These Worms Left the Ocean Floor and Never Looked Back

The average scale worm trudges along the seafloor like a tiny armored tank. The worm’s overlapping scales shield its backside from predators, while bristled appendages help it scuttle through the mud. This is a good life for a worm, said Katrine Worsaae, a marine zoologist at the University of Copenhagen.

“Worms love mud,” Dr. Worsaae said.

But some scale worm species have evolved to leave the mud behind and swim up into the water column. Some even live their whole lives suspended in water, never needing to touch the ground. To accomplish this grand liftoff, the worms evolved less muscle mass and elongated appendages that stroke through the water like oars, according to a paper published on Wednesday in the journal Royal Society Open Science.

The paper is a collaboration between researchers at the University of Copenhagen, the Smithsonian National Museum of Natural History in Washington, D.C., and the Monterey Bay Aquarium Research Institute in California.

Elizabeth Borda, an evolutionary biologist at Texas A&M University-San Antonio who was not involved with the research, compared the scale worms’ evolution of swimming to insects’ evolution of flight. “Annelids are usually thought of as creepy crawlies,” Dr. Borda wrote in an email. “This work provides insight into the evolutionary transitions that took place to achieve an entirely new lifestyle.”

For many organisms, the bottom of the sea, where food sinks, is a great place to eat. It’s also a great place to be eaten. “Everything crawls around there and eats each other,” said Karen Osborn, a zoologist at the Smithsonian and an author on the paper. “If you can learn to swim even just a little bit, you can use the water column as a refuge.”

Many scale worms can swim in brief spurts, perhaps to escape a predator or catch prey. “They can swim when they want,” said Greg Rouse, a marine biologist at the Scripps Institution of Oceanography in San Diego, Calif., who was not involved with the research, adding that he often sees the worms “take off” when startled by a remotely operated vehicle.

A Gesiella species of scale worm swimming in the waters of the Túnel de la Atlántida, an undersea lava tube in waters off the Canary Islands.CreditCredit…Allentoft-Larsen et al., Royal Society Open Science, 2021

But for most scale worm species, their heavy bodies inevitably sink back down to their muddy homes. “To live your entire life cycle up in open water, you need a whole new body type,” Dr. Worsaae said.

There are around 2,000 species of scale worms, only a “handful” of which are known to swim continuously, Dr. Worsaae said. But there are “obviously far more scale worms that swim than we thought before,” Dr. Osborn said.

The idea to collaborate on research was sparked when Dr. Worsaae and Dr. Osborn were chatting about — what else? — worms. Dr. Worsaae had become fascinated by waterborne scale worms after seeing a species called Gesiella jameensis swimming in a lava tube in the Canary Islands — the third species of swimming worm she’d seen in caves. Dr. Osborn mentioned collecting many scale worms in several hundred yards up in the midwater, the gigantic swath of ocean between the seafloor and the surface.

A worm swimming hundreds of yards above the seafloor is doing something very different than a crawling worm occasionally fleeing the seafloor. Moreover, the bodies of the swimming worms looked very different. “They tend to be transparent,” Dr. Osborn said. “They have lighter bodies and longer appendages.”

The researchers decided to compare the swimming worms in the caves to their close relatives in the deep sea. They examined five species of scale worms with swimming aptitudes, in locations as varied as the Faroe Islands, the Canary Islands and Turks and Caicos.

The researchers also took microCT scans of dead specimens of each species to reconstruct the muscles inside the worms’ appendages. Marc Allentoft-Larsen, then a masters student in Dr. Worsaae’s lab, led the analysis.

Initially, Dr. Worsaae wondered if the worms might need more bulk to swim, as human swimmers do. “We thought they must have invented some fancy muscles that are really good for swimming,” she said.

But the scans revealed the worms that could swim best actually had drastically less muscle mass and density and were baggier and more gelatinous than their mud-dwelling counterparts. This is the ideal body type, at least for swimming scale worms.

The swimming scale worms also had elongated appendages, which they could extend to stay buoyant and float in the water column. “They look like little porcupines,” Dr. Worsaae said, adding that the worms would slick the appendages back down while chasing prey through the water.

The appendages also functioned almost like a set of oars. Dr. Worsaae compared this adaptation to snorkeling fins, which increase the surface area of people’s limbs, making it easier to swim.

“It is much more efficient to put on a fin than to go to the gym and get double-size thighs,” Dr. Worsaae said. “It seems to be a very simple solution to a very difficult task.”

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