SYDNEY: Some baby fish manage to find their way to their home coral reef across kilometres of open ocean by using their sense of smell, researchers say.
The discovery, made on Australia's Great Barrier Reef by a team of U.S. and Australian scientists, shines a new light on how the breathtaking diversity of fish on coral reefs has arisen, and has major implications for the management of reefs.
"The babies of many coral fish species are swept off their home reef by ocean currents within days of hatching," said Mike Kingsford of James Cook University in Townsville, Queensland, a member of the research team. "Ordinarily you'd expect them to be thoroughly mixed up and this would mean the population of one reef would be pretty much the same, genetically, as another."
"But that is not the case," he said. "There are major genetic differences between fish of the same species on reefs only a few kilometres or even just hundreds of metres apart.
"This genetic separation between reefs may be what gives rise to so many different species in coral reef systems," said Kingsford, who believes that the lack of interbreeding between groups of fish from the same species on different reefs may, over time, have caused them to evolve into separate species.
The impetus for the research came from the researchers' interest in how tiny damsel and cardinal fish, swept off of their home reef, manage to find their way back - braving strong currents and ferocious predators during 20 days at sea - all when only a centimetre or so in size.
"We tested several ideas, but the most attractive seemed to be that they could smell the unique trace of their home reef - rather like salmon can smell the home river," said Kingsford. "We know these late stage fish larvae … already have developed noses - but the question was whether they could use them to recognise what the home reef smelt like, when they left it only a day or so after hatching."
The team exposed tiny fish larvae in a tank to pure streams of water from four different reefs. To their amazement, within minutes a surprisingly high percentage of baby fish had congregated in the water flow from their home reef.
"It was a lot more than you'd expect to happen by pure chance - and it applied, in differing degrees, across several species of fish," said Kingsford.
The fish could also be responding to other stimuli, including distant noise off a reef and the behaviour of other fish, but the team concluded that smell was probably the dominant factor leading the babies home.
"Every reef gives off its own unique chemical signature, a rich mixture of the proteins and amino acids emitted by corals, all the plankton and mucus from its life," said Kingsford. "We think baby fish can pick this up and distinguish it from other reefs.
"We think some fishes then choose currents that smell like 'home' and swim up them. The ones that cannot do this perish. The ones that get home preserve the unique 'ethnic' make-up of their tribe - and so continue the process of evolving into separate new species."
How the fish learn the unique smell of home remains a mystery. In their paper, published last week in the U.S. journal Proceedings of the National Academy of Sciences, the researchers theorise that the smell is imprinted on a baby fish either when it is an unfertilised egg inside its mother, a fertilised egg on the bottom, or a newly-hatched fry.
"An egg, even a fry, hasn't a fully developed sense of smell, but it may have a way of absorbing the local molecules and then recognising their signature as 'home' when it grows up a bit and is ready to settle," said Kingsford. "This evidence that individual coral reefs play such a key role in the emergence of new species is a fresh reason to take even greater care in how we look after them."