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Why corals do calisthenics

Pulsating motion appears to flush water for better photosynthesis

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Like hyperactive flowers, xeniid corals open and then clench their little branched tops every few seconds much of the day and night. What makes these coral calisthenics worthwhile, experiments now suggest, could be the way they mix and freshen water to improve coral nutrition.

All that flexing roils the water near the corals and sweeps it upward and away, says marine ecologist Maya Kremien of Hebrew University of Jerusalem. That boosts the photosynthetic capture of energy from the sun to make food for the corals, Kremien and her colleagues report April 22 in Proceedings of the National Academy of Sciences.

Near Israel’s Interuniversity Institute for Marine Sciences in Eilat, Kremien and her colleagues observed the motions and water flow in swaths of the coral Heteroxenia fuscescens growing wild in what she describes as “amazing pulsating carpets.”

As animals, the coral polyps don’t photosynthesize themselves, but rely on live-in algae to do so. In the lab, clusters of H. fuscescens with algae in place more than doubled their net photosynthetic rate when flexing versus resting.  

The boost may come from the way pulsing coral keeps excess oxygen from building up. Photosynthetic algae release oxygen, which competes with carbon dioxide for access to a key enzyme used in photosynthesis. So waving away water full of oxygen can dial up photosynthetic efficiency.

To test this idea, Kremien and her colleagues raised the oxygen concentration in aquarium tanks containing coral. As suspected, highly oxygenated water slowed algal photosynthesis.

Keeping photosynthesis robust may be particularly important to this coral family because algae apparently provide most, or maybe all, of the corals’ food. Many other kinds of corals supplement their diets by snagging little planktonic creatures wafting by. Kremien and her colleagues find little evidence of food snagging in H. fuscescens.

Photosynthesis aside, stirring up water can improve nutrient supplies by mixing in seawater that nearby polyps have not yet depleted of essentials such as nitrogen.

Only corals of the family Xeniidae, including the Heteroxenia in this study, pulsate. They’re one of the families of what are called soft corals, clusters of flexible polyps that don’t create great stony architecture.

“I have been wondering why Heteroxenia pulsates for decades — great to see the mystery solved,” says reef ecologist Katharina Fabricius of the Australian Institute of Marine Science in Townsville. “It creates another mystery though: Why don’t more soft corals do it?”

Shown moving at their natural speed, the flowerlike tops of soft coral polyps in the Xeniidae family open and close for much of the day and night. Keeping the water mixed for better nutrition may be what makes all this work worthwhile.
Credit: M. Kremien et al/PNAS 2013

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