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photo - scuba diver under water

Researcher Cody Clements places bottle caps into the rocky sea floor off Votua Reef on the Coral Coast of the Fiji Islands. The caps are used to anchor small colonies of coral for experimentation to understand how crown-of-thorns sea stars and seaweed affect coral growth. The bottle caps allow for the coral colonies to be removed for accurate weighing. Photo by Cody Clements.


On the coral reef, knowing your friend from your enemy can be a little complicated.

Take seaweed, for instance. Normally, it’s the enemy of coral, secreting toxic chemicals, blocking the sunlight, and damaging the coral with its rough surfaces. But when hordes of hungry crown-of-thorns sea stars invade the reef, the seaweeds appear to protect the coral from the marauding creatures, reported a study published in the journal Proceedings of the Royal Society B.

photo - corals under waterA crown-of-thorns sea star is shown eating a coral from the genus Acropora, which is a preferred meal for the organism. Photo by Cody Clements.

The findings demonstrate the complexity of interactions between species in ecosystems, and provide information that could be useful for managing endangered coral reefs.

“On the reefs that we study, seaweeds reduce coral growth by both chemical and mechanical means,” said Mark Hay, a professor in the Georgia Tech School of Biology. “But we found that seaweeds can benefit corals by reducing predation by the crown-of-thorns sea stars. Corals surrounded by seaweeds were virtually immune to attack by the sea stars, essentially converting the seaweeds from enemies to friends.”

Crown-of-thorns sea stars are a major problem in the Pacific. On the Great Barrier Reef, coral cover has declined by more than 50 percent over 25 years, and the voracious spine-covered creatures get much of the blame.

“You don’t have to see the crown-of-thorns to know they have been on the reef,” said Cody Clements, a Georgia Tech graduate student in Hay’s lab and the paper’s first author. “You can see where they have been because they leave trails of bleached white coral.”

To study the relationship between sea star attacks and seaweed cover, Clements photographed the amount of sea star damage to different coral colonies, and related the damage to the amount of seaweed on corals in the attacked areas. Coral colonies that had been attacked had, on average, just eight percent seaweed coverage, while nearby colonies of the same species that had not been attacked averaged 55 percent coverage of seaweeds.

Experimentally, Clements fabricated 10 cages in which he placed two Montipora coral colonies, one surrounded by varying levels of seaweed and the other lacking adjacent seaweeds. Into each cage he placed a sea star, then observed how much of each coral would be eaten. At the highest levels of seaweed, the sea stars were completely deterred from eating the coral.

The research was supported by the National Science Foundation, the National Institutes of Health, and the Teasley endowment at Georgia Tech.

— John Toon

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