Starfish larvae found eating, running by creating water whorls

A study has identified that starfish larvae, each smaller than a grain of rice, create whorls of water either to bring food to them or propel themselves to better feeding grounds.

In experiments starting in the summer of 2015 at Stanford University's Hopkins Marine Station in Pacific Grove, California, researchers fed starfish larvae nutrient algae and observed their movements with video-enabled microscopes.

The larvae spend 60 days and 60 nights paddling the open ocean before metamorphosing into the familiar star shape.

"Our first eureka moment came when we saw the complex vortices flowing around these animals," said Vivek Prakash, a postdoctoral scholar in bioengineering. "This was beautiful, unexpected and got all of us hooked. We wanted to find out how and why these animals made these complex flows."

The vortices were puzzling because they seemed to make no evolutionary sense and it took a lot of energy to create spiral flows of water, said William Gilpin, first author on a paper published in the journal Nature Physics and a graduate student in the lab of Manu Prakash, an assistant professor of bioengineering.

To resolve the mystery, the experiment zeroed in on the larva's cilia, from the Latin word for eyelashes, which work like the oars that might be used to row an ancient galley, except that each larva has about 100,000 oars, arranged in what the researchers call ciliary bands that gird the organism in a pattern far more complex than any galley's oars.

The cilia had three potential actions: forward, reverse and stop. And just as with oars, the cilia moved in different synchronized patterns to create different motions. Presumably orchestrated by its nervous system, the larva beats its 100,000 eyelashes in certain patterns when it wants to feed, so as to swirl the water in a way that brings algae close enough to grab. Then, with a different flutter of eyelashes, the larva creates a new pattern of whorls and speeds off.

Realizing that they were observing a previously unknown mechanism that improved the larva's odds of survival, the researchers theorized that the physical structure of the starfish larva, controlled by its nerves, allows it to make feed-versus-speed tradeoffs: lingering whenever algae are plentiful, then darting off should nutrients grow scarce.

"We have shown that nature equips these larvae to stir the water in such a way as to create vortices that serve two evolutionary purposes: moving the organisms along while simultaneously bringing food close enough to grab," bioengineer Prakash, a member of Stanford Bio-X and Stanford ChEM-H and an affiliate of the Stanford Woods Institute for the Environment, was quoted as saying in a news release. Endit