Nature

14
Jan

How Sea Stars See

It sounds like the description of an alien in a sci-fi horror film. The creature moves around on tube feet, can regrow its arms, and disgorges its stomach into prey that’s larger than itself. Oh, and it has eyes on each of its many arms.

Yet its not the fabrication of an overactive scriptwriter’s mind. It’s a sea star. We’ve known about many of sea stars’ remarkable traits for years, and scientists suspected that structures on the end of each arm functioned as eyes. But they didn’t know whether the eyes were of any use to the sea stars.

linckia
A blue sea star, Linckia laevigata, on the reef.

To test how sea stars may use that vision, Anders Garm and Dan-Eric Nilsson, of the University of Copenhagen and Lund University in Sweden, respectively, first examined the structures of blue sea stars (Linckia laevigata) in detail, revealing a compound eye on each foot. Those eyes, however, are relatively crude. They don’t have lenses and can’t see in great detail.

But Garn and Nilsson suspected that those simple images were enough to help the sea stars get around.

Sarah Zielinski, writing for Science News:

So here comes the gruesome part: They went to a reef in Japan where the sea stars live and used a pair of scissors to cut off the eyes on five of them, along with a couple of tube feet and ossicles. And as a control, the scientists did a sham operation on five more starfish that removed tube feet and ossicles only. (This seems really mean, I know, but starfish can grow back whole arms, so it’s not quite as bad as it sounds.)

After letting the animals heal overnight, the researchers placed the starfish about a meter away from a reef and watched as the sea stars began to move slowly. Nearly all of the sham-operated starfish made a beeline for the reef, but the eyeless sea stars just wandered. Similarly, intact starfish left a meter from the reef on a moonless night also failed to find their way home.

Clearly, there’s more to sea stars than meets the eye. Information about sea star vision not only contributes to our growing understanding of the evolution of sight—which has evolved on many different occasions—but it could also be helpful for future research into sea star ecology. Garm is already planning to apply this research with a different species, the crown-of-thorns sea star (Acanthaster planci), which can have a devastating effect on coral reefs. By determining how the echinoderm finds a reef, he hopes to create traps that would capture the predators before they cause any lasting damage.