Jellyfish-like salp are amazing swimmers, converting their gelatinous bodies into efficient undersea propulsion systems. But these tiny creatures can move even faster and further when banding together to create long, snake-like chains. Researchers have finally figured out how this unusual configuration makes salp even better swimmers.
Video: University of Oregon
Salps are tiny, barrel-shaped marine invertebrates that move through the water by pumping liquid through their gelatinous bodies with pulsed contractions. At the same time, filters along the salps' bodies strain the incoming water, providing the animals with nutritious phytoplankton. These creatures are capable of living in equatorial, temperate or cold seas, and are most abundant in the Southern Ocean near Antarctica, where they form enormous swarms. They can often be found in long inter-chained links, with colonies up to 4.5m long.
New research published in the Journal of the Royal Society Interface shows that salp become much more hydrodynamic when they move as a complete unit. Though their jet spurts may seem chaotic and uncoordinated, the random pulsing of each salp in the chain results in a steady swimming velocity, with significantly decreased drag through the water.
Marine biologists are familiar with salps and their remarkable marine propulsion systems, but previous research into these animals focused solely on the amount of thrust each individual was able to produce, while ignoring the convergent effects of multiple, asynchronous pulses along the chain. The new research, led by University of Oregon biologist Kelly Sutherland, overcomes this limitation.
Using high-speed, high-resolution video, Sutherland's team was able to study the movements of salp both in the wild and at the Liquid Jungle Lab, a research station located on a Pacific island off Panama. Green fluorescent dye was used to visualise the jet wakes, revealing the timing and strength of each individual pulse along the chain. With the help of aerospace engineers, the researchers showed how the snake-like structures produced a "smoother velocity profile" as a result of the uncoordinated, asynchronous swimming movements of individuals.
"Individual jellyfish swim using pulsed jets, and previous work has shown that this is an efficient means of moving through the water," noted Sutherland in a release. "One disadvantage is that pulsed-jetters like jellyfish speed up and slow down with each pulse. Colonial animals with multiple jets — like salps and siphonophores — can time their jets so a whole colony moves at a constant speed."
Sutherland says this explains why salps that swim together can migrate downwards some 1000m during the day, and return to surface at night. It's not immediately clear if (or how) the salps are deliberately "timing" their pulses, but they're definitely able to avoid synchronous pulsing, which would cause the whole structure to lurch back and forth in undesirable ways.
The researchers say their work might help to explain how jellyfish move through the water (salps are not jellyfish, they're chordates, but both animals have simple body forms and eat plankton). Interestingly, salps could also inspire the creation of novel underwater propulsion systems.
"We haven't really moved beyond the propeller when it comes to underwater vehicles," said Sutherland. "Multi-jet vehicles present a highly effective means of transport and also allow for swarm-like behaviour where individual units could break apart from the colony to carry out different objectives."
Autonomous swarms of robotic salp definitely sounds like an idea whose time has come.