Despite their name, horseshoe crabs—bizarre, ancient aquatic critters—don’t look like horseshoes and they’re definitely not crabs.

They’re arachnids. Just like spiders, scorpions, amblypygids, mites and a bunch of other leggy, land-dwelling animals.

That’s what scientists recently concluded after sequencing an enormous amount of genetic material from horseshoe crabs and arachnids, and then using those sequences to draw the most plausible family tree. (Read more about horseshoe crab evolution.)

“This particular part of the tree of life has always been quite challenging to solve,” says study leader Jesús Ballesteros of the University of Wisconsin-Madison, who reported the results recently in the journal Systematic Biology.

“But one of the things that was surprising in this analysis is that regardless of how