August 14, 2005
There are fish in the sea, but many species are over-exploited, aren’t evenly distributed and some, like the clownfish of “Finding Nemo” fame, are in high demand for tropical aquariums. Understanding how marine populations grow and spread is essential to protect threatened species, yet tracking fish movements has posed an enormous challenge to science. An international team of researchers may have helped solve the mystery — using the human antibiotic tetracycline.
In a recent study published in Current Biology, Woods Hole Oceanographic Institution (WHOI) biologist Simon Thorrold and colleagues from Australia and France followed larvae of the panda clownfish (Amphiprion polymnus) in Kimbe Bay, Papua New Guinea, to determine how far from home they wandered before settling down for good. Easily recognizable with bright, broadly striped bodies, clownfish live on coral reefs in peculiar harmony with sea anemones, finding protection among the anemone’s usually poisonous tentacles. Clownfish are limited, therefore, to habitats that also support anemones, making them good candidates for dispersal studies.
Many marine organisms spend a portion of their lifecycle as pelagic or free-floating larvae. During this phase, larvae have an opportunity to migrate beyond the immediate neighborhood of their birth sites, populating new areas or joining other established communities. Were the younger members of a community born there? Were they the offspring of local adults, or did they move in from a different neighborhood? And if so, from where?
To find the answers, the researchers marked all larvae in the embryonic stage originating at a given birth site with the antibiotic tetracycline to determine whether a juvenile settling at that site had been born there. Tetracycline is known to darken the teeth of human babies when taken by mothers at certain stages of pregnancy. In the same way, it marked the otoliths or ear bones of the developing fish, labeling them as originating in the study area.
As a further indicator of the origins of new residents, the researchers used genotyping to establish the parentage of recently settled juveniles. By comparing newcomers’ DNA to that of adults previously established in the community, they were able to determine if the new members had settled close to home or had drifted in from elsewhere.
Thorrold and colleagues Geoffrey Jones of James Cook University in Australia and Serge Planes of the Universite de Perpignan in France say that measuring larval dispersal is the greatest challenge facing marine ecologists and managers. Understanding how discrete populations are connected to each other is important to making complicated decisions about the size and location of areas to be set aside as marine preserves.
“Ideally, preserves would do more than protect organisms living within their boundaries,” Thorrold said. “Properly sited, preserves could serve as seed areas for nearby open fishing grounds, helping to maintain a sustainable harvest. ”
Among the team’s findings: clownfish tend to be homebodies. Although none of the offspring studied settled into the same anemone as their parents, one third and possibly more of the successfully settled juveniles had established homes within 100 meters (about 330 feet) of their birth sites. The origins of the other two-thirds have not been determined. Since the nearest anemone habitat outside the study area is more than 10 kilometers (about 6 miles) away, it seems likely that newcomers to the neighborhood traveled a considerable distance to get there.
This study provides new information about one particular species under pressure, but the methods used by Thorrold and his colleagues may be adapted to shed similar light on the habits of other species. About 80% of marine fish species in U.S. waters are either fully or partially overexploited.