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Coral Withstands Bleaching By Feeding

coral, bleached



JON WITMAN/BROWN UNIVERSITY
Coral reefs are the backbone of marine life. Now, these rainbow-hued constructions of millions of tiny polyps are being bleached bone-white by pollution and climate change, among other causes. The exquisitely sensitive creatures that make up the reefs live in symbiosis with algae, which give the reefs their color and the polyps their food. But when ocean temperatures rise too high, the corals must expel their algae and, when their stores of food energy run out, die. In 1998 alone, 16 percent of corals worldwide blanched thanks to an El Nino event and the United Nations predicts that 60 percent of all reefs will disappear by 2030. But researchers have discovered one coral that survives bleaching events by feeding on a different food source: plankton.

Geologist Andrea Grottoli of Ohio State University, biologist Lisa Rodrigues of Villanova University and ecologist James Palardy of Brown University teamed up to understand why some corals seem to bounce back from warmer seas more quickly than others. They harvested eight branches of each of two different types of coral in the waters off Oahu--Porites compressa and Montipora capitata--and placed them in eight tanks. Four of the tanks mimicked natural conditions while the temperature was raised to 30 degrees Celsius in the rest.

As expected, both corals bleached and, after 30 days, the scientists measured the polyps' remaining energy stores. Without the algae that fed them, they were starving. The researchers then returned the samples to the sea for another six weeks of recovery. P. compressa regained a bit of its color in this time but still had little energy. But M. capitata managed to regain all its strength and more while remaining entirely algae-free.

To understand this miraculous recovery, the researchers restricted both corals access to ambient food. By measuring the amount of plankton each had eaten, they discovered that M. capitata had increased its feeding on plankton more than fivefold. In effect, this polyp had gone from an efficient grower of algae-provided food to an efficient hunter of plankton.

"These superfeeders have an ecological advantage, one that may protect reefs from extinction," Palardy says. "In evolutionary terms, corals that eat more may win." In fact, the so-called rice coral had enough energy reserves to spawn while its competitors lagged far behind. More research will be needed to determine if other corals can switch their feeding patterns as well, but the finding reported in today's Nature provides at least a bit of hope for the survival of the reefs.

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