The Mesmerizing Motions of Starfish Larvae [Video]

This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American

Want an easy (and mesmerizing) art project? Gently squish a starfish larva between a glass slide and cover slip and add tiny beads to the surrounding water. Place under microscope. This will be the result:

As you can see baby starfish (a.k.a. larvae) not only look nothing like starfish, they are covered in beating hairs called cilia whose motions make them look much more like single-celled microbes than nearly the closest living relatives of vertebrates.

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These videos were captured by Stanford scientists William Gilpin, Vivek Prakash, and Manu Prakash, authors of a paper that appeared in Nature Physics last December. They were inspired to study the larvae while taking a summer class in embryology at Stanford's Marine Station in Pacific Grove, California. They noticed the little animals had a curious shape and decided to investigate why that might be. Only once they brought the larvae into their lab and began feeding and observing them under the microscope did they discover to their astonishment the mesmerizing flow fields the larvae generate.

Like the drifting larvae of many marine invertebrates, a starfish larva has bands of cilia on its sinuous margins. It is hard to believe, but a single larva bristles with 100,000 cilia.

As you saw in the video, the larvae can throw short sections of their ciliary bands into reverse. When sections of cilia alternate stroke direction, vortices form.  By shifting which sections of cilia beat in which direction, the larvae can alter the pattern of vortices that form around them in ways that foster different activities. As the video suggests, these particular baby starfish seem to have two primary gears: feed and swim.

The difference between those two gears is the number of ciliary band sections that reverse, and hence the number of vortices. Each vortex creates eddies that pull plankton toward the surface of the larva. More vortices = more eddies = more food captured.  

When the larva is in feeding mode and an alga nears the surface of the larva, the sudden reverse stroke of a single cilium can deliver the hapless prey to its doom. Once it touches the larva, prey are delivered to the larva’s mouth either by water currents moving mouthward or by being passed from cilium to cilium. When enough algae have collected around the mouth, the baby starfish takes a bite. 

However, more cilia paddling away from the direction in which the larva would like to travel is obviously a bit of a drag. When the larva decides it’s time to move to greener currents, it beats more of its cilia in the same direction and creates fewer vortices. Fewer vortices equals more forward motion. It's a classic example of a trade-off, the authors wrote, one substantiated by mathematical experiments and computer modeling they conducted..

It’s also possible, the authors say, that these larvae have lesser-used third or even fourth gears, and can use their vortex fields as camouflage by masking “mechanical strain fields” that attract predators. I assume, though am not certain, that that’s a fancy way of saying they can use their cilia to disturb the water around them less or to blend in with the surrounding currents if they are trying to avoid being noticed. That would be amazing. It would be as if they larva had a cloaking device, a la Star Trek. For now, this possibility is still just (fun) speculation.

Reference

Gilpin, William, Vivek N. Prakash, and Manu Prakash. "Vortex arrays and ciliary tangles underlie the feeding-swimming trade-off in starfish larvae." Nature Physics (2016).