The single-celled algae that set up house inside hard corals and give reefs their vibrant colors may be able to see, a new study says.
The algae—called zooxanthellae—have mysterious crystal-like deposits, which are made of uric acid, a common element in light-reflecting structures in insect and animal eyes.
The substance in the algae had been previously misidentified as calcium oxalate, which is often found in plants, the researchers say.
The algae's crystal clusters strongly reflected light in lab experiments, suggesting that "this is really a functional eye," study co-author Kazuhiko Koike, of Japan's Hiroshima University, said in an email.
Each of the single-celled orgamisms also contains a photoreceptor molecule, which creates an "eyespot."
Eyespots are light-sensitive patches that allow simple organisms, such as jellyfish and some other algae, to sense their environments.
(Related: "Brittle Star Found Covered With Optically Advanced 'Eyes.'")
Other types of dinoflagellates—one-celled aquatic organisms that include zooxanthellae—have at least four variations of eyespots, Koike said.
But he believes the newfound type of eyespot is unique to the coral-dwelling life-forms.
House Hunting
In shallow tropical waters of the world's oceans, zooxanthellae and reef-building coral polyps have evolved to be dependent on one another.
The corals' reefs give the algae natural havens and ingredients for photosynthesis.
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