Cornell University researchers say the fish look alike and have the same genetic makeup, but have very different electrical signals and will only mate with fish that produce the same signals. The Cornell neurobiologists say they believe the different electrical signals are the fishes' first step in diverging into separate species.

"We think we are seeing evolution in action," said Matt Arnegard, a neurobiology postdoctoral researcher.

The electric fish -- known as mormyrids -- emit weak electric fields to sense their surroundings and communicate with other fish. Each species of mormyrid produces a characteristic electric impulse.

While the fish may be able to understand other species' impulses, said Arnegard, "They seem to only choose to mate with other fish having the same signature waveform as their own."

However, when researchers were about to publish descriptions of two separate species, Arnegard decided to take a genetic look at those particular fish -- and he couldn't find any differences in their DNA sequences.

The study appears in the Journal of Experimental Biology.

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