Turbulence Helps Sea Urchins Settle in Right Place

Researchers at the University of California, Davis, reveal how the larvae of purple sea urchins settle in the right place in order to grow into adults.

Researchers at the Bodega Marine Laboratory have found that when purple sea urchins hit the rocky shore while tumbling in the waves, they realize it's time to look to a perfect place to grow into an adult.

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

Purple sea urchins, also known as Strongylocentrotus purpuratus, have a two-stage life cycle. When compared to adult sea urchins, the younger ones are microscopic and often float in the upper levels of the ocean. They continue to float for nearly a month until they find a rocky shore in order to transform into spiny adults.

On finding the correct place, they stick to a rock and go through changes in their body to convert into a spiny juvenile sea urchin.

The larvae respond to chemical traces over short distances. They are quick in responding if the substance is from rock, which has thick algae growth and other food that the growing urchins use.

The researchers found that the larvae lack the resources to swim for miles along the beach in hunt of slimy rock. However, on being carried by currents near a wave-swept rocky reef, the high turbulence acts like an indicator to start the search for a suitable location.

With the help of a device called a Taylor-Couette cell, the researchers checked how the urchin larvae responded on being tossed by force, when compared to those in waves that hit the rocky shore. The device consists of a rotating cylinder placed inside another with a layer fluid separating the two. As the cylinders spin relative to each other, they create a shear force in the fluid.

The study was conducted by Gaylord and co-authors Jason Hodin of Stanford University's Hopkins Marine Station and Matthew Ferner of San Francisco State University. 

On spinning the sea urchins through a Taylor-Couette cell and later on exposing them to potassium, they noticed that the larvae responded to the chemical signal earlier. In fact, the researchers were surprised to see them respond at a stage at which they thought the larvae could not settle.

The researchers are not sure how the larvae detect the turbulence. They predict that they are able to do so with the help of receptors that respond to stretching or flexing.