ScienceTake: The Beauty of a Starfish Larva at Lunch

James Gorman
James Gorman
SCIENCETAKE

It was one of those lovely nights when the moonlight on Monterey Bay makes you wonder: What would happen if I put some microbeads in with the starfish larvae?

At least that’s what you wonder if you are Manu Prakash, who runs a laboratory at Stanford University and is intrigued by the way life is shaped by the laws of physics.

He was actually in a lab that night, with the bay just outside. He and his colleagues had collected the starfish larvae from the bay with other invertebrates that they were studying.

The larvae were from a species called the bat star and they propel themselves, like many other small invertebrates, by the beating of many, many hairlike cilia. They are exotic and alluring in shape. “They look like alien starships,” Dr. Prakash said.

He put the beads in the seawater with the larvae under a microscope to watch the turbulence they produce around them as they swim. The beads, smaller than red blood cells, follow even small swirls of water and reflect light, so the lines of water flow are visible. What he saw entranced him.

The next day he showed it to William Gilpin, a graduate student in his lab, and Vivek N. Prakash, a postdoctoral researcher. They were all “smitten by the beauty of the patterns,” he said.

So began a yearlong project to understand what the larvae were doing, and why. The researchers reported the results in the journal Nature Physics in December.

The cilia on the surface of a larva, Dr. Prakash said, look “almost like an ornament — a line that goes around the edge of the animal, a high-density band of cilia.”

If they all beat together, the larva moves as fast as it can. But if some patches of cilia beat against the prevailing motion they create vortexes, swirling eddies that the researchers found bring algae close to the surface of the larva, and eventually, to its mouth.

The larva varies its speed by the number of vortexes it creates. It may make as few as two, in which case it swims along at a good clip and eats little, or as many as six, slowing down to munch the daisies, or algae. The Stanford group’s work involved producing detailed mathematical descriptions of how this all works.

A three-minute video produced by the researchers, “Eat, Prey, Swim: Dynamic Vortex Arrays Created by Starfish Larvae,” won a Milton van Dyke Award at the 2016 meeting of the American Physical Society’s Division of Fluid Dynamics, and a 22-second video won first place in the 2016 Nikon Small World in Motion Competition.

One other interesting aspect to the story, Dr. Prakash said, is that the vortexes pull in particles of a certain size, so that the tiny animal gets the food it wants. This kind of filtering is very different from the sieves and nets that human technology uses to separate out particles of a certain size.

It is used by not just starfish larvae, but countless billions of other microscopic invertebrates that filter food from the oceans.

Studying it made Dr. Prakash think that the principles behind this filtering, if well understood, could be adapted to new technologies, filtering particles of a known size without a filter.

This is just speculation so far, but it is a potential practical use of knowledge that came out of pure scientific curiosity of the sort that could seem like idle wool gathering to the nonscientist.

The research, Dr. Prakash said, started “with a very pure question of shape and beauty and form.”