Molecular and functional diversity of visual pigments: clues from the photosensitive opsin–like proteins of the animal model Hydra

The primary event of vision is the absorption of photons by photosensitive pigments, which triggers the transduction process producing the visual excitation. Although animal eyes and eyeless photoreceptive systems developed along several levels of molecular, morphological and functional complexity, image–forming rhodopsin family appears ubiquous along visual systems. Moreover, all Metazoa have supplementary extraocular photoreceptors that regulate their temporal physiology. The investigation of novel non-visual photopigments exerting extraretinal photoreception is a challenging field in vision research. To study molecular and functional differences between these pigment families, we propose the cnidarian Hydra, the first metazoan owning a nervous system, as a powerful tool of investigation. Hydra shows only an extraocular photoreception lacking classic visual structures. Our findings provide the first evidence in a phylogenetically old species of both image– and non–image–forming opsins, giving new insights on the molecular biology of Hydra photoreception and on comparative physiology of visual pigments.