Corals only ‘choose’ reefs with a healthy smell

Coral reefs in the Red Sea in Egypt, where myriad life forms thrive. Picture: Getty Images
Coral reefs in the Red Sea in Egypt, where myriad life forms thrive. Picture: Getty Images
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BABY corals and fish can smell the difference between good and bad reefs, a study based in Fiji has revealed.

When offered a choice of two water samples in the lab, the animals turned away from the stench of seaweed that invades depleted reefs but were drawn to the smell of healthy coral.

It is the first time that corals have been shown to react over long distances to chemical “smells” in the water.

The findings suggest that controlling seaweed is key to repopulating reefs.

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

Once a coral reef has decayed and seaweed takes over, stopping fishing in the area may not be enough to revive the coral. “If you’re setting up a marine protected area to seed recruitment into a degraded habitat, that recruitment may not happen if young fish and coral are not recognising the degraded area as habitat,” Dr Danielle Dixson from the Georgia Institute of Technology, the study’s lead author, said.

Dr Dixson’s research, published in the journal Science, made use of three marine protected areas off the coast of Fiji, which contain healthy coral reefs but are immediately adjacent to heavily fished areas dominated by seaweed.

“We’ve got these fished and unfished areas that are small and immediately adjacent to each other, so it’s a nice experimental setting,” Professor Mark Hay said.

Water from both healthy and weedy areas was taken to Prof Hay’s lab, where fish were offered a choice between the two. Young fish from 15 different species all chose the water from the healthy reefs, spending more than 80 per cent of their time there.

Doing the same sort of test with coral was ground-breaking, Prof Hay said. “For fish, people weren’t too surprised that it happened. They have tails, they have a nose. But for corals… they thought we were nuts.”

The idea was challenging because baby corals are so simple. “They’re kind of like bags of snot,” Prof Hay said. “We didn’t think it would happen either.”

However, three species of coral larvae showed the same behaviour, swimming into the better-smelling water by waggling their tiny hairs or “cilia”.

The researchers also identified key ingredients by mixing things up: if they contaminated water from a healthy reef with the smell of specific seaweeds, the fish would avoid it just as much as water from an abandoned reef.

Similarly, the aroma of certain healthy corals is enough to make bad water attractive.

These results reveal new complexity in the way coral behave.

Researchers had previously seen this happen only over short distances, when the coral effectively contact a good or bad surface. “This is the first time that we’ve seen coral’s ability to assess this on a large scale, when they’re floating around,” Prof Hay said.

“They can’t do much against a current. So what we think is going on is that they’re drifting through these different reefs, and if it smells good, they go down.

“It’s a very strongly selected behaviour. Over aeons, there’s been good reefs and bad reefs and if you settle on the bad ones, you die.”