Seaweed Bodyguards Coral Against Bullying Sea Stars

Crown-of-thorns sea stars are an "underwater swarm of locusts" that devour coral—unless the coral is protected a layer of seaweed. Christopher Intagliata reports.

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Warmer, more acidic oceans in the South Pacific. But the corals also have more visible foes: such as . "It's an underwater swarm of locusts with a stomach that can be turned wrong side out and digest you as it walks across." Mark Hay, a marine ecologist at Georgia Tech. "You just look in front of them and there's good corals, and you look behind them and there's these white skeletons."

Hay says the corals in Fiji's marine protected areas are particularly vulnerable to attack. So he and his colleague Cody Clements took a closer look at the underwater ecosystem there. And they discovered something weird. Neighboring seaweeds usually compete for resources with corals to the point where they will whip corals with their fronds and poison them with toxins. But the researchers found that, in this case, the seaweeds were saving the corals, blocking the marauding sea stars. "And so these competitors were really acting as kind of bodyguards for the corals, once things got bad." Hay and Clements replicated those observations in underwater experiments, in which even fake seaweed did the trick—suggesting that seaweed is simply passively blocking the predators. The findings appear in the . [Cody Clements and Mark Hay, ]

The marine reserves in Fiji are relatively small. And they border lots of degraded reefs, which Hay suspects are playgrounds for baby sea stars. When the stars get older and hungrier, he says, pristine corals are right next door. "They kind of act like little piles of candy that you put out in the kindergarten. Everybody just recruits to them, and eats them quickly." The reserves don’t have much seaweed, either, so the coral lack their bodyguards—except for an interested human:

"In one of the villages there's a guy that's particularly supportive of the marine protected area named Aquila. And he has sort of a floating wheelbarrow, and he goes out every now and then and fills it up with crown-of-thorns starfish and he comes back and builds a fire and burns them, on the shore, to keep them from regenerating." This study suggests Aquila’s system of killing the starfish to save the coral is on the right track. And Hay says his aggressive management style might be worth emulating elsewhere in Fiji: to ensure that marine protected areas still have something to protect in years to come.

—Christopher Intagliata