They might look like rocks with tails, or messed-up giant bugs, but horseshoe crabs are super old, super cool, and they deserve your respect.    They may have already saved your life.   First of all, they aren’t crabs. They’re members of their very own family of arthropods -- arthropods include insects, arachnids, and crustaceans -- known as Limulidae, and there are only four species of them in the world.    They’re more closely related to arachnids, like spiders and scorpions, than anything else, but really, they’re their own deal.    They’re also mega-ancient. The oldest horseshoe-crab fossil we’ve found is around 450 MILLION years old! That means they have survived, virtually unchanged, for nearly half a billion years, outliving pretty much everything else.    These guys are survivors.    So what makes them so tough? Well, part of it has to do with their amazing blood.    For one thing, it’s blue. Baby blue. Our blood is red because the substance it uses to ferry oxygen around, called hemoglobin, contains iron. But horseshoe crabs use hemocyanin, which is copper-based. So, just like iron turns red when it oxidizes, when copper meets oxygen, it turns blue.   But that’s not even half of what makes horseshoe crab blood so cool.   So seawater is chock full of bacteria, and unlike mammals, horseshoe crabs don’t really have an immune system -- no infection-fighting white blood cells at all. So if they get, say, a crack in their shell, all that bacteria just gets into their bodies and could wreak all kinds of havoc.      But what they do have is a glorious chemical -- found only in their blood cells -- that binds to and inactivates unwanted bacteria, viruses, and fungi.    When the blood cells sense bad-guy invaders, they release a protein called coagulogen that forms a gooey barrier around the infecting agents, preventing their spread.    That clot of gooieness also essentially creates a physical barrier, sealing up the wound and preventing further infection. It’s an amazing internal defense mechanism.   Humans: we have no such ability. So for the past 50 years, we’ve been stealing it from the horseshoe crabs.   Ever since American physician Fredrick Bang discovered the weird power of horseshoe crab blood in the 1960s, these ancient sea creatures have played an essential, yet largely-unknown, role in human medical treatment.    If you have ever received any kind of drug or vaccine injection, it’s because these crabs made sure that injection was bacteria-free.    The whole thing starts when horseshoe crabs are collected from the wild, transported to one of only five production labs in the world, and then rigged up for a blood drive.    Needles are inserted around the heart, and up to 30 percent of the animal’s blood is removed. This stuff is liquid gold, and can go for $15,000 a liter.    From there, scientists extract the coagulogen from the blood cells, and use it to test solutions that are used in injectable drugs and vaccines.    If the crab-blood extract finds unwanted bacteria in the solution, it immobilizes it in a gooey clot. So if your sample doesn’t form any clots, you know that it’s bacteria-free and safe for human use.    This simple procedure, called the LAL, or Limulus amebocyte lysate test, is nearly instantaneous, and no other test works as well.   As for the crabs themselves, most seem to bounce back several days after being released back into the wild, but some folks are concerned that the blood-letting weakens them more than we think.    So LAL manufacturers tend to only collect blood once a year, to ensure they’re not double-dipping on the same crabs, and research is being done to come up with a synthetic alternative so that we can eventually leave them alone.   Until then, next time you get a shot, give thanks to all those old horseshoe crabs who made your good health -- and your sore arm -- possible.   Thanks for watching this SciShow Dose -- especially our Subbable subscribers. To learn how YOU can help us keep sharing wonderful science like this, just go to Subbable.com. And don’t forget to go to YouTube.com/scishow and subscribe!