Deep Sea “Purple Sock” Finally Placed On The Evolutionary Tree

The creature is described by researchers as looking like a “purple sock” strewn on the floor. Rouse et al. 2016
Josh Davis 04 Feb 2016, 21:02

With no eyes, no brain, no gut, and no anus, a bizarre deep-sea worm has been puzzling biologists for more than 60 years. What actually is it, and where does it fall on the tree of life? Described as a deep sea “purple sock,” and with so few external body features, scientists have had very little to go on. Initially it was classed as a flatworm, then a highly reduced mollusc. Some suggest it’s related to vertebrates, others echinoderms.

During the discovery of four new species of the weird creatures, scientists conducted DNA tests that showed the worms are actually very early forms of what are known as bilateral animals, which include you and me. This simply means it belongs to the group of animals that have mirror symmetry down the middle, in contrast to those, such a jellyfish and anemones, which have multiple planes of symmetry.

The four newly described species in their natural habitat, X. monstrosa (a), X. churro (b), X. profunda (c), X. hollandorum (d). Rouse et al. 2016

This places the creatures, known technically as Xenoturbella, at the base of the evolutionary tree that eventually gave rise to the majority of living animals, from spiders and oysters to birds and fish. It also indicates that despite previous suggestions, the creatures are not the result of a once complex organism losing features over time, but that it represents an incredibly primitive branch on the tree of life.

The new study, published in Nature, brings the number of known species of Xenoturbella from one up to five. The animals were first described in 1949 after being found in the cold waters off the Swedish coast. But while sampling the deep sea surrounding California and Mexico, the researchers discovered another four species living near deep-sea cold seeps, hydrothermal vents, and whale carcasses. Ranging in size from 2.5 centimeters (1 inch) to over 20 centimeters (7.8 inches), the new species add considerably to the diversity of the group.

X. churro is perhaps not as delicious as its namesake. Rouse et al. 2016

While the researchers have described them as akin to socks strewn on the floor, they’ve also compared them to the tasty Spanish treat of deep-fried dough, calling one of the new species Xenoturbella churro. Living at depths of up to 3,700 meters (12,100 feet), what they do and how they live still remains a considerable mystery.

Despite collecting a few samples from the sea floor, due to the lack of a gut, the researchers are still unable to determine what they actually eat. They do suspect, however, that where previous DNA-testing efforts identified the animals as molluscs, they may have actually been sampling the DNA of what the worms had eaten. How they might go about eating hard-shelled molluscs with a lack of teeth or any discernable parts of their mouths that could aid, simply confuses things further. 

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