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Abstract

Swimming behaviour and distribution of Daphnia magna were studies in relation to various concentrations of the food alga Scenedesmus acutus. The experiments were done in a ring-shaped flow through chamber in the light and in the dark. Daphnia behaviour was monitored with a video camera sensitive to infrared light. The daphnids showed rapid swimming in visible light and at low food concentrations. At higher food concentrations the swimming speed decreased. Only slow swimming was observed in the dark despite the food concentration. In a food-gradient, the animals congregated in the areas with the highest algal densities when there was light, whereas animals in the dark showed a weaker reaction. Daphnids do not seem to have a directed food search guide by vision or chemical signals, but seem to swim horizontally in the light until they meet food patches