There’s teamwork—NASA putting people on the moon, for instance, or the Mighty Ducks triumphing over Team Iceland—and then there’s teamwork. A gelatinous sea creature called a salp knows this better than anyone, forming long chains of neurologically connected individuals that work together for the greater good. That is, eating and not dying.
A new study helps unravel the complexities of the salp’s jet-powered, aggregate lifestyle, showing how a creature that’s actually dozens of individuals manages to get around at all. Fascinating stuff in and of itself, and potentially big news for designers of underwater vehicles.
Salps have a goofy way of going about life. Each individual in a chain can reproduce sexually to produce a solitary individual, which you can think of as a barrel. Through one end it sucks water, filtering out planktonic food. It fires the water out the other end as a jet, propelling itself forward. This solitary salp reproduces asexually to make another chain of salps.
How a solitary salp gets around is fundamentally different from how a fish moves. “When a fish wants to produce thrust, it ‘wiggles’ its body and fins, with the side effect of increasing drag from the ideal, stretched straight hydrodynamic shape,” says aerospace engineer Daniel Weihs of the Technion-Israel Institute of Technology, a coauthor of the study. Salps, on the other hand, largely maintain their shape as they jet around. Plus, living together in long chains reduces the surface area exposed to the water, further reducing drag.
So as an aggregate, you’ve got what is essentially a chain of engines, which you might think could get messy—logistically speaking. So do the individual salps coordinate their bursts? As it happens, no, not usually. Their locomotion is asynchronous.
“What we showed with the experiments is that during kind of normal steady state swimming, each individual is essentially behaving as an individual,” says biologist and study coauthor Kelly Sutherland of the University of Oregon. “It’s pulsing at its own frequency, but from this individual behavior you get this emergent behavior that’s really efficient.” (The aggregate will, however, coordinate its bursts when it feels threatened and needs to book it.)
That’s because what you end up with is a community that by its collective jetting can better maintain a steady speed unlike, say, a jellyfish, which has more of a burst thing going on. Think of it like city versus highway driving—you’re going to get better mileage at a steady 65 mph than you would decelerating and accelerating all the time. (I’m not calling out the jellyfish for inefficiency, by the way. It’s actually quite efficient in its own right.)
Weihs and Sutherland also studied the structure of the wake that individuals leave behind. After all, efficiency might take a hit if those wakes get mixed up. “When you compare between the solitaries and the aggregates, the jet wakes really look pretty similar,” says Sutherland. “The key thing is just that you don’t get interactions between those jet wakes.” Everything seems to get along fine.
So in the end you have an aggregate salp that benefits from uncoordinated pulsing yet still manages to generate efficient wake. That could be of great use to anyone designing underwater vehicles. Propellers have their place, of course—good luck moving a cruise ship with salp-like bursts—but certain situations require a more delicate approach, like exploring sunken ships or sneaking around for nefarious purposes.
Not that you should go and slap a bunch of salp-inspired jets all over your underwater robot. But by looking to nature, engineers can mimic the collective wonders of gelatinous sea creatures. After all, there is no “i” in team—or salp for that matter.