Research Highlight: Deep-Sea Churro Finds its Evolutionary Home

Churros, the delicious snacks covered with cinnamon sugar and served with hot chocolate, have a doppelganger in the deep sea. The deep-sea worm named after the deep-fried sticks of dough was recently discovered in a 1,700-meter (5,577-foot)-deep cold seep in the Gulf of California, an unforgiving environment where methane and other hydrocarbon-rich fluids seep out from beneath the seafloor.

Xenoturbella churro was one of four new flatworm-like species found by scientists, including Greg Rouse from Scripps Institution of Oceanography at UC San Diego, living near deep-sea cold seeps, hydrothermal vents, and whale carcasses off the coasts of California and Mexico. These new finds have helped stabilize the placement of the worms, not the snack, on the animal tree of life.

Researchers from Scripps, the Western Australian Museum, and the Monterey Bay Aquarium Research Institute (MBARI) described these elusive creatures, all in the genus Xenoturbella, in the Feb. 4 issue of the journal Nature, and helped properly identify them through genetic analysis.

“The findings have implications for how we understand animal evolution,” said Scripps marine biologist Greg Rouse, the lead author of the study. “By placing Xenoturbella properly in the tree of life we can better understand early animal evolution.”

The search to understand these pinkish-purple animals, which also slightly resemble chewed-up bubble gum, began in 1949 when a single species, Xenoturbella bocki, was found in waters off Sweden.  What it was, and how it came to exist, would continue to puzzle biologists for the next six decades.

It was first classified as a flatworm, then, in the 1990s as a simplified mollusk. It was later discovered that it wasn’t a mollusk at all, but somehow eats mollusks. In recent years, Xenoturbella has been regarded as either close to vertebrates and echinoderms, or as a more distant relative on its own branch further away. Knowing exactly where Xenoturbella belongs is important to understand the evolution of organ systems, such as guts, brains and kidneys, in animals.

Xenoturbellas have only one body opening¾the mouth. They have no brain, eyes, gills, eyes, kidneys or anus. They appear to be evolutionarily simple rather than having lost these organs over time.

"When Greg first spotted the worms gliding through a clam field in Monterey Bay, we jokingly called them purple socks," said MBARI scientist Robert Vrijenhoek, a co-author of the study who led the deep-sea expeditions using remotely operated vehicles.

For the next 12 years, the researchers were on deep-sea hunts to find more “purple socks.” By 2015, they had found all four new species and collected specimens for anatomical and genetic analysis.

The analysis of the animals' genes conclusively identified them as evolutionarily simple members near the base of the evolutionary tree of bilaterally symmetrical animals, which are distinguished by having matching halves through a line down the center. 

This recent discovery also greatly expands the diversity of the known species from one to five.  The largest of the new species, Xenoturbella monstrosa, measuring 20-centimeters (8-inches) long, was found in Monterey Bay and the Gulf of California. The smallest, the 2.5-centimeter (1-inch)-long Xenoturbella hollandorum, was found in Monterey Bay and named in honor of Scripps evolutionary biologists Linda and Nick Holland. The deepest species, Xenoturbella profunda, was discovered in a 3,700-meter (12,139-foot)-deep hydrothermal vent in the Gulf of California. All four species are thought to be associated with deep-sea cold seeps and vents since it’s where their main prey, clams and other bivalve mollusks, are found. How exactly they can get inside the shell using just their mouth still puzzles scientists.

“I have a feeling this is the beginning of a lot more discoveries of these animals around the world,” said Rouse.

Specimens of Xenoturbella churro and the other new species have a new home in the Scripps Benthic Invertebrate Collection

– Annie Reisewitz

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