Many amphibian tadpoles hatch and swim before their inner ears and sense of spatial orientation differentiate. Tadpoles are heavier than water and if touched while lying on the substrate they reliably swim upwards, often in a tight left or right handed spiral which is not fixed for individual tadpoles. In water, their centre of mass was ventral to the muscles in the yolk mass. Measurements in tethered tadpoles showed that dorsal body flexion, which could produce a dorsal pitch torque, is present during swimming and increases with swimming frequency. 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.
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. 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 increased for up to 20 minutes during sustained dimming. We suggest that the increase in pineal discharge during dimming increases the probability of upwards swimming and in this way increases the probablity of tadpoles attaching to objects higher in the water column which cast shadows. One of our central goals is to define the anatomy and function of all spinal neuron classes in the Xenopus tadpole.