Fish Brain

The fish brain is not a shrunken mammalian brain. It has its own architecture optimised for aquatic sensing and rapid response. The optic tectum handles vision (a role given to the visual cortex in mammals). The cerebellum coordinates fine motor movements. Recent research has demonstrated learning, tool use, social recognition, and even the passing of the mirror test in cleaner wrasse - overturning centuries of assumptions that fish were reflexive automatons.

Five main regions: olfactory bulb (smell), telencephalon (behaviour, learning), diencephalon (endocrine), mesencephalon with optic tectum (vision), and rhombencephalon with cerebellum (motor coordination) and medulla oblongata (autonomic). Brain-to-body mass ratio varies from 1:1000 in some lampreys to 1:100 in mormyrid elephantfish - the highest ratio in fish.

Sensory integration, motor coordination, endocrine regulation, and learning. Cleaner wrasse pass the mirror mark test - suggesting self-recognition. Archerfish learn to compensate for light refraction to shoot down insects. Zebrafish show associative learning that persists for weeks.

Mormyrids (elephantfish) have huge cerebellums and use active electroreception. Deep-sea fish have reduced brains overall. Sharks have surprisingly large brains, especially the olfactory lobes.

Mormyrid elephantfish have the largest brain-to-body-mass ratio of any fish - larger than some mammals. Their brain consumes nearly 60% of their oxygen budget.