Physiology of Finfish and Shellfish (2+1)

3.3.7. Pineal gland

The pineal gland dorsally located on the brain also has light sensitivity.
Blind cavefish (Astyanax mexicanus) can sense light when young, even though their eyes lost their function over a million years of evolution. Scientists have found that the fish larvae can detect an overhead shadow and seek shelter by swimming towards it. The researchers found that the behaviour is driven by the pineal gland, a light-sensitive organ sometimes known as the ‘third eye’. This gland contains a light-sensitive pigment called rhodopsin, and when it was removed the fish’s shadow response disappeared.