Samplings: July–August 2008

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Gentile Francesco Ficetola collects water samples to identify DNA fragments. Three 15-milliliter samples are sufficient to detect the presence of bullfrogs, an invasive species, in this large pond.

Claude Miaud

It’s easy for small creatures to hide in a pond—what with all the murky water, vegetation, rocks, and logs—so biologists who want to catalog them must do a lot of mucking around. A new technique might make that task a lot easier: just collect half a tablespoon of pond water and examine the DNA therein. Bullfrogs, for starters, shed enough DNA into the water to enable their detection, according to a new study.

Ecologist Gentile Francesco Ficetola, now at the University of Milano–Bicocca in Italy, and three colleagues studied American bullfrogs (Rana catesbeiana), which have invaded European wetlands and displaced many native amphibians. Ficetola and others had already documented the invaders’ distribution in France by surveying more than 2,500 wetlands. For the present study, Ficetola’s team collected small water samples—just half an ounce each—from ponds known either to harbor bullfrogs or to be free of them. The team found bullfrog DNA in every sample from ponds with the animals, but never in samples from ponds without. What’s more, the amount of detectable bullfrog DNA in a sample signaled whether a pond is teeming or only sparsely populated.

The free-floating DNA, Ficetola says, probably comes from mucus, feces, urine, or decomposing bodies. The next steps will be to extend the technique to additional species and to determine how long a species’ DNA “calling card” persists after animals have left a pond—or been successfully eradicated. (Biology Letters)

Web Links:

• Laboratoire d’Ecologie Alpine
• Claude Miaud