Finding Nemo with electricity

Researchers have discovered an electrifying love story in the turbid rivers of the lower Congo rapids. Elephant-nosed electric fish find and recognize their mates through crackling communications, according a study published today (Nov. 25) in Biology Letters.

Campylomormyrus elephant-nosed fish have a specialized muscular organ that emits millivolt-strength electric signals -- too weak for humans to much notice but powerful enough to help the fish navigate and communicate. In 2006, Philine Feulner, an evolutionary biologist at the University of Sheffield, UK, and her colleagues showed that the electric discharges were species-specific, and could be used to tease apart cryptic Campylomormyrus species. Now, her team has found that females choose their mates based on the electric serenades, too.

A Campylomormyrus compressirostris male

"We knew they communicated [through electric discharges]," Phillip Stoddard, a zoologist at Florida International University in Miami, who was not involved in the study, told The Scientist. "The missing part of all of this was showing that the receiver responded meaningfully to these things."

Feulner's team performed a series of female mate-choice experiments with reproductively ripe females of one species, C. compressirostris, presented with either live males or recorded male electric signal playbacks from their own species and two other Campylomormyrus species. They found that females could differentiate between both males and recorded playbacks when confronted with a closely related species that lives in the same river, but they could not tell the difference in a distant relative not normally encountered in the wild.

The female elephant-nosed fish (center) chooses between two potential electrifying lovers

Because females prefer the charged communiqués of their own species, the electric signals also probably drive the early stages of speciation, noted Feulner. Females choosing mates based on these signals "could lead to reproductive isolation, an important step in speciation," she wrote in an E-mail.

But the electric signals are also used for other functions, such as prey detection and navigation, and it's not clear whether electric discharges were first used for mate recognition and then co-opted for ecological purposes, or the other way around, said John Sullivan, an evolutionary ichthyologist at the Academy of Natural Sciences in Philadelphia, who was not involved in the research. "What comes first in the diversification and speciation of this group?" he asked.

"This study shows that there is discrimination going on, but the mechanism is not elucidated," added Sullivan. He noted that the sensory electroreceptor found in the skin of weakly electric fish, known as the knollenorgan, has been implicated in species recognition in other weakly electric fish species. Thus, it's possible that C. compressirostris' knollenorgan is finely tuned to its own species' signal, which has a short wavelength, rather than the close relative's more drawn-out discharge, he said. Conversely, because the distant species' signal has a similar waveform, the knollenorgan might not be able to distinguish between the species.

Jenny Boughman, an evolutionary biologist at the University of Wisconsin-Madison who also was not involved in the research, cautioned that the sample size of only 6 or 7 females was too small to test whether the fish could discriminate their own signal from that of the distantly related species'. "I'm not convinced that there's no discrimination," she said.