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Why do electric fish have such unusually large brains?

After mapping the brain of African mormyrid fish with great precision, a team of researchers has found that these fish have a larger cerebellum compared to their relatives. The experts believe this enlarged cerebellum may be linked to electric discharges that are used by the fish to communicate and locate prey.

Bruce Carlson is professor of Biology in Arts & Sciences at Washington University in St. Louis. He explained that the size finding in itself is not particularly surprising for those who follow this fish. This is because African mormyrid fishes are known for having a brain-to-body size ratio that is similar to humans.

“It had almost become a truism,” said Professor Carlson. “In mormyrids, at least, the thinking went, ‘big brains mean big cerebellums.’”

The researchers are excited to find out how their new measurements may help to shed light on longstanding mysteries in neuroanatomy, such as whether all brain regions scale up in a predictable way as brains get bigger or if natural selection influences brain regions independently of each other.

“When you look at human brains, the cerebral cortex has become this giant part that has engulfed the other regions of the brain,” said study first author Kimberley V. Sukhum. “We see something similar with the cerebellum in the mormyrids. But it wasn’t known how this region had gotten so big – or even if all of the species had a very big cerebellum.”

For the investigation, the team compared individuals from six mormyrid species, one species that was considered a close relative, and three other distantly related fish species.

The researchers used micro-CT technology to collect full scans of the soft tissue of the fishes’ brains, creating 3D maps which allowed them to measure the volume of individual regions of the brain with remarkable accuracy.

Among all of the fish examined in the study, the team found that the sizes of individual regions within the brain increased in a predictable fashion as the total brain size increased. In the mormyrids, however, the size of the cerebellum could not be fully predicted by the total size of the brain.

“Here, we see the mosaic increases only in the mormyrids, and not in the outgroups,” said Sukhum. “And one of the things that we see in the mormyrids and not the outgroups is the electrosensory system. There is a potential relationship between these two things.”

While the study did not produce a definitive answer for why mormyrids have such big cerebellums, Professor Carlson believes that this part of the brain may be responsible for most of the mormyrid’s motor planning, and also for anticipating any sensory feedback that will be received after the fish’s electrical charges are released.

The study is published in the journal Current Biology.

By Chrissy Sexton, Staff Writer

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