Extraocular, Non-Visual, and Simple Photoreceptors: An Introduction to the Symposium.

It has been recognized for decades that animals sense light using photoreceptors besides those that are devoted strictly to vision. However, the nature of these receptors, their molecular components, their physiological responses, and their biological functions are often obscure. Only recently have researchers begun to learn how critical these non-visual or very simple visual responses are to organismal function. New approaches, including high-throughput molecular genetic techniques, have led to a revolution in our understanding of the evolution, anatomical distribution, physiology, and-in some cases-function of non-visual photoreception in diverse organisms. In the following papers, we bring together specialists from throughout the field to review the current state of knowledge regarding extraocular, non-visual, and simple photoreceptors in a large diversity of organisms ranging from protists through vertebrates and invertebrates.