Breaking the rules: Atypical photoreceptors with diverse functions

Abstract

As the Earth rotates around its axis, the Sun illuminates different parts of the planet with varied strengths and amounts of light exposure. Primary producers, from unicellular cyanobacteria to redwood trees, harness the light energy and provide the basis for food chains in the Earth's ecosystems. Light, however, has other functions that are important for survival, which include phototaxis in unicellular organisms, measuring day length (photoperiodism) in plants and animals, and vision. Although the eye is a highly specialized organ that contains the photoreceptive machinery to mediate vision, organisms such as fish, birds and lizards rely on extraocular photoreceptors to co-ordinate long-term effects of light, including synchronization of daily rhythms to the solar day, photoperiodic measurements and hormonal regulation. Mammals, however, rely solely on the eyes and a specialized photoreceptive neuronal structure, the retina, to receive all light information for image and non-image forming visual functions.