Pineal photosensitivity. A comparison with retinal photoreception.

Pineal photoreceptors of poikilothermic vertebrates possess numerous anatomical, physiological and biochemical similarities to retinal photoreceptors, including the rhythmic melatonin biosynthesis, with nocturnal peaks and low day-time levels. This brief outline will survey the photoreceptor properties of the pineal organ of poikilothermic vertebrates, which suggest that the pineal is not only a simple light detector (that acts as a kind of photometer), but that it is capable of processing the light information and to discriminate it from informations that have no meaning for its assumed photoperiodic function.