We know that humans breathe through their lungs and fish breathe through their gills—but where exactly does that leave sea spiders?

Though they might appear to share much in common with land spiders, sea spiders are not actually arachnids. And, by extension, they don't circulate blood and oxygen the way you'd expect them to, either.

A new study from Current Biology found that these leggy sea dwellers (marine arthropods of the class Pycnogonida) use their external skeleton to take in oxygen. Or, more specifically: They use their legs. The sea spider contracts its legs—which contain its guts—to pump oxygen through its body.

Somehow, these sea spiders hardly take the cake for Strangest Spider Alive (especially because they're not actually spiders); check out, for instance, our round-up of the 10 strangest spiders, and watch the video from National Geographic below: