Two groups each of approximately 100 Moenkhausia dichroura, a schooling characid, showed a long-lasting, constant-oriented swimming when placed in a light-centred circular channel. This apparatus consists of a I-m diameter circular channel illuminated by either a central or a peripheral light system, so that the light angle is constant all around the channel. With the central light at a fixed angle, fish swam for several months in one direction and reversed direction at a certain date. When the light angle was increased by 10" every other day between 0" (horizontal) and 90" (vertical), swimming direction was reversed at a particular angle in each experiment. This response to artificial light suggests that this small schooling fish uses the sun as an orientation clue in its seasonal migrations.

SynopsisFish groups were tested both in a circular and in a figure eight-shaped channel. In both cases fish showed a long lasting, constant direction swimming provided that illumination was maintained at a constant angle around the channel. In the circular channel, fish did not reverse direction, as would be expected, when light angle was shifted from one side to the other in the channel. However, direction reversals did occur when these illumination shifts were performed on the eight-shaped channel. We suggest that constant-oriented swimming reflects a sun-compass oriented behavior, but swimming at a constant angle in the circular channel produces an irreversible disarrangement of the inertial-orientation system, which does not occur in the eight-shaped channel due to the geometrical relationship between the light and the shape of the channel.

SynopsisSmall microcharacids Cheirodonpulcher were netted during the dry season in pools of the savanna 'llanos' at a considerable distance from the main river channel. Fish could have moved from the river to these pools using sun-compass orientation. Orientation responses of these fish were tested in a four-armed experimental device, exposed to direct sunlight or to azimuth-inverted sunlight reflected by a mirror. Groups of 20 fish were placed in a central area, released and then counted in the arm traps. When exposed to direct sunlight, fish oriented south, both in the morning and afternoon. Nevertheless, when the sun azimuth was inverted they oriented north, indicating that they use a sun-compass and that they compensate for sun-azimuth inversion at noon in a way that geographic headings remain constant throughout the day. These results reinforce previous laboratory interpretations which suggested that these flood plain species migrate inland during the high-water period and return to the main river channels before the dry season begins.