DNA reveals where sharks were born

Shark populations worldwide have been threatened by the demand for shark fins, with tens of millions of the animals killed each year to make a soup prized by Asian consumers. Since many sharks travel the globe on their own — and others are caught and transported across oceans to shark-fin markets — conservationists face a problem: how to pinpoint where particular sharks were born.

Now, a group of scientists has an answer. By examining genetic material that a mother passes to her offspring, they can say with precision where a given shark was born. In doing so, they can identify the extent to which certain endangered species are being targeted for the shark-fin trade.

“By analyzing part of the genome that is inherited solely through the mother, we were able to detect differences between sharks living along different continents — in effect, their DNA Zip codes,” said Demian Chapman, who led the research team and serves as assistant director of science of Stony Brook University’s Institute for Ocean Conservation Science. “This research shows that adult females faithfully give birth along the continental region where they were born.”

The researchers — who come from the United States, United Kingdom, Australia, Canada, South Africa, Peru and New Zealand — took tissue samples from nearly 400 dusky and copper sharks. The animals came from fishing boats, research cruises and beach meshing nets as well as from markets in Hong Kong, the global trading center for shark fins.

The International Union for Conservation of Nature classifies the dusky shark population off the East Coast and in the Gulf of Mexico as endangered because it has declined by more than 80 percent over the past two decades even though it has been protected since 2000. Globally, the copper shark is listed as “near threatened” by the IUCN. (Last week, the National Oceanic and Atmospheric Administration’s fisheries service determined that scalloped hammerhead sharks in the western Atlantic are overfished and under intense fishing pressure.)

The DNA findings — which are being published in the journals Endangered Species Research, and Marine and Freshwater Research — will allow experts to estimate to what extent a given population of sharks is being targeted for the fin trade.

While this method of analysis could help authorities curb finning — in which vessel operators typically slice off a shark’s fin and toss the body overboard because shark meat is much less valuable — it also highlights the challenge of restoring a depleted shark population to health.

The dusky shark in the western Atlantic is threatened not only because it has been overfished but also because it takes up to 20 years to mature sexually and reproduce. And even if dusky sharks fare better elsewhere on the planet, they won’t breed with the endangered ones because the females will give birth only in their own territory.

“Here in the United States, it took only a few decades to nearly wipe out our dusky sharks, and it will probably take a few centuries for their stocks to be replenished,” said Martin Benavides, who served as lead author on both of the journal articles and serves as a research assistant at the Institute for Ocean Conservation Science. “The sight of a dusky shark swimming off our shores will be a rare experience for generations to come.”

Eilperin is the author of "Demon Fish: Travels Through the Hidden World of Sharks," forthcoming from Pantheon Books in June.

Juliet Eilperin

Hair can tell where you were yesterday

Take note, crime scene investigators: Researchers may have found a way to figure out where a person has been — and even hints about what they’ve been eating — simply by zapping their hair with an ultraviolet laser. In a demonstration reported online this month in Rapid Communications in Mass Spectrometry, a team burned three 50-micrometer holes in a horse hair. Analyses of the vaporized material revealed variations in the carbon isotope ratios incorporated into the hair as it grew. Those changes were caused by changes in the horse’s diet — for instance, the isotope ratio in oats the horse ate a week ago may be quite different from the ratio in alfalfa it consumed last month. Besides providing insights into dietary changes, such analyses can provide hints about animals’ migratory habits. For humans, techniques of this sort could help discern the recent travel habits of a criminal suspect or identify the place of origin for an unidentified set of remains.


ScienceNOW is the daily online news service of the journal Science, which can be read online at at www.sciencemag.org.