BioEd Online

Hungry corals have an unexpected trick up their sleeve that could help reefs to escape destruction at the hands of warming seas. Marine biologists have discovered that some corals can weather 'mass bleaching' events by gorging on marine animals.

Corals usually gain sustenance from the tiny algae that live alongside them and convert energy from sunlight into nutrients. These algae give coral reefs their dazzling array of colours and are the reason that reefs are generally found in shallow, sunny waters.

But corals are also armed with stinging tentacles that can reach out a short distance to snag prey, allowing them to eat without the help of algae. Biologists suspect that relying on partner algae is a far more efficient way to live. But there may be times when the latter method of feeding comes to the fore.

White out

Rising water temperatures, for example, can lead to deadly mass-bleaching events, in which many corals shuck off their partner algae through stress. This often causes the majority of corals on a reef to starve. Some ecologists predict that such events could kill up to 60% of the world's coral over the coming decades.

Researchers led by Andréa Grottoli of Ohio State University in Columbus wondered whether some corals might be able to ramp up their tentacle feeding during such hard times, to help them survive and recover from a bleaching event.

They discovered that the coral species Montipora capitata, when experimentally bleached in the lab, can indeed sustain itself by catching tiny marine creatures called zooplankton. They report their results in this week's Nature¹.

Hot times

Grottoli and her team collected a range of corals from Kaneohe Bay, Hawaii, and placed them in tanks containing water warmed from the corals' preferred temperature of 27 ºC to a toasty 30 ºC. After a month the corals had completely bleached. The researchers then returned them to the sea and monitored their feeding to see how they fared.

M. capitata regained a healthy mass and energy level once out in the sea, even though it hadn't built up an algae population. It seems this coral had mastered the technique of fishing for zooplankton and ramped up its hunting once the algae were gone. The other two species tested, Porites compressa and P. lobata, continued to lose weight for weeks after being returned to the sea, suggesting that they were relying on stored energy rather than replenishing themselves.

"This suggests there are some corals out there that can survive," says Grottoli. Those that actively feed, rather than relying on dwindling energy reserves, should recover more quickly from bleaching.

It is not yet clear whether other coral species can also use this skill. "Montipora might be the only one on the whole planet that does it," Grottoli admits. Rare or not, corals with this skill might be expected to become predominant as climate change proceeds and others fall by the wayside in the world's warming waters.

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