Propelled by Jellies

Oceanographer Kelly Sutherland is studying a species of sea jelly (please don’t call them jellyfish) that uses multiple “swimming bells” to propel itself through the water, as depicted by the photo illustration above. These water-shooting jets set the species, found in the Puget Sound and known as Nanomia bijuga, apart from other colonial siphonophores (marine invertebrates that swim together in chainlike formations).

“This is relatively rare in the animal kingdom,” notes Sutherland. “Most organisms that swim with propulsion do so with a single jet.”

Each of these creatures is composed of between four and 12 separate, but genetically identical, units. The coordination of the units could provide inspiration for next-generation underwater vehicles that use multiple engines for propulsion. “They can turn on a dime, and very rapidly,” Sutherland says.

The organisms are small—rarely more than two inches long—with tentacles that extend up to a foot. Sutherland says they look a bit like a bunch of small jellies strung together. The siphonophores use a coordinated effort to move through the water, with a clear division of labor.

“The younger swimming bells at the tip of the colony are responsible for turning,” Sutherland says. “They generate a lot of torque. The older swimming bells toward the base of the colony are responsible for thrust.” Their tentacles capture zooplankton, the tiny organisms that these jellies consume, she adds.

To understand how these jellies pulse water to maneuver, researchers placed sample colonies in small, custom-built tanks. The jellies’ movements were captured with high-speed digital photography—at 1,000 frames per second. The data were analyzed with particle image velocimetry, a technique that provides instantaneous velocity measurements.

Most sea animals and human-engineered submarine vehicles alike rely on jet thrusters that turn to change direction, a practice that, Sutherland said, is complicated from a design or engineering standpoint.

“These jellies have a slight ability to turn their individual jets, but they don’t need to,” she says. “With multiple static jets they can achieve all the maneuverability they need. Designing a system like this would be simple yet elegant. And you have redundancies in the system. If one jet goes out, there would be little loss of propulsion.”

Can jelly-powered submarines be far behind?

—By Kelly Sutherland

Kelly Sutherland holds appointments at the Oregon Institute of Marine Biology and the Robert D. Clark Honors College. She collaborated with lead author John “Jack” H. Costello, of Providence College in Rhode Island, and colleagues from several other institutions on the research, which was supported by a grant from the National Science Foundation. Their findings were published in the journal Nature Communications.