Feeding, Digestion and Oxygen Consumption in Daphnia Magna a Study in Energy Budgets

Abstract

The feeding behaviour of Daphnia magna was investigated using fluorescent tracer particles to test simple model predictions for the ingestion rate of an alga, Scenedesmus subspicatus. The model assumes that ingestion rate is proportional to the squared length of the daphnid, that gut volume, measured as cell numbers, is proportional to the cubed length and independent of food density and that the functional response is hyperbolic. Gut residence time for food particles is therefore proportional to the length of the daphnid and depends on food density. Experimental results and data from the literature confirm these model predictions. A model for digestion, which is consistent with the observed feeding behaviour, is proposed. It shows that energy gain per food particle is independent of body size if the production of unstable enzyme is proportional to the surface area of the gut. Constraints have been formulated for complete digestion. Microbial proliferation is found unlikely to be a major factor in the digestion process. The oxygen consumption of individual daphnids was measured to test model predictions relating its dependence on energy reserves (modulated by starvation) and size. The measurements, which are taken to indicate the rate at which energy is used from reserves, confirm the model predictions on size dependence: the rate can be written as a physiologically interpretable weighted sum of a squared and a cubed length measure. The results are not conclusive for competing theories about the use of energy reserves during starvation.