Microbeads reveal how larvae move, eat
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 look like alien starships,” Prakash said.
He put the beads in the seawater with the larvae under a microscope to watch the turbulence they produce 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. Under the microscope, cilia on the surface of a larva look “almost like an ornament — a line that goes around the edge of the animal,” Prakash said.
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. After a year of study, Prakash and his colleagues recently produced detailed mathematical descriptions of how this all works.
The vortexes also 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 humans use 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. Yet the research was never intended to explain a biological phenomenon, Prakash said, beginning only “with a very pure question of shape and beauty and form.”
The starfish larva varies its speed by the number of vortexes it creates.