News in Science

Thousands of fish ID'd from glass of water

DNA soup A glass full of water is all fish detectives needed to identify approximately 13,000 fish living in the larger body of water from which the sample was taken, report researchers.

The secret is in DNA that water-dwelling organisms regularly release into their environment.

"It might be unpleasant to think about when going for a swim in the ocean, but the water is a soup of cells shed by what lives there," says study lead author Ryan Kelly of the University of Washington.

He explains that fish shed cells from their skin. They and other organisms also release unneeded things, such as damaged tissues and bodily waste.

"Every one of those cells has DNA and, if you have the right tools, you can tell what species the cell came from," says Kelly. "Now we're working to find the relative abundance of each species present."

He and his colleagues tested the technique out at the Monterey Bay Aquarium's 450,000 litre Open Sea tank, which is among the 10 largest aquarium exhibits in the world.

This tank was suggested because the inhabitants are known and could be compared to what the new technique revealed was present, giving the authors a way to judge the method's accuracy.

The approach successfully identified the eight species of bony fishes in the tank and determined that tuna and sardines made up the greatest amount of biomass in the tank.

The technique turned out to be so finely tuned that it also picked up DNA from long-dead menhaden from the Atlantic Ocean, fish that had been processed, transported and added to the tank as food.

It was a surprise when an Atlantic species turned up, until the researchers realised where the DNA was coming from, and then they made calculations to take that into account.

Out in the open water

The researchers next hope to try the technique out in a natural setting: San Francisco Bay. It could be used in any number of locations, however. The goal is to have an easier, more accurate marine life census tool.

Once obtained, the resulting census info could then be used to help scientists better monitor and manage aquatic habitats.

It could, for example, reveal the arrival of invasive species before they become a major problem, or provide new ways to look at marine food chains and other basic ecosystem features.

The study is published in the online journal PLOS ONE.