Thresholds of retinal and extraretinal photoreceptors measured by photobehavioral response in catfish, Silurus asotus

Light thresholds of retinal and extraretinal photoreceptors in catfish were examined by the photobehavioral response using a method in which reflex body movements are recorded. Thresholds were determined in six groups, (A) intact, (B) ophthalmectomized, (C) pinealectomized, (D) ophthalmectomized + pinealectomized, (E) ophthalmectomized, pinealectomized and skinless over the brain (skinless fish) and (F) ophthalmectomized, pinealectomized and dorsally covered with aluminum foil over the brain (covered fish). All these fishes displayed short term activity to white light stimulation after being dark adapted for more than 5 h. The lowest threshold was obtained in the intact group (2.0×10−4 μW/cm2). The thresholds of lateral eyes and the pineal organ were 3.4×10−3 and 1.5×10−2 μW/cm2, respectively. Without lateral eyes and pineal organ, catfish still responded to light, indicating the possible existence of extraretinal nonpineal photoreceptors (ENPs). The threshold of ENPs was 3.3 μW/cm2. The localization of ENPs was assumed to be in the brain from the experiment with the combination of skinless and covered fish.