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 we treated the data, we consistently found the same results … the horseshoe crabs are always nested inside the arachnids [on the family tree].”

A tale of two sisters

For decades, scientists have known that horseshoe crabs and arachnids, which both belong in the subphylum Chelicerata, are closely related, but it’s been tricky to sort out just how closely related.

Horseshoe crabs, the blue-blooded burrowers in the genus Xiphosura, first appeared in the fossil record about 450 million years ago. Around that same time, arachnids crawled into existence.

As the old version of the story went, arachnids and horseshoe crabs descended from a common ancestor, an aquatic chelicerate of some sort, and then split into sister branches. One lineage quickly moved onto land and diversified into as many as 100,000 species, becoming today’s arachnids. (See pictures of fossil spiders found with “glowing” eyes.)

Another, the horseshoe crabs, stayed put and survived major mass extinction events essentially as-is; today, there are only four surviving species of horseshoe crabs found throughout the world’s oceans, some of which can be more than a foot long.

“The problem with Xiphosurans is that there are only four extant species,” says Matthias Obst of Sweden’s University of Gothenburg, who also studies the evolutionary relationships within chelicerates.

So there’s “very little internal diversity, which usually helps to stabilize certain groups in the tree of life.”

Arachnocrabs

But in more recent years, genomic sequencing work by Ballesteros and others has challenged this story, suggesting that instead of existing as a sister group to arachnids, horseshoe crabs are arachnids.

This most recent study reached that conclusion after assessing the family trees that can most easily explain how genetic sequences are distributed among 53 species of arachnids, horseshoe crabs and sea spiders (which, despite the name, are not actual spiders, but another chelicerate lineage), as well as various crustaceans and insects.

View Images Horseshoe crabs, found throughout the world's oceans, are a familiar sight on beaches of the eastern U.S. Photograph by Joel Sartore, National Geographic Photo Ark

The majority of those gene trees—roughly two-thirds—place Xiphosura well within the arachnid lineage, appearing as a sister group to hooded tick spiders.

But Rosa Fernández, who also studies arachnid evolution at the Center for Genomic Regulation in Barcelona, notes that there are lingering uncertainties. For instance, one-third of the sequenced genes keep horseshoe crabs outside arachnids.

Horseshoe Crabs Mate in Massive Beach "Orgy" June 16, 2014 - Horseshoe crabs have been on Earth an estimated 450 million years, pre-dating the dinosaurs by some 200 million years. They are one of the few living creatures with blue blood, which is used by the pharmaceutical industry for testing drugs. Horseshoe crabs live in the ocean year-round, but they make one annual visit to the shoreline to lay eggs in sandy, wet beaches. In the United States, the highest concentration of egg laying is along the Delaware Bay.







“The authors have done a magnificent job,” says Fernández. But “I would not be surprised if alternative methodologies yield a different result in the future.”

By land or by sea?

If that result holds, the story of arachnid evolution needs a bit of tweaking—in essence, scientists need to explain how one small branch on the family tree ended up in the sea while everyone else is hanging out on land. Did horseshoe crabs split from terrestrial arachnids and return to sea? Or did arachnids move onto the land more than once? (See 10 beautiful photos that will make you love spiders.)

“Until now, we—or at least myself—had the idea that Xiphosurans evolved from a marine trilobite-like ancestor of all arthropods,” Obst says, referring to the group of organisms that includes insects and crustaceans, in addition to arachnids and others. “Now it looks like horseshoe crab ancestors lived on land and were rather small.”