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. But for years, there was doubt about their relationship to arachnids. Then, a team of evolutionary biologists from the University of Wisconsin-Madison analyzed a collection of genetic data in 2019 and confirmed that horseshoe crabs are in fact related to arachnids.
"By showing that horseshoe crabs are part of the arachnid radiation, instead of a lineage closely related to but independent of arachnids, all previous hypotheses on the evolution of arachnids need to be revised," Jesús Ballesteros, a postdoctoral researcher, wrote in the study. "It's a major shift in our understanding of arthropod evolution."
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.