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The average-sized worm slithers across the seafloor like a small armored tank. The worm’s overlapping scales protect its hindquarters from predators, while its hairy appendages help it burrow through mud. This is good life for an earthworm, said Katrine Worsaae, a marine zoologist at the University of Copenhagen.
“Worms love mud,” said Dr. Worsaae
But some types of worms have evolved to leave the mud behind and swim into the water column. Some even live their entire lives suspended in water, never needing to touch the ground. To perform this big take-off, the worms developed less muscle mass and longer appendages that hit like rowing through the water, according to an article published Wednesday in the journal. Royal Society Open Science.
The article is a collaboration between researchers at the University of Copenhagen, the Smithsonian National Museum of Natural History in Washington DC, 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 in the research, compared the swimming evolution of scale worms to the flight evolution of insects. Dr. “Annelids are often thought of as creepy reptiles,” Borda wrote in an email. “This work provides insight into the evolutionary transitions that have taken place to achieve an entirely new lifestyle.”
For many organisms, the bottom of the sea where food sinks is a great place to eat. Also a great place to eat. “Everything there creeps up and eats each other,” said Karen Osborn, a zoologist at the Smithsonian and author of the paper. “If you can learn to swim a little, you can use the water column as a shelter.”
Many scale worms can swim in short bursts, perhaps to escape a predator or capture prey. “They can swim whenever they want,” said Greg Rouse, a marine biologist at the Scripps Institution of Oceanography in San Diego, California, who was not involved in the study, adding that he often sees worms “blow up” when startled. with a remote control vehicle
But for most scale worm species, their heavy bodies inevitably end up back in their muddy home. Dr. “You need a whole new body type to live your entire lifecycle in open water,” Worsaae said.
Dr. There are about 2,000 species of scale worms, of which only a handful are known to swim constantly, Worsaae said. However, Dr. “Obviously, there are a lot more floating scale worms than we previously thought,” Osborn said.
The idea of collaborating on research came from Dr. Worsaae and Dr. It came up while chatting about Osborn – what else? – worms. Dr. Worsaae was fascinated by waterborne scaleworms after seeing a species called Gesiella jameensis swimming in a lava tube in the Canary Islands – the third type of swimming worm she’d seen in caves. Dr. Osborn talked about collecting many scale worms a few hundred meters above midwater, the huge strip of ocean between the seafloor and the surface.
A worm floating hundreds of meters above the seafloor is doing something very different than a crawling worm that occasionally escapes from the seafloor. Moreover, the bodies of the floating worms looked very different. Dr. “They tend to be transparent,” Osborn said. “They have lighter bodies and longer appendages.”
The researchers decided to compare the cave-dwelling worms with their close relatives in the deep sea. They studied five species of scale worms with swimming abilities in places as diverse as the Faroe Islands, the Canary Islands, and the Turks and Caicos Islands.
The researchers also took microCT scans of dead specimens of each species to reconstruct the muscles inside the worms’ appendages. At that time, Dr. Marc Allentoft-Larsen, a graduate student in Worsaae’s lab, led the analysis.
Initially, Dr. Worsaae wondered if worms needed more mass to swim like human swimmers. “We thought they must have invented some fancy muscles that are really good for swimming,” he said.
But the scans revealed that the best swimming worms actually have much less muscle mass and density, and are baggy and more gelatinous than their mud-dwelling counterparts. This is the ideal body type, at least for floating scaly worms.
Swimming scale worms also had long appendages that they could extend to stay afloat and swim in the water column. “They look like little hedgehogs,” said Dr. Worsaae added that the worms will slide the appendages backward as they chase their prey in the water.
The extensions also functioned almost like a series of oars. Dr. Worsaae likened this adaptation to humans’ snorkel fins, which increase the surface area of their limbs and facilitate swimming.
Dr. “It’s much more efficient to wear fins than go to the gym and get double the thighs,” Worsaae said. “It seems like a very simple solution to a very difficult task.”
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