First identification and localization of a visual pigment in Hydra (Cnidaria, Hydrozoa)

The cnidarian Hydra does not possess identified photoreceptive structures o r specialized cells for light detection; nevertheless, it shows a marked ph otosensitivity. So far no evidence has been previously reported about the l ocalization of the proteins involved in the photoresponse. We used polyclon al antibodies and immunofluorescence microscopy on whole-mount Hydra to ide ntify a putative rhodopsin-like protein. Our results show an immunoreactivi ty in the ectodermal layer of Hydra, which corresponds in position to the n ervous epidermal sensory cells. These data provide the first identification of a rhodopsin-like protein in a phylogenetically old invertebrate and giv e a new insight into the Hydra photoreceptive response.