Unraveling Brain Evolution Through the Study of Teleost Fish Intelligence

Higher order brain function in different animals

Once, higher-order intelligence was thought to be exclusive to primates, as identified by a number of complex cognitive functions such as advanced tool use and self-recognition. However, these hallmarks of intellect have since been recognized in a number of other animals, even outside of mammals. Notably, corvids and parrots have exhibited highly advanced behaviors including creative tool use, which necessitates a combination of complex reasoning, planning, and fine motor skills.

The development of higher brain function is coupled with an increase in relative brain mass, termed encephalization. In primates, this corresponded to an increase in the size of the cerebral cortex compared to other mammals. It would be natural to assume that similar increases in intellect would occur in the same way in birds, but instead, it was found that they had an increase in neuronal density in the pallium (a region of the brain which also encompasses the cortex). This seems to indicate that higher cognitive function can develop in markedly different ways in different animal classes.

Phylogenetic tree of the teleost species sampled by P. Estienne, et al. Figure reproduced under CC BY 4.0.

Wrasse fish intelligence

Wrasses are a family of marine fish in the teleost fish infraclass, a sub-section of Actinopterygii or “ray-finned fishes.” Despite the relatively low encephalization of teleosts, and fish in general, wrasses have been found to exhibit tool-use. Since they lack limbs, they use their mouths to grab shellfish and break them by repeatedly hitting them against a hard flat surface such as a rock or coral.

To determine how this higher-order behavior corresponded to encephalization, researchers at the Paris-Saclay Institute of Neuroscience compared the body mass, brain mass, and brain structure of various wrasses and teleosts.

Left) The five major regions of the brain examined in this study, highlighted on the brain of a cichlid, a relative of the wrasse. Right) The same regions highlighted on the wrasse brain. Figure adapted from P. Estienne, et al. under CC BY 4.0.

Brain visualization with Amira Software

In order to visualize these intricate brain structures, micrometer thick slices of teleost brains were imaged and recombined digitally into 3D representations with Thermo Scientific™ Amira™ Software. Using manual segmentation, the individual regions of the brain could be identified, including lobes and connective fibers. The researchers found that wrasses have a larger telencephalon and general forebrain/midbrain region compared to other teleost fish. Notably, this included a much larger inferior lobe, a structure that has no direct analog in mammals and birds. This lobe also showed unique and substantial connectivity to the pallium, which has already been linked to higher-order thinking in other animals. Overall, this showcases yet another unique way in which higher-order thinking can develop in animals, expanding the ways in which we understand brain function and evolution.

Comparison of the interconnectivity of the pallium and the inferior lobe (IL) in wrasses and zebrafish. The IL of wrasses is distinctly larger compared to the zebrafish, indicating a high degree of encephalization. Figure adapted from P. Estienne, et al. under CC BY 4.0.

Understanding brain evolution through imaging

This study has provided unique, novel insights into the development of intelligence in different animal classes, and how intelligent behavior can evolve along different pathways in the brain. These observations were enabled by high-resolution imaging and 3D visualization, which provided the researchers with an intimate view of the entire brain and its intricate structures.

Read the full article in Nature >>

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