[Structural variations of the eye linked to daily vertical migrations of Daphnia longispina]

The role of the eyes, and more precisely that of the ommatidian pigments, in the control of daily rhythms of movement of Daphnia longispina is investigated. In the laboratory, under permanent light (LL), the pigmentary modifications observed are globally similar to those observed in situ, except in their timing, around dusk, LL cycles do not coincide with the ascent of daphnids but precede it. This is the expression of an endogenous free-running rhythm. Therefore, in D. longispina, an internal oscillator controls DVMs, which are circadian and not nycthemeral, and, in situ, illumination at dawn suffices to synchronize migrations on dark/light alternation.