Spark in fish brain electrified evolution

28 April 2011
Posted by Will Hamilton.

Bigger brain regions caused a 'big bang' in the evolution of African electric fish, new research has shown.African electric fish

The fish, known as mormyrids, produce weak electrical impulses to communicate with one-another. The electric signals are detected by receptors on the fish's body and interpreted by a region of their brains that can either be small and simple, or bigger and more complicated, depending on the species.

"We hypothesised that this larger, more complex brain anatomy might be associated with an enhanced ability to detect subtle differences in the electric signals that the fish generate," said Bruce Carlson, one of the authors of the study.

The scientists tested their hypothesis by placing electric fish collected from Gabon, in West Africa, into experimental tanks that could generate electrical pulses in the water.

When these pulses of electricity were first turned on, the fish responded by altering their own electricity production, but as the pulses continued, they gradually lost interest and resumed their previous activity.

This phenomenon, called 'habituation', is common amongst animals. If a stimulus is repeated many times then animals tend to stop paying attention to it, like when people ignore background noise. But if the stimulus changes suddenly, as when someone shouts above the general hubbub of a noisy room, it usually gets noticed.

The scientists took advantage of this by slightly altering the electrical nature of the pulses and then seeing whether the fish reacted.

"If we inserted a new stimulus and the fish suddenly started to respond again, then that [would] suggest the fish could tell the difference between the two," said Carlson.

The results showed that electric fish species with larger, more complex brain anatomy could detect subtle changes in electrical signalling with greater sensitivity than their smaller-brained relatives.

TGabon, West Africahis observation helps to explain why species with advanced brain anatomy are over-represented in the electric fish family, with 175 identified species compared to just 33 for the rest of the family put together.

"We have a very dramatic difference in the total extent of species diversity in relation to this difference in brain anatomy," said Carlson.

The authors theorise that an enhanced ability to detect electrical signalling allowed larger-brained fish to distinguish small differences between electric signals produced by other fish. This then acted as a divisive force between groups of fish with slightly varying electrical signals, intensifying divergence into new species.

"It's going to reinforce boundaries between these different groups of fish and facilitate species diversification. Whereas if you can't tell the difference between different signals you're going to have less boundaries set up in terms of who you choose to mate with".

This gives insight into how species originate, and why certain lineages diversified enormously during evolution while others remained stable over millions of years.

And although African electric fish may not look spectacular, they clearly have a lot to teach us about animal communication, behaviour and evolution.

Bigger brains triggered evolution in African electric fish, as Bruce Carlson, from Washington University in St. Louis, explains.

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