Baby fish 'smell their way home'

Marine scientists working on Australia's Great Barrier Reef have uncovered evidence that baby fish, only millimetres long, manage to find their way to their home coral reef across miles of open sea by using their sense of smell.

Remarkable in itself, the discovery by a team including Professor Mike Kingsford of the ARC Centre of Excellence for Coral Reef Studies and James Cook University and colleagues from Woods Hole, USA, also shines a new light on how the breathtaking diversity of fish on coral reefs has arisen. This has major implications for how reefs are managed.

"The babies of many coral fish species are swept off their home reef by ocean currents within days of hatching. 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," he says.

"But that is not the case. There are major genetic differences between fish of the same species on reefs only a few kilometres or even hundreds of metres apart."

This diversity between populations of the same fish species is what drives evolution on the Reef and underpins the spectacular richness of its sea life, Prof. Kingsford says. "This genetic separation between reefs may be what gives rise to so many different species in coral reef systems."

The researchers were intrigued how tiny damsel and cardinal fish, born on one reef, managed to find their way back home to preserve such remarkable population differences, braving strong currents and ferocious predators in their 20 days at sea – all when only a centimeter 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.

"We know these late stage fish larvae, generally between about 9 and 14mm long, 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," Mike says.

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. 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 is a mystery still. The researchers theorise that it is imprinted on them either when they are an egg inside their mothers, a fertilised egg swept around on the bottom, or new-hatched fry loose in the stream or brooded in their parents' mouths.

"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 recognizing their signature as "home" when it grows up a bit and is ready to settle," Mike says.

"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."

Source : James Cook University