Colonial jellyfish have an unusual feature: multiple water-shooting jets. This simple but elegant system makes them great swimmers.
A single colonial siphonophore (Nanomia bijuga) resembles a bunch of tiny jellyfish strung together and is much different than larger, single-jet-powered jellyfish. Individual colonies have four to 12 jet-like swimming bells, or structures known as nectophores.
“This is a very interesting system for studying propulsion, because these jellies have multiple swimming bells to use for propulsion,” says Kelly Sutherland, an oceanographer with the Oregon Institute of Marine Biology at the University of Oregon. “This is relatively rare in the animal kingdom. Most organisms that swim with propulsion do so with a single jet. These siphonophores can turn on a dime, and very rapidly.”
Torque and thrust
To understand how these jellies pulse water to maneuver their barely two-inch-long bodies, researchers placed some in small custom-built tanks and added neutrally buoyant seeding particles as tracers.
With the tanks lit with thin, 2D laser sheets, scientists captured the jellies’ movement with high-speed digital photography—at 1,000 frames per second. The data was analyzed with particle image velocimetry, which provides instantaneous velocity measurements.
Most animals and human-engineered vehicles rely on jet thrusters that are turned to change directions, a practice that, she says, is complicated from a design or engineering standpoint.
“The younger swimming bells at the tip of the colony are responsible for turning,” says Sutherland. “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 jellyfish consume, she adds.
‘Simple yet elegant’
The research gives insight on how animals can achieve complex levels of maneuverability and performance with relatively simple components, says John “Jack” H. Costello of Providence College in Rhode Island, the lead author of the study in Nature Communications.
“The nectophores of these jellies appear to be fairly simple jet-producing structures,” he says. “When swimming forward, the jets are essentially stereotypic in direction—they appear to jet in a consistent direction. The complexity of turning is achieved by alternating which units contract and how strongly they jet.”
These jellies, Sutherland says, can slightly turn their jets but don’t need to do so.
“With multiple static jets they can achieve all the maneuverability they need,” she adds. “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.”
Nanomia bijuga are members of the phylum Cnidaria, whose members have specialized stinging cells that are used mainly for capturing prey. They can swim hundreds of meters each night, when they migrate to the surface. By day they swim to the depths.
Under a National Science Foundation grant to Sutherland, the researchers gathered the jellies from Puget Sound off Washington’s San Juan Island.
Other coauthors on the paper are from Rhode Island’s Roger Williams University, the University of South Florida, and Stanford University.
Source: University of Oregon