Turns out, horseshoe crabs are in fact related to spiders not crabs

Horseshoe crabs (Xiphosura, if you want the technical name) are fascinating creatures. They have survive essentially unchanged for over 450 million years, shrugging off several global mass extinctions in the process. They have pale blue blood, they are ultra-sensitive to toxins, and they look like nothing else in existence. But here's the kicker: it turns out that horseshoe crabs are not crabs at all, but arachnids.
Researchers at the University of Wisconsin-Madison, studying the genetic material of the four existing species of horseshoe crabs from genome sequencing projects have concluded that they are in fact aquatic arachnids, related to the spiders and scorpions that evolved on land (and most closely related to the hooded tick spiders).
Well, I wasn't expecting that!