, but the horseshoe crab has a terrestrial pedigree more than 440 million years long. It's the nearest living relation to the extinct trilobite. And it's not really a crab at all but is actually more closely related to the spider.
Horseshoe crabs' blood is blue: Instead of hemoglobin, they distribute oxygen using a copper-based molecule called hemocyanin. That system has a lot to recommend it, including the blood's ability to clot very quickly around bacterial contaminants. This has made it an extremely valuable commodity to the pharmaceutical and medical-supply industries, which pay up to $15,000 a quart for horseshoe crab clotting agent.
The horseshoe crabs get nothing, of course, but at least they usually survive the blood donation. Up until the 1970s, they were routinely gathered on East Coast beaches, where they come to spawn, and then ground into fertilizer or pig feed. More recently, these remnants of the ancient order of Xiphosura were harvested for eel bait. Now protected, horseshoe crabs are making a modest recovery--as are the small sandpipers known as red knots, which need to refuel on horseshoe eggs at Delaware Bay midway through their 18,000-mile spring migration from Tierra del Fuego to the Arctic tundra.
Photo by iStock/ShaneKato