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Tszzzzzt! Electric fish may jam rivals' signals

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11:58am, November 16, 2005
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For the first time, researchers say, they've found an electric fish sabotaging another fish's electric signals.

The brown ghost knifefish (Apteronotus leptorhynchus) generates a weak electric field that it uses to detect obstacles and to communicate with other knifefish. When confronting a rival knifefish, both males and females can raise the frequency of their own electric signals close enough to the other fish's to distort its electric field, reports Sara Tallarovic of the University of the Incarnate Word in San Antonio. In previous experiments, such jamming blinded fish-guidance systems.

Tallarovic and Harold Zakon of the University of Texas at Austin recorded electric frequencies as one knifefish darted in an unfriendly way toward another fish or a dummy emitting an electric signal. The lunger often locked jaws with its opponent or snapped at its electric organ, as if trying to bite it off (view the video clips here).

Decades of experiments had shown that knifefish tailor their electric field frequencies to make them differ from those of a field applied by experimenters, so the idea of intentional jamming has been a surprise, says Tallarovic.

"That's very novel. I wouldn't have expected it," says electric-fish specialist Leonard Maler of the University of Ottawa.

Brown ghost knifefish don't pack enough of a zap to stun their prey. Their electric organs, which are collections of neurons that fire 600 to 1,000 times a second, create an electric field that changes slightly when it encounters something with conducting properties different from water's. Receptors scattered over the fish's body pick up the field distortion.

Also, males "sing to the females in the electric frequencies," says Tallarovic. She's recorded hours of nighttime serenades of intensified, wavering frequencies (listen to a sample here).

The fish grows about 6 inches long and has become popular in aquariums. It hunts at night and, in the wild, lives in brackish waters from Mexico to Uruguay.

A fish of this species occasionally raises its frequency but never lowers it, Tallarovic says. She suspected signal jamming when she noticed upward frequency shifts as one fish attacked another. "Everybody just told me, 'No, it's got to be an artifact,'" she says.

So, she and Zakon monitored fishes' electric fields in several scenarios, the team reports in an upcoming Animal Behaviour. When researchers put two fish in an unfamiliar tank or used a field-emitting dummy to mimic an intruder in a fish's home tank, both males and females tended to raise their electric-field frequencies as they attacked. The changes' timing and context convinced the researchers that the attacking fish was jamming the other's signals.

The evidence that Tallarovic and Zakon present "leaves little doubt that their interpretation of the response as aggressive signaling is correct," comments electric-fish researcher Kent Dunlap of Trinity College in Hartford, Conn.

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