Eyeless zebrafish larvae may still find their way out of darkness by the activation of light-sensitive neurons deep inside the brain.
Harold Burgess at the National Institute of Child Health and Human Development in Bethesda, Maryland, and his team found that the transparent zebrafish larvae (Danio rerio; pictured, with certain brain cells in green) swim gradually towards the illuminated areas of their tank, even after their eyes have been removed. This behaviour suggests the presence of light-responsive neurons outside the conventional visual organs. Engineered eyeless fish that express less Opn4a — a light-sensitive molecule in the brain — responded poorly to light, whereas fish that produced more Opn4a performed better.
The researchers conclude that neurons expressing Opn4a in the preoptic area of the brain may support simple, and perhaps primordial, light-seeking behaviours.
Curr. Biol. http://dx.doi.org/10.1016/j.cub.2012.08.016 (2012)