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.
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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