Does the pineal eye help tadpoles to find objects that cast shadows?
When the light is dimmed, the pineal eye of hatchling Xenopus laevis tadpoles excites the central pattern generator for swimming but the behavioural significance of pineal excitation was unclear. Tadpoles spend 99% of their time hanging from the surface meniscus or solid objects using mucus secreted by a cement gland on the head. Attachment inhibits swimming but unattached tadpoles swim spontaneously. Provided their pineal eye is intact they attach closer to the water surface in the dark than in the light and attach preferentially to the underside of floating objects which cast shadows. Dimming causes tadpoles swimming horizontally to turn upwards (see picture), and is very effective in initiating upwards swimming in unattached tadpoles. Similar pineal dependent responses during swimming are present up to stage 44 but pinealectomy blocks responses to dimming at all stages. Recordings in immobilised tadpoles reveal that dimming leads to faster swimming and that pineal activity is increa
