Coral and fish can 'smell' bad reefs

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.
Once a coral reef has decayed and seaweed takes over, stopping fishing in the area may not be enough to bring the coral back.
"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 recognizing the degraded area as habitat," said Dr Danielle Dixson from the Georgia Institute of Technology, the study's first author.

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

"We've got these fished and un-fished areas that are small and immediately adjacent to each other, so it's a nice experimental setting," said Prof Mark Hay, the study's senior author.
Water from both healthy and weedy areas was taken to Prof Hay's lab, where fish placed in a special chamber were offered a choice between the two.
Very young fish from 15 different species all chose the water from the healthy coral reefs, spending more than 80% of their time on that side of the chamber.
Doing the same sort of test with coral was a ground-breaking experiment, Prof Hay said.
"For fish, people weren't too surprised that it happened, even as babies. 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 told the BBC. "We didn't think it would happen either."
But sure enough, three species of coral larvae showed exactly the same behaviour, swimming into the better-smelling water by waggling their little hairs or "cilia".
"Dani and I were sitting around, smiling at each other and just laughing, going - can you believe this? This is so cool. Nobody's going to believe it. We better do a lot of replicates."

The researchers also identified the 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.
Breaking the cycle These results reveal new complexity in the way that coral behave.
Previous work had showed that coral larvae would settle in some places and not others, based on molecular cues. But researchers had only seen this happen over very short distances, when the coral effectively contact a good or bad surface.