Color-changing brittle stars can see — even without eyes

January 3, 2020, 12:46 p.m.
The red brittle star, Ophiocoma wendtii.
Photo: Lauren Sumner-Rooney

The red brittle star doesn't have eyes in the traditional sense. But this marine creature still is able to use vision to find its way through coral reefs thanks to what scientists call a "neat color-changing trick."

Related to sea stars and sea urchins, Ophiocoma wendtii lives in the bright reefs of the Caribbean Sea. Earlier, scientists discovered that this brittle star is covered in thousands of light-sensitive cells, but they weren't sure what they were for. New research, published in the journal Current Biology, suggests the brittle star uses its color-changing powers for vision.

For years, an international team of researchers has been conducting experiments to test the brittle star's "eyesight."

"These experiments gave us not only the first evidence that any brittle star is able to 'see', but only the second known example of vision in any animal lacking eyes," study co-author Lauren Sumner-Rooney, a research fellow at Oxford University Museum of Natural History, said in a news release.

The brittle star Ophiocoma wendtii shelters in a crevice The brittle star Ophiocoma wendtii shelters in a crevice. (Photo: © Heather Stewart)

In the experiments, the brittle stars look for areas of contrast. The researchers think that these contrasts in the wild may signify structures that could offer safety from predators.

The animals' vision is limited during the day and is nonexistent at night, researchers found.

"We were surprised to find that the responses we saw during the day disappeared in animals tested at night, yet the light-sensitive cells still seemed to be active," said Sumner-Rooney.

In their experiments, researchers compared O. wendtii to a close relative, Ophiocoma pumila, which is also covered in light sensors but doesn't change colors. The red brittle star turns red during the day, but becomes beige at night. The paler, non-color-changing species failed all the eye tests.

"It's a very exciting discovery," said Sumner-Rooney. "It had been suggested 30 years ago that changing colour might hold the key to light-sensitivity in Ophiocoma, so we're very happy to be able to fill in some of the gaps that remained and describe this new mechanism."