Le role de l'oeil dans les rythmes migratoires de Daphnia magna et Daphnia longispina (Cladoceres)

Abstract

Vertical migrations of Daphnia coincide with changes of light intensity. These displacements persist under constant conditions, suggesting the existence of an endogenous component acting in the migration mechanisms. This component was investigated in the eye of Daphnia. Organisms were submitted to various light conditions: alternating light/dark or constant light. They were fixed at different times of the day and the structure of the eye was studied with an image analyseur. Under alternating light/dark, the photoprotector pigments are more concentrated around the cristallin before sunset and are around the upper part of the rhabdom later on. This phenomenon persists under constant light conditions, suggesting the presence of an endogenous component in the mechanism of the pigment distribution. We propose the hypothesis that pigment variation determines the vertical migration of Daphnia. The alternating light/dark in blue light inhibits migrations and variations of pigment density are no more significant. The above results seem to confirm that the displacements of pigment along the rhabdom are part of the internal oscillator, or are an expression of it.