"Settlement selection is one of their most important decisions, and it really dictates whether they'll live or die," Dixson said.
When dropped into the same two-flume tank, coral larvae overwhelmingly chose to swim in the water from healthy areas, the researchers found.
"Seaweed has practically become synonymous with the degradation of coral reefs globally," Bob Steneck, a professor in the School of Marine Sciences at the University of Maine, who was not involved in the study, told Live Science. "This is a very elegant experiment suggesting that the larval coral and the fish are choosing not to be in a harmful environment. It makes perfectly good evolutionary sense."
Dixson and colleagues also showed that coral have very nuanced preferences for the surfaces they choose to call home. In the field, the researchers put raised, square tiles in both the protected and nonprotected habitats. They found that in the nonprotected areas, the corals tended to settle on the artificial tiles, a sign that the animals were avoiding the seaweed-strangled natural reef. But the tiles in the protected areas remained coral free, indicating the corals were joining the rest of the coral colony, the researchers found. [In Photos: Bizarre-Looking Reef Fish]
When sniffing out a potential home, fish and corals can even smell which kinds of corals and seaweed are already living there. The researchers found that the creatures were more attracted to water from degraded habitats if an Acropora coral had been soaked in the tank before the experiment.
Acropora corals are very sensitive to changes in temperature. They're usually the first to get eaten when crown-of-thorns starfish invade a reef, and they're vulnerable to bleaching, a phenomenon in which corals kick out the tiny symbiotic algae that provide them with food. Because of their vulnerability, these corals can only thrive in the healthiest reefs, and the coral larvae and fish seemed to sense that.
"That's a very a surprising result," Steneck said. "I take graduate students down to the Caribbean every year, and most of them can't identify coral species as well as coral larvae can."
The researchers also found that the fish and baby corals were less attracted to water that had been imbued with chemical cues from the common seaweed Sargassum polycystum, which blooms and can take over a reef.
The findings, published in the journal Science today (Aug. 21), suggest conservation efforts that involve removing harmful seaweed from reefs might be most effective at boosting healthy coral populations.
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