Okay, I know people as a general rule tend to not care about invertebrates as much as cute, fuzzy mammals, but this is a must-read if you care about animal welfare. The short version is that horseshoe crab blood has been used for decades in medicine as a way to test whether something is truly sterile; the blood clots in the presence of bacteria. Since then millions of horseshoe crabs have been captured and drained of blood, even though a synthetic alternative was developed a few years ago.
They go through a pretty brutal experience in the process. They're caught by fishermen who often throw them by their tails into a pile in the open air, and they're then trucked to a bleeding facility where they're strapped down and their blood is removed with needles jabbed directly into their hearts. Over half their blood may be taken, after which they're supposed to be returned to the ocean. However, it's likely many of them never make it back, instead turned into fish bait and sold by the same fishermen who caught them in the first place.
Apart from the fact that this is a horrific thing to put any animal through, the attrition due to fatalities has put a serious dent in horseshoe crab numbers. This is compounded by massive habitat loss, pollution, and the capture of horseshoe crabs as food, particularly as the females of one species are considered a delicacy. And other animals that rely on horseshoe crabs are suffering, too. The American rufa subspecies of the red knot, a medium-sized shorebird, is critically endangered as the horseshoe crab eggs it must have in order to successfully complete migration have become increasingly scarce, and it is likely the bird will become extinct if trends continue.
While there are guidelines for medical horseshoe crab harvest, they're considered optional. The few laws that exist are poorly enforced. Short of a complete ban on horseshoe crab blood in favor of the synthetic alternative, these animals are in very real danger of going extinct after a history spanning over 400 million years on this planet.
Thankfully, this article is not the first to bring forth the issues surrounding horseshoe crab harvest. Here are a few resources for further information and action (US based, though horseshoe crabs are threatened throughout their entire range):
