Jet-Propelled, Snake-like Salp Colonies Trace Huge Helices in the Ocean

WOODS HOLE, Mass.— Colonies of gelatinous sea animals swim through the ocean in giant corkscrew shapes using coordinated jet propulsion, an unusual kind of locomotion that could inspire new designs for efficient underwater vehicles, researchers report this week in Scientific Advances,

The research involves salps, small creatures that look like jellyfish and take a nightly journey from the depths of the ocean to the surface. Observing that migration with special cameras helped scientists from University of Oregon (UO), the Marine Biological Laboratory, Woods Hole, and their colleagues capture the macroplankton’s graceful, coordinated swimming behavior that traces huge helices in the ocean.

“The largest migration on the planet happens every single night: the vertical migration of planktonic organisms from the deep sea to the surface,” said Kelly Sutherland, associate professor in biology at the UO’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 their resemblance to jellyfish, salps are barrel-shaped, watery macroplankton that are more closely related to vertebrates, said Alejandro Damian-Serrano, an adjunct professor in biology at the UO. They can live either as solitary individuals or operate in colonies, he said. Colonies consist of hundreds of individuals linked in chains that can be up to several meters long.

“Salps are really weird animals,” Damian-Serrano said. “While their common ancestor with [humans] 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 both daytime and nighttime SCUBA dives, when they encountered an immense flurry of different salps that were doing their nightly migration to the surface. 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. 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. They have contracting muscle bands, like those in the human throat, that pump water sucked in from one side of the body and squirted out the other, to create thrust.

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 said.

“My initial reaction was really one of wonder and awe,” she said. “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 said, 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 said. A multijet design also may be energetically advantageous for saving fuel, he said.

Beyond microbes, larger organisms like plankton have yet to be described in this way, Sutherland said. 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 said. “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?”

Co-authors John Costello of Providence College and Sean Colin of Roger Williams University, who are Whitman Center scientists at the Marine Biological Laboratory in Woods Hole, Mass., developed and tested some of the methods used in this study for underwater videography of marine animals and analysis of their locomotion patterns. Their collaboration with Sutherland is “part of a long-term goal of studying delicate, oceanic gelatinous animals in their natural environment to collect data that has previously been inaccessible using conventional oceanographic techniques,” Colin said.

The study also included collaborators from Louisiana Universities Marine Consortium and University of South Florida.

This work was supported by Gordon and Betty Moore Foundation and Office of Naval Research.

Citation:

Kelly R. Sutherland, Alejandro Damian-Serrano, Kevin T. DuClos, Brad J. Gemmell, Sean P. Colin and John H. Costello (2024) Spinning and corkscrewing of oceanic macroplankton revealed through in situ imaging. Science Advances, DOI:10.1126/sciadv.adm9511

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The Marine Biological Laboratory (MBL) is dedicated to scientific discovery – exploring fundamental biology, understanding marine biodiversity and the environment, and informing the human condition through research and education. Founded in Woods Hole, Massachusetts in 1888, the MBL is a private, nonprofit institution and an affiliate of the University of Chicago.