Jelly sea creature ‘jet propulsion’ could give robots a boost

Scientists have discovered that colonies of gelatinous sea animals swim through the ocean in giant corkscrew shapes using coordinated jet propulsion.

It’s an unusual kind of locomotion that could inspire new designs for efficient underwater vehicles.

Salps are small creatures that look similar to jellyfish that take a nightly journey from the depths of the ocean to the surface. Observing that migration with special cameras helped researchers capture the macroplankton’s graceful, coordinated swimming behavior.

“Salps are really weird animals.”

“The largest migration on the planet happens every single night: the vertical migration of planktonic organisms from the deep sea to the surface,” says Kelly Sutherland, an associate professor in biology at the University of Oregon’s Oregon Institute of Marine Biology, who led the research.

“They’re running a marathon every day using novel fluid mechanics. These organisms can be platforms for inspiration on how to build robots that efficiently traverse the deep sea.”

Despite looking similar to jellyfish, salps are barrel-shaped, watery macroplankton that are more closely related to vertebrates like fish and humans, says Alejandro Damian-Serrano, an adjunct professor in biology. They live far from shore and can live either as solitary individuals or operate in colonies, he says. Colonies consist of hundreds of individuals linked in chains that can be up to several meters long.

“Salps are really weird animals,” Damian-Serrano says. “While their common ancestor with us probably looked like a little boneless fish, their lineage lost a lot of those features and magnified others. The solitary individuals behave like this mothership that asexually breeds a chain of individual clones, cojoined together to produce a colony.”

But the most unique thing about these ocean creatures was found during the researchers’ ocean expeditions: their swimming techniques.

Exploring off the coast of Kailua-Kona, Hawaii, Sutherland and her team developed specialized 3D camera systems to bring their lab underwater. They conducted daytime scuba dives, “immersed in infinite blue,” as Damian-Serrano described, for high visibility investigations.

They also performed nighttime dives, when the black backdrop allowed for high-contrast imaging of the transparent critters. They encountered an immense flurry of different salps that were doing their nightly migration to the surface—and many photobombing sharks, squids, and crustaceans, Sutherland says.

Through imaging and recordings, the researchers noticed two modes of swimming. Where shorter colonies spun around an axis, like a spiraling football, longer chains would buckle and coil like a corkscrew. That’s called helical swimming.

Helical swimming is nothing new in biology, Sutherland says. Many microorganisms also spin and corkscrew through water, but the mechanisms behind the salps’ motion are different.

Microbes beat water with hair-like projections or tail whips, but salps swim via jet propulsion, Sutherland says. They have contracting muscle bands, like those in the human throat, that pump water sucked from one side of the body and squirted out the other end to create thrust, Damian-Serrano says.

The researchers also noticed that individual jets contracted at different times, causing the whole colony to steadily travel without pause. The jets were also angled, contributing to the spinning and coil swimming, Sutherland says.

“My initial reaction was really one of wonder and awe,” she says. “I would describe their motion as snake-like and graceful. They have multiple units pulsing at different times, creating a whole chain that moves very smoothly. It’s a really beautiful way of moving.”

Microrobots inspired by microbial swimmers already exist, Sutherland says, but this discovery paves the way for engineers to construct larger underwater vehicles. It may be possible to create robots that are silent and less turbulent when modeled after these efficient swimmers, Damian-Serrano says. A multijet design also may be energetically advantageous for saving fuel, he says.

Beyond microbes, larger organisms like plankton have yet to be described in this way, Sutherland says. With Sutherland’s new and innovative methods of studying sea creatures, scientists might come to realize that helical swimming is more pervasive than previously thought.

“It’s a study that opens up more questions than provides answers,” Sutherland says. “There’s this new way of swimming that hadn’t been described before, and when we started the study we sought to explain how it works.

“But we found that there are a lot more open questions, like what are the advantages of swimming this way? How many different organisms spin or corkscrew?”

The study is published in Science Advances. Additional coauthors are from Louisiana Universities Marine Consortium, University of South Florida, Roger Williams University, Marine Biological Laboratory, and Providence College.

The Gordon and Betty Moore Foundation and the Office of Naval Research supported the work.

Source: Leila Okahata for University of Oregon