From an ecological viewpoint, diel vertical migration (DVM) of zooplankton is an adapted response to environmental factors. DVM coincides with changes in light intensity, but persists also under constant illumination conditions. This fact suggests the existence of an internal clock. In Daphnia longispina, it could be located in the eye. Indeed, the spatial distribution of the pigments containing in the eye ommatidia exhibits significant changes over time when Daphnia are kept under light/dark (LD) conditions. These variations coincide with displacements of the organisms. Furthermore, it is observed that the blue/dark conditions, which inhibit DVM, also stop changes in the distribution of pigments in the ommatidia. The modifications of the eye observed under LD white light represent either the internal clock or its manifestation.

We investigated pigment granule migration in the eye of the crustacean, Daphnia longispina. Pigment granule positions were indirectly observed by monitoring the pigment granule surface area (PGSA) under constant illumination as well as under natural lighting conditions. Significant variations were observed in the PGSA during the course of the 24-h night-day cycle. These phenomena, which persisted under continuous illumination, suggest that an endogenous mechanism is involved in bringing about the observed diurnal morphological changes in the compound eye of Daphnia longispina.

Le rôle des yeux, et plus précisément des pigments ommatidiens, dans le contrôle des rythmes quotidiens de déplacement de Daphnia longispina est étudié. En éclairement constant (LL) au laboratoire, les modifications pigmentaires sont globalement identiques à celles observées in situ, hormis dans le temps, au voisinage du crépuscule. Les changements dans l'oeil en LL précèdent la remontée des daphnies, au lieu de coïncider avec elle. Ceci est l'expression du rythme propre d'un oscillateur en libre cours. Ainsi, chez D. longispina, un oscillateur interne contrôle les DVM, qui sont circadiennes et non nycthémérales et, in situ, l'illumination à l'aube suffit à synchroniser les migrations sur l'alternance jour/nuit.