THE ROLE OF THE EYE IN THE MIGRATORY RHYT HMS OF DAPHNIA-MAGNA AND DAPHNIA-LONGISPINA (CLADOCERA)

Vertical migrations of Daphnia coincide with changes of light intensit y. These displacements persist under constant conditions, suggesting t he existence of an endogenous component acting in the migration mechan isms. This component was investigated in the eye of Daphnia. Organisms were submitted to various light conditions : alternating light/dark o r;constant light They were fixed at different times of the day and the structure of the eye was studied with an image analyseur. Under alter nating light/dark, the photoprotector pigments are more concentrated a round the cristallin before sunset and are around the upper part of th e rhabdom later on. This-phenomenon persists.under constant-light cond itions, suggesting the presence of an endogenous component in the meca nism of the pigment distribution. We propose the hypothesis that pigme nt variation determines the vertical-migration of Daphnia. The alterna ting light/dark in blue light inhibits migrations and variations of pi gment density are no more significant. The above results seem to confi rm that the displacements of pigment along the rhabdom are part of the internal oscillator, or are an expression of it.