Crash of the waves calls sea urchins home

The purple sea urchin knows it is time to settle down when it senses the turbulence of breaking waves on a rocky shore, according to a new study by a team of California researchers.

To test this idea, the scientists took groups of sea urchin larvae for a spin, inside a unique laboratory device that mimics the toss and turn of a high-energy surf zone.

A photo of a purple sea urchin

Purple sea urchin

As larvae, the urchins float freely in the open ocean, said co-author Matt Ferner, a research scientist at San Francisco State University’s Romberg Tiburon Center for Environmental Studies. Until now, it’s been something of a mystery as to how the larvae find their way back to the right kind of shoreline where they develop spines and the tube feet that anchor them to rocks as adults.

“We tried to fill that black box between the dispersal of larvae offshore and the recruitment of juvenile urchins onshore,” explained Ferner, who studies the ecology of marine invertebrates at the San Francisco Bay National Estuarine Research Reserve.

Urchin larvae receive chemical cues from the shoreline -- dissolved in the seawater and bound to the rocks -- that tell them that it is time to settle down. But these signals only emanate over tiny distances, typically no more than a few centimeters. So the researchers looked for other environmental cues that might guide the drifting larvae to a more permanent home.

Since adult urchins often live in high-turbulence zones of breaking waves, the researchers decided to find out whether turbulence itself could be a signal to the urchins that they were near a shoreline favorable to settlement.

They placed larvae inside a device called a Taylor-Couette cell. The cell consists of one stationary cylinder nested within another rotating cylinder, separated by a layer of fluid. When the outer cylinder spins, it creates predictable shear forces and eddies in the fluid that resemble the turbulence urchin larvae might experience along a rocky shore.

Larvae tossed about in the device were dramatically more likely to settle down, after receiving the proper chemical cues, than those who spent their time in a calm fluid. “When you expose them to high levels of turbulence and then give them a standard chemical inducer, “ Ferner said, “within minutes they decide it’s time to settle.”

Turbulence alone doesn’t tell the larvae to settle, Ferner and his colleagues concluded, but it appears to accelerate the larvae’s transition into a state where they are receptive to the necessary chemical signals that trigger settlement.

The researchers haven’t identified the exact molecular mechanism the larvae use to sense turbulence. Luckily, there is a wealth of information about the biology of the animal that scientists can draw on to explore this question, Ferner said. “The purple sea urchin is more than just a tidepool creature that folks have seen as they walk along the coast out here. It’s also sort of a lab rat. Its genome has been completely sequenced.”

The researchers would also like to compare related species that live in different kinds of shore environments, to see if their larvae are “tuned” to sense differences in turbulence. “Species that live in calmer water may respond to even lower levels of turbulence than in our experiment,” Ferner said, “because that would be reflective of their adult habitat.”

Ferner is especially curious to learn what becomes of the urchin larvae when they get the signal to settle too soon, before they are ready to complete the transformation into their juvenile form. “When they settle too early, the juvenile urchins don’t have well-developed spines or well-developed tube feet,” he explained, “and so they’re probably less able to hold on to the rocks when the next wave comes and bashes them.

“They may not be able to defend themselves as well against predators, or gather food effectively, because they’ve committed to becoming an adult when they’re still too small.”

The new study, “Turbulent shear spurs settlement in larval sea urchins,” was published in the April 9 early edition of the Proceedings of the National Academy of Science.

-- University Communications