purple sea urchin Strongylocentrotus purpuratus
April 11, 2013

Turbulent Times: Turbulence Necessary For Settling Sea Urchin Larvae

Alan McStravick for redOrbit.com - Your Universe Online

The purple sea urchin (Strongylocentrotus purpuratus) has been receiving a bit of attention lately on the pages of redOrbit and in academic journals. As it turns out, this sea creature is particularly efficient at adaptive success. redOrbit´s own Lee Rannals wrote yesterday about how this spiny creature seems suited to a future where climate change plays a significant role. And today, we learn that even in their youngest state, these creatures read the turbulence of the sea in which they drift to help them determine where to settle and transform themselves into adolescence.

The study, which focused on the purple sea urchin larvae, was conducted by researchers at the University of California, Davis, Bodega Marine Laboratory and has been published in the journal Proceedings of the National Academy of Sciences.

"How these animals find their way to the right habitat is a fascinating problem," said Brian Gaylord, professor of evolution and ecology at UC Davis and a researcher at the Bodega Marine Lab. "The turbulence response allows them to tell that they're in the right neighborhood."

The life-cycle of the purple sea urchin, like many shoreline creatures, is one that involves two stages. The young are typically microscopic. Their appearance is completely different from how they will look once they reach adulthood. As larvae, they drift the open sea on the upper levels of the ocean for about a month. They transition into adulthood once they settle upon a rocky shore. It is here they take on the more familiar appearance as a spiny adult.

"Once they decide to settle, they attach to a rock and undergo body remodeling into a juvenile sea urchin with spines," Gaylord said.

When they are within range of a rocky shoreline, the larvae can sense the chemical traces in the water that are given off by algae covered rocks, letting them know this is a particularly suitable area for sustenance and maturation.

The UC-Davis researchers, however, wanted to find out how the larvae knew they were close enough to one of these habitable shorelines to begin searching for these chemical signals in the first place.

The California and Oregon coastlines have several rocky headlands which are the preferred environment for the spiny urchins. However, interspersed among the rocky shores are long stretches of beach. The smooth beaches offer little turbulent activity to the incoming sea. And the larvae simply can´t swim the long beaches looking for an algae covered rock. However, the sea provides a more turbulent environment when it encounters a rocky reef. The scientists learned it is the increased turbulence that signals the larvae to begin their search for a rock on which to rest.

The team of researchers, including Jason Hodin of Stanford University´s Hopkins Marine Station and Matthew Ferner of San Francisco State University, employed the use of a device called a Taylor-Couette cell to see how urchin larvae responded to being churned by shear forces comparable to those in waves breaking on a rocky shore.

The Taylor-Couette cell is a device consisting of one rotating cylinder inside another, with a layer of fluid in between. Typically used in the field of fluid dynamics, this cell was able to create a turbulent situation similar to that which the larvae might experience as they approach a rocky beachhead.

Once the larvae were put into the Taylor-Couette cell, the researchers exposed them to potassium which is known to act as a chemical signal responsible for triggering larvae to begin their search to settle.

What they found was larvae that had been exposed to a turbulent environment responded earlier to this chemical signal than larvae that didn´t experience the turbulence. In fact, the researchers claim the larvae reacted at a stage in their development that was previously believed a time the larvae couldn´t possibly settle.

The team states their results were especially telling in that neither the turbulence nor the chemical signal on their own could prompt the larvae to settle at the earlier developmental stage. They had to exist together.

Gaylord commented, “The experiment shows that the shift from living free in the ocean to living on a rock is a two-step process.” The first step, exposure to turbulence initiates an abrupt transition to a state in which the larvae are considered competent to settle. A chemical signal then triggers the second step.

It's not yet clear how the larvae detect turbulence, Gaylord said. That might happen through receptors that respond to stretching or flexing. The two-step settlement process might occur in other species that settle on shorelines, he said.

The work was supported by the National Science Foundation and the National Oceanic and Atmospheric Administration.

Image 2 (below):  Top: The larva of the purple sea urchin drifts in the ocean for a month before settling on a rock. Bottom: Then the larvae change shape and grow spines as they prepare to settle down on a rocky shore. Credit: Brian Gaylord, UC Davis