Decades-Old Question About This 'Purple Sock' Is Answered

Since it was first discovered 60 years ago off the coast of Sweden, biologists have wondered exactly where the deep sea creature that resembles a crumpled purple sock belongs in the animal kingdom's family tree. Now the discovery of four new species in an entirely different ocean has effectively answered that question—but a big one remains. A team led by the Scripps Institution of Oceanography at UC San Diego reports in the journal Nature on four newly found species in the genus Xenoturbella living off the coasts of California and Mexico. Their discovery allowed for more genetic tests, and those tests established all five species' place on the family tree—and it's low. Xenoturbella are, this much is clear, quite primitive deep sea dwellers situated right near the base of the evolutionary tree of animals.

The brainless, eyeless, and gutless creatures have only one hole (yes, what goes in must come out of the same place). They were first mistaken for flatworms in the 1950s, then identified as mollusks in the 1990s, per a press release. Problem was, scientists had inadvertently sequenced the DNA of what it had eaten. "We find it where these mollusks are, and when we sequence it, we find these mollusks, their DNA, is inside," lead researcher Greg Rouse tells the BBC, referencing the "great unsolved mystery": how does the creature eat? Researchers have opened them only to find an empty gut, and their single hole is small, toothless, and void of "any sucking proboscis structure." (Check out how deep these whales dive.)