Newly discovered deep-sea worms, including one named ‘churro,’ could shed light on animal evolution

Here’s one churro you won’t want to eat.

A team led by California researchers has discovered a pink, flatworm-like critter in the deep seas off the Pacific that they’ve named Xenoturbella churro, after the long, delicious fried-dough treat that you will never look at in the same way again.

The discovery of X. churro and three other related species, described this week in the journal Nature, help settle a longstanding mystery about where these strange critters belong on the tree of life.

Xenoturbella are remarkably simple animals — with no brain, no gonads, not even an anus. The first species, Xenoturbella bocki, was described around 1949 off the coast of Sweden, and lumped in with the flatworms. In the 1990s, people decided that it could be a mollusk, based on DNA analysis — until it became clear that the DNA was present because the animals actually ate mollusks.

It’s a question that has stymied researchers for decades, and it’s one that could have pretty big implications for how we understand the evolution of organs such as brains, kidneys and those in the digestive tract.

“One would expect them to find a home among the acoels — similarly simple animals thought to lie at the base of the evolutionary tree of Bilateria, bilaterally symmetrical animals,” Henry Gee, a senior editor at Nature who was not involved in the research, wrote in a commentary. “Yet Xenoturbella have caused puzzlement since they were first described in 1949, because quibbles about their ultrastructure and mitochondrial DNA sequences have meant that the worms have never sat entirely happily in their assumed station.”

Now, two new studies in Nature finally help to settle the issue. Scientists at UC San Diego’s Scripps Institution of Oceanography and the Monterey Bay Aquarium and Research Institute have described four new species in the same group that are eking out a living in the depths of the Pacific by noshing on clams and other mollusks.

The orangey-pink X. churro is four inches long and was discovered in a 5,577-foot-deep cold seep in the Gulf of California. X. monstrosa, which hails both from the gulf and from Monterey Submarine Canyon, is purple or pink and comes in at a lengthy eight inches. X. profunda (which is Latin for “deep”) was found in a 12,139-foot deep hydrothermal vent in the gulf. X. hollandorum, the tiniest of the lot, measures a mere one inch and was found in the canyon near the bones of a gray whale.

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Their characteristics (and an analysis of the genetics of 11 species whose members included Xenoturbella, published in the same issue) have helped scientists finally nail down where these species should be — within a group known as Xenacoelomorpha that lies near the base of the Bilateria family tree.

“The authors’ anatomical and phylogenetic studies on these new forms add weight to the idea that these worms were the earliest to branch from other bilaterians,” Gee wrote.

Bilateria are an enormous collection of animals whose bodies typically show bilateral symmetry — a group that includes both flatworms and vertebrates like humans. Because Xenoturbella are now thought to lie very close to the common ancestor of all Bilateria, they could shed light on early animal evolution.

In the meantime, now that they’ve found these weird critters, the scientists say they expect to find more related species.

“These discoveries highlight the possibility of further Xenoturbella discoveries in deep-sea environments that sustain their likely food source, bivalve mollusks,” the study authors wrote.

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