New maps suggest how electric fish got their big brain

Headsets of the world's freshwater fish, African mormyrid fish are known to have a similar brain-body size ratio to that of humans.

But the size of mormyrid brains varies a lot. These differences offer the opportunity to examine what lies behind the mass.

Researchers at the Washington University in St. Louis have mapped brain regions in mormyrid fishes in great detail. In a new study published in the November 15 issue of Current biology, they report that the part of the brain called the cerebellum is larger in members of this family of fish than in related fish – and this may be associated with their use of low electric shocks to locate their prey and communicate with each other .

The discovery of size in itself is not particularly surprising for those who follow this fish, said Bruce Carlson, professor of biology in the field of arts and sciences. "It had almost become a truism," he said. In mormyrids, at least, they thought, "big brain means big cerebellum".

Instead, researchers explain how their new measures can help shed light on long-standing issues in neuroanatomy.

As the brain grows, do all brain regions grow predictably? Or does natural selection act independently on distinct regions of the brain, so that parts of the brain grow in animals for which there are additional reasons for using them?

"When you look at the human brain, the cerebral cortex has become that giant that has engulfed the other brain regions," said Kimberley V. Sukhum, the first author of the new study, who has just completed his PhD. in biology at the University of Washington. She works as a postdoctoral researcher at the Washington University School of Medicine in St. Louis.

"We see something similar with the cerebellum in mormyrids," said Sukhum. "But we did not know how this area had become so big, or even if all the species had a very big cerebellum."

There are over 200 species of mormyrids. Some have been the subject of extensive studies because of their special electrosensory systems. Researchers have long sought answers on how these fish use electrical impulses for communication and tracking. Other species of the family Mormyridae have been largely ignored.

For this analysis, Carlson and Sukhum chose to compare individuals from six species of mormyrids, a closely related species and three other closely related fish species.

Sukhum and Carlson used micro-scanner technology to collect complete sweeps of the soft tissues of fish brains, and then minutely identified landmarks that delineated different regions. The 3D maps they created allowed them to measure and compare with great precision the volume of individual regions of the brain.

On all the fish compared, they found that the increase in total brain size also resulted in a predictable increase in the size of individual regions in the brain. However, with the mormyrids, they found that the size of the cerebellum could not be fully predicted by the total size of the brain.

In other words, electric fish had a much larger cerebellum than fish without a zipper. Electric fish also had lower limbs larger than their non-electrical related cousins.

If it is possible to demonstrate that one part of the brain is sized independently of the others, neuroanatomists then declared that it was an example of evolution. mosaic. Mosaic movements are thought to be relatively rare, and much of the current research on this subject is focused on identifying the types of situations in which these changes occur in nature.

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

Some of the mormyrids that they measured had an extremely large brain, up to three times larger than the mormyrids with the smallest brain in the study. Despite this difference in total size, there was no evidence of mosaic changes in the cerebellum within the group. This is proof that mosaic changes are not simply the result of the development of a large brain. Instead, the changes are probably related to the evolution of new traits, such as the electrosensory system.

This study can not say for sure why mormyrids have such a large cerebellum, but only that they have them.

But Carlson has a presentiment. He believes that this part of the brain is doing the lion's share of Mormyride's work in motor planning, as well as how the fish anticipates the sensory reactions that it will receive during its movements and that he sends electrical impulses.

"There is a lot more planning and impact to manage than for your average fish," Carlson said.

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More information:
Current biology (2018). DOI: 10.1016 / j.cub.2018.10.038