Pineal eye and behaviour in Xenopus tadpoles

Abstract

THE pineal complex of lower vertebrates contains photoreceptors similar to those in the retina¹. These photoreceptors are in synaptic contact with ganglion cells whose impulse discharge is decreased by light and enhanced by dimming in most of the animal groups examined (Agnathans, Elasmobranchs, Teleosts and Amphibians)². In Amphibians, pineal photoreceptors have been implicated in control of pigmentation, synchronisation of circadian activity patterns, phototaxis, compass orientation and in the perception of linearly polarised light³. However, no simple reflex responses have been reported. It is therefore interesting that in young tadpoles of the clawed toad (Xenopus laevis), pineal photoreceptors seem to be responsible for a simple escape response. The evidence presented here indicates that the pineal eye can initiate swimming when the illumination is dimmed.