A lined chiton sits on coralline algae along the California coast. The creature has hundreds of tiny-eye like structures dotting the plated shell on its back.
Credit: Chad King/Monterey Bay National Marine Sanctuary
A close-up image of the eye lenses on chiton shells. Researchers have found that chitons can actually see through these hundreds of tiny lenses, which are formed from aragonite, a carbonate mineral.
Credit: Kevin M. Kocot
SYDNEY: They may not be too bright but U.S. scientists say the humble chiton's hundreds of crystal eyes are complex enough to watch out for predators.
Chitons are small, rock-dwelling molluscs found around coastlines worldwide. While other organisms have eyes with lenses of calcite, a carbonate mineral, the West Indian fuzzy chiton ( Acanthopleura granulata) is the first known creature to form its eye lenses from the closely related aragonite. The discovery reveals new insights into the diversity of eyes.
"Chitons ... simply make their lenses out of the same material as the shell plates in which their eyes are embedded," said lead author Daniel Speiser, a biologist at the University of California, Santa Barbara. "Animals are clearly quite resourceful and opportunistic in fashioning eyes out of whatever materials are handy."
Working eyes play fewer tricks
The study, which will appear in Current Biology, found the chiton's hundreds of eyes form images, rather than simply being able to detect changes in light - an ability present in other chiton species which may have photosensitive abilities rather than developed eyes.
"The animal is able to distinguish between passing shadows and actual objects that might represent predators," said Speiser. "This means they won't get tricked into giving a defensive response every time a cloud passes overhead."
Surprisingly their eyesight, while limited, works equally well in water and on land, a tricky feat because light interacts differently with air and water, Speiser said, yet makes senses because they live at the shoreline.
Eye test for chitons
In order to test their vision, Speiser placed individual chitons with and without eyes on a flat surface. He then held a black disk - or target - against a white background followed by a grey screen that blocked the same amount of light, and gauged their reactions.
The chitons with eyes were unresponsive to the grey screen, but were observed clamping down their armoured, oval-shaped bodies beneath the presence of the black target. These tests were repeated when the chitons were submerged. This response shows that the eyed chitons were able to see the disk, said Speiser.
He suggested that spatial vision could work as a defence mechanism for the slow-moving chitons, allowing their many upward-facing eyes to survey a wide field of view without moving around.
Eyesight helps chitons feed
Karen Gowlett-Holmes, a marine scientist from Australia's national science organisation CSIRO, said it's not surprising that the small creatures have formed their eye lenses from aragonite given the complex structures already found on their shells.
"The fact that the lenses are birefringent [doubly refractive], using both refractive indices of aragonite, and therefore equally effective in air and water, means that the material obviously has a lot of advantages," said Gowlett-Holmes.
She said chitons with eyes tend to be the ones best adapted to living in intertidal zones or shallow water, where the advantage of eyesight can be maximised to help them feed.
Being able to differentiate between changes in light and approaching predators means less time and energy is wasted on false alarms and more energy is spent grazing, she said. "As the time they can spend feeding is usually restricted by tides, anything that can maximise this is a real advantage."
