Endogenous rectilinear guidance in fish: Is it adjusted by reference to the sun?

Previous works have shown that small migratory fish, when placed in an annular channel with central symmetrical lighting, swim continuously in one direction. This has been interpreted as a sunlight orientation response which assists in straight-line migration. The purpose of the present work is to test whether these fish have an endogenous rectilinear mechanism (inertial guidance) and if this mechanism is in some way associated with the sun directional orientation response. A group of small schooling fish, Cheirodon pulcher, was placed in an annular channel with central symmetrical lighting. Fish swam steadily in a clockwise direction during two weeks. They were then placed individually in a diffuse-light arena for 90 min and their movements were video-recorded. The number of right and left turns was counted. Fish rotated in the same direction as in the annular channel. In contrast, fish from a holding tank made equal numbers of right and left turns. The maintenance of turning behaviour after removal of the turning-promotive conditions suggests that an endogenous rectilinear guidance mechanism was modified while fish were swimming at a constant angle to light in the annular channel. This mechanism would be normally adjusted by reference to the sun.