Sickly Coral Reefs Fail the Smell Test

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Young corals and fish in the Pacific Ocean can smell a bad neighborhood. When looking for a place to settle down, these animals use chemical cues to avoid reefs that are littered with seaweed and flock to healthy habitats instead, according to a new study.

Scientists have seen corals decline around the world over the past several decades, and the new findings help explain why some reefs aren't recovering or recruiting new corals, despite conservation efforts.

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"The reefs in Fiji have such a stark contrast between the healthy areas and the degraded areas," said Danielle Dixson, an assistant professor of biology at the Georgia Institute of Technology in Atlanta, who led the study.

Dixson and colleagues studied the waters off of three villages along the southern side of Fiji's main island, Viti Levu, which each managed a small marine protected area, or MPA, next to another area where fishing was allowed. Each MPA was a little less than a square kilometer in size (about 0.3 square miles) and had someone on patrol to enforce no-fishing laws 24 hours a day, Dixson told Live Science. Life thrives inside the MPAs, but the nonprotected areas often lack the large populations of herbivores, such as parrotfish, which would normally trim the seaweed from corals and keep them healthy, Dixson explained.

"If you're snorkeling on those reefs, it's almost like you can see where the line of protection stops," Dixson said.

Snorkelers and scientists aren't the only ones who are able to notice the difference; fish and corals can sense it, too, even in a lab setting.

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Dixson and colleagues collected 15 different species of fish — 20 specimens of each — from both the healthy, protected areas and the degraded, nonprotected areas. The team set up a tank that had one plume of water from the healthy habitat and another plume from the degraded habitat. Given a choice, the fish consistently preferred to swim in streams of water from the healthy habitats, even if they had grown accustomed to swimming in a degraded, seaweed-choked habitat in the wild.

The same was true for coral larvae. Before they settle on a reef and morph into hardened polyps, coral larvae look like free-floating, popcorn-shaped blobs covered in hairlike cilia. They can't see or swim, but they can decipher chemical cues in their surroundings and control when they settle down.