Electric fish may have switched from AC to DC

Until now, the two types of knifefish were often mistaken for each other and placed together in fish collections. Both communicate with weak electric discharges and "electrolocate" – special receptor cells in their skin can pick up distortions in electric currents made by objects in the water.

Image of the two new species with graphs of their electric organ discharges (EODs). Brachyhypopomus walteri (top) has a longer, thinner tail and produces an EOD with both positive and negative phases. Brachyhypopomus bennetti (bottom) has a shorter, thicker tail and produces only positive phase EODs.

The researchers report that one of the species, Brachyhypopomus bennetti, has a large electric organ and a short fat tail and produces direct current, or a monophasic, signal; the other, Brachyhypopomus walteri, has a more typical electric organ, a long thin tail and an alternating current, or biphasic signal.

"The most striking differences between these two similar species have to do with their electric organs and their electric organ discharges," said lead author John Sullivan, curatorial affiliate at the Cornell University Museum of Vertebrates.

From a boat, study co-author Jansen Zuanon of the National Institute of Amazonian Research in Manaus, Brazil, pulls a net underneath floating grasses and water hyacinth, capturing specimens of the two new species of electric fish.

Most electric fish, like B. walteri, produce pulse-type, alternating current (AC) signals that sound like "pop-pop-pop" when recorded underwater, said Sullivan. These biphasic charges alternate currents of positive and negative volt phases, which cancel each other out in the water and may "cloak" such fish from predators that possibly hunt by sensing electric currents, said Sullivan.

The more rare type of signal, exhibited by electric eels and B. bennetti, consist of only a positive, monophasic pulse in which the direct current (DC) is not canceled out.

All of B. bennetti's relatives, including B. walteri, have biphasic electric signals. "For that reason, we know that this trait evolved in B. bennetti's lineage," said Sullivan. "The interesting question is why."

In 1999 a Florida International University researcher suggested that B. bennetti may be mimicking the electric eel, which has a powerful charge for stunning prey and protecting itself, but also a weak charge for electrolocating and communicating. The signal of B. bennetti closely matches that of the electric eel. This type of mimicry, where a benign creature mimics the appearance or qualities of a noxious organism, is called Batesian mimicry. For example, viceroy butterflies mimic the colors of foul-tasting monarchs, or benign marmalade hoverflies flies look like black and yellow wasps.