Generally humans gave little damns about them as well. Fishermen use them as bait when fishing for conch, but other than that, they remained as black and white photos decorating your local Red Lobster. However, once tests were run on the copper based blood (ours, and pretty much everything else's is an iron based red, except that royal family in the movie Stardust, they apparently bleed blue as well), some scientist got the vapors. The extremely primitive immune system of the crab works in an extremely simple manner: if the animal receives an injury or a cut and bacteria or some other toxins attempt to infiltrate the animal, the blood congeals and forms a gelatinous barrier that protects the crab from infection. Think about that the next time you eat grape jell-o. So now scientists, and pharmacuticalists, and other interested ists "harvest" horseshoe crabs (obviously they are related to wheat?) drain about 1/3 of their blood and return them to the wild to be caught by those same conch fishermen before. Studies guess that there is only a 10% mortality rate for the blood donors, but who really knows. I mean, 100% of the ones used as bait expire. So, do they carry donor cards and have fishermen release them until their 30 day replenishing is up? Doubtful. But that is the sacrifice they make, bloodletting to help a species that has only been around a fraction of their species' time on this planet.
Maybe we should blame the sand piper birds, after all they are the ones that fly in and devour millions of horseshoe crap eggs ever year at the annual horseshoe crab beach orgy. This has even been shown around National Geographic and Planet something narrated by Sir David Attenborough. Interestingly enough, there is a new book out about these guys written by retired paleontologist Richard Fortey. Put it on your summer reading list, read it at the beach and then tell your kids about how awesome that leggy writhing beach rock with a sharp tale actually is, and make sure to bring some blue jell-o.