Optical Orientations

Abstract

Why vision in an optical empty environment? Why orientation in a vast homogenous pelagic environment by animals having a small action radius only? For a Calanus in the middle of the ocean, even for a Daphnia in the middle of a lake, it is without sense to swim 10 m to the left or to the right or in whatever horizontal direction. On the other hand, orientation in the vertical plane is of paramount importance, otherwise there would be no diel vertical migrations. In the literature of the first half of the last century, geotaxis was thought as an important mechanism to distinguish between up and down. Geotaxis was, however, in experiments difficult to distinguish from phototaxis and the two have never been properly separated. Therefore, the role of geotaxis in vertical plane orientation became questionable (Cushing, 1951). Another environmental factor must be available for finding the vertical direction in the pelagic environment and downwelling light is a reliable one making optical orientation plausible. The following experiment with different angular light distributions illustrates this.