Despite their namesake, horseshoe crabs are not crabs at all. In fact, they aren't even crustaceans, being more closely related to arachnids like spiders than to anything else.
Horseshoe crabs have been around for about 450 million years. They are truly alien-looking — they even have blue blood. Horseshoe crabs also have a unique immune system, and studying them has led to breakthroughs in cancer research and spinal meningitis treatments, among other things.