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It's not easy to look at a sea spider and see an animal so representative of its kind that it may help scientists sort out the evolution of almost everything with eight legs. But that's the potential a new study finds in these spindly, strikingly strange bottom-dwellers.

After all, once you're done counting the legs, you quickly run out of resemblances between the 1,300-some known species of sea spider and their relatives, like actual spiders, scorpions, ticks, mites and horseshoe crabs.

Sea spiders breathe through their skin, moving oxygen around their body using a kind of peristalsis (muscle contractions similar to how you squeeze food down your throat). When it's time to make babies, the males cement the fertilized eggs onto themselves and carry them around on their bodies until they hatch.

There isn't even much body to speak of, because sea spiders look like plumbing schematics. They're all tubes, mostly because they have no abdomen—that back end that bears the scorpion's stinger, that stores all that blood in a well-fed tick, and that gives tarantulas their bulbous, hairy mass.

"They're weird," says Prashant Sharma, a researcher who specializes in that sort of thing. His University of Wisconsin–Madison lab is intermittently stocked with blind arachnids that live only in a handful of Eastern Mediterranean caves, scorpion-shaped vinegaroons that spray acid from their butts, and daddy longlegs that have … short legs.

"Sea spiders are just incredibly cool and understudied animals. So, that's what draws us to them," Sharma adds.

That interest has revealed a more scientific reason to be drawn to sea spiders. They are a useful anchor for the genetics of the chelicerates, a group that includes all of the charismatic and consequential, many-legged animals mentioned above.

Sharma, a UW–Madison professor of integrative biology, studies the evolution of chelicerates, digging deep into their genes to understand better how their varied, intricate bodies have developed. He's studied how and when they picked up tricks like venom and shown that the horseshoe crab belongs to this cohort just as much as the house spider.

What many of his animal subjects have had in common over the years is a twist in their evolution that strikes geneticists as a bit of an advantage: free DNA real estate. Somewhere along their line, they went through a process called whole-genome duplication.

"There are a few mechanisms for whole-genome duplication," Sharma says, including a misstep in cell division or combining genomes with a close relative. "But the result is a species getting an extra copy of all of its chromosomes. You can look at all those extra genes as more places where new traits, new functions could develop."

Sharma and collaborators—including former lab members Siddharth Kulkarni and Emily Setton, and scientists at the Arctic University of Norway— the first high-quality genome of a sea spider species in the journal BMC Biology. The work was spearheaded by their colleagues at the University of Vienna, most notably Georg Brenneis, one of the very few people on the planet working on sea spider development.

The study centers on the knotty sea spider, Pycnogonum litorale, which is widespread on rocky sea beds across the North Atlantic Ocean and looks a lot like a tiny, tangled ginger root. The researchers found that this specific sea spider has never experienced whole-genome duplication.

Because whole genomes, once duplicated, tend to keep traces of that doubling of genes, this places P. litorale somewhere near the base of the entire chelicerate family tree relative to all those branches that include species with duplicated genomes. It's a steady point to which scientists can trace back the progression of variation across modern spiders and related species.

"They are an important reference for the evolution of all these species, which include some of the most significant agricultural pests, like mites, and vectors for human disease, like ticks," says Sharma.

The researchers also may have uncovered sea spiders have no abdomen. They are missing a gene, handily called "Abdominal-A," from a group of genes called the Hox cluster known for its importance to organizing body parts. As a result, sea spiders have stuffed all the usual contents of an abdomen—stomachs, reproductive organs, the stuff they use to breathe—into their legs.

Weirder still, there are fossil sea spiders from tens and hundreds of millions of years ago that do sport an abdomen.

"We don't know quite when that structure was lost. We know they started out looking more like modern arthropods," says Sharma, referring to the wider group of animals with exoskeletons and segmented bodies, including beetles and crustaceans and bees and his chelicerates. "And then, at some point, they just went totally bizarre. So weird."