Once spawned, reef fish larvae are at the mercy of the ocean's currents, which propel them far from their birthplaces. At least that was the theory. New evidence, however, indicates that some reef fish species recognize the odor of their natal reef and use it to sniff their way home. This behavior may help explain the incredible biodiversity of fish seen on the ocean's reefs.
Biologists have had a tough time explaining how reef fish have become so biodiverse, considering that larval dispersal should scatter relatives far from each other. New species form when populations become isolated from one another and develop their own genetic identities over time. In order for this to occur, the subpopulation must have enough genetically similar members--such as close relatives--who can interbreed.
Biologists Gabriele Gerlach and Jelle Atema of the Marine Biological Laboratory in Woods Hole, Massachusetts, wondered if these fish were somehow able to find their way back to home reefs and their closest kin. Drawing on previous studies, which showed that juvenile salmon "imprint" on odors associated with their native streams, the team tested the homing ability of three species of reef fish living on five closely spaced reefs of Australia's Great Barrier Reef. The researchers captured late-stage larval fish of each species, including the spiny damselfish and a cardinalfish, that were either freshly settled, or about to settle, at various reef sites. Using a specially constructed flume, they then exposed each fish to water samples from the different reefs and compared how long it spent in water from its home reef versus that of the other reefs.
Two species showed clear preferences for their home waters, spending, on average, roughly twice as much time there compared with water from "foreign" reefs. The findings suggest that odors--not yet identified--could guide larvae either to reefs in general or to their home reef. Returning to their home reef allows these fish to keep their population genetically isolated from other populations on different reefs, thus contributing to the overall diversity of tropical reef fish, the researchers report online this week in the Proceedings of the National Academy of Sciences.
Biologist Timothy Tricas, of the University of Hawaii, Manoa, applauds the study for "showing for the first time that larval fish can chemically distinguish--very likely by olfaction--between waters of their settlement reefs and adjacent areas." It seems larval reef fish are "anything but passive drifters with currents," he adds.