30 July 2009

Coral zooxanthellae have eyes!

A type of eyespot unique to the coral-dwelling zooxanthellae has been discovered in a recent study.
Living reefs of Terumbu Raya
Zooxanthellae are single-celled algae that live inside corals. Zooxanthellae not only give the coral host its vibrant colours but also produce nutrients through photosynthesis which the coral host needs to grow and stay healthy.

Zooxanthellae can live outside of corals, and may use their eyespot to find a suitable coral host. Young corals, in turn, may be using unknown "attraction mechanisms" to entice zooxanthellae. What's more, coral-dwelling zooxanthellae have eyes only when they are seeking their reef homes. They lose their sight once they are living inside their hosts.

Coral Algae Have "Eyes," Study Says
Christine Dell'Amore, National Geographic News 28 Jul 09;
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 were found to be 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.

Four other types of dinoflagellates—one-celled aquatic organisms that include zooxanthellae—have 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.

The algae, in turn, create oxygen for the coral animals, remove waste, and provide nutrients necessary for survival.

Considering how crucial this partnership is, it may be that roving zooxanthellae use their eyespots to scope out the most desirable digs—a possibility "we think is quite interesting," Koike said.

Young corals, in turn, may be using unknown "attraction mechanisms" to entice zooxanthellae to inhabit the reefs.

What's more, coral-dwelling algae have eyes only when they are seeking their reef homes, Koike added.

The organisms lose their sight once they are living inside their hosts.

By contrast, other types of algae that live inside giant clams keep their eye-like structures while inside their hosts.

Koike speculates this could be because the clam-dwelling algae want to escape the grip of the clam, which "farms" the algae and eats some of them each night.

Climate Concerns

Overall, Koike added, the more scientists know about how corals and their resident algae pair up, the better the chances of preventing corals' ongoing decline due to climate change.

Warmer seawater often causes corals to eject their colorful zooxanthellae roommates, "bleaching" the reefs and leaving the nutrient-deprived corals to die slowly.

"We must understand how this relationship is initiated … as soon as possible," Koike said.

Research appeared July 17 in the journal PLoS One.


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