[Photo] Aerial image shows one of the study sites, One Tree Island, part of Australia’s Great Barrier Reef. Credit: Michael Kingsforde

Ocean currents can whisk tiny larval fish a long way from home. Turns out, the little ones follow their noses back to their native reefs.

Jelle Atema, a sensory biologist at Boston University, and Gabriele Gerlach of the Marine Biological Laboratory in Woods Hole, Mass., analyzed small gene units called microsatellite markers in three species of reef fish, including the cardinal fish, spiny damsel fish and the neon damsel fish, living on five reefs within Australia’s Great Barrier Reef.

The markers indicated that more returned to their home reef than random chance would predict. The most obvious was the cardinal fish, which showed clear genetic differences from one reef to another.

“Even though the [cardinal fish] larvae are dispersing out in the ocean, there are distinct differences between reefs as if they were not dispersing,” Atema explained. “That means they either have to, by great majority, all go home every generation, or those that do go to other reefs die off and they don’t reproduce.”

To find out what was steering the cardinal fish home, the scientists placed individual fish larvae into a flume containing “lanes” of water samples collected from different reefs. The cardinal fish larvae preferred water from their home reefs and spent more time in that part of the flume compared with water from other reefs.

The researchers suggest the young creatures sniffed out familiar scents dispersed in the water to guide them home.

The three-week-old larvae smell in a way that’s similar to humans. “The only difference is water instead of air passes over [the smelling organ] and so they smell dissolved substances in the water,” Atema said.

For most coral reef fish, the female lays her eggs and the male fertilizes and guards them until they hatch into pinhead-size larvae. During their first week, the na??ve larvae float and swirl around, catching rides on nature’s currents.

As they mature, their transparent bodies start to look more like adult fish, and they become super swimmers reaching speeds of more than 1 mph in short bursts. Even with a speedy fin-stroke, the larvae would surely get lost without directions. “Good swimming is one thing. If you don’t know where you’re going, it’s no good,” Atema points out.

Questions remain, such as how the larvae “learn” the odor from their home reefs or when they pick it up. Plus, the researchers hope to figure out the chemical make-up of the home scent, which Atema said will be tricky.

“It is just like you go to your grandma’s house and you go to your aunt’s house; and you can smell the difference. You tell me what the difference is in this mixture of smells,” Atema said.

The research is published in the Proceedings of the National Academy of Sciences.