Studying Daphnia feeding behavior as a black box: a novel electrochemical approach

We present a novel approach for examining the complex feeding behavior of a filter feeder at a previously unexploited scale. A Daphnia lives in a viscous environment and thus creates a feeding current with a distinct laminar inflow and a repetitive pulsed outflow. We propose that by treating the feeding apparatus as a black box, and using the pulsed outflow current as a surrogate to the inside working of the apparatus, we can calculate feeding rate in near real time. The structure of the outflow is interpreted as a direct representation of the organism’s response to its environment. Therefore, we examine how the work performed by an organism’s feeding apparatus is altered according to environmental factors and metabolic demands. Our approach is an integration of optical (Schlieren system) and electrochemical (chronoamperometry) techniques that allow for real time visualization and temporal analysis of flow systems, respectively. As electrochemistry requires a tracer chemical, we employed low dopamine concentrations (≤ 1mM), and tested the effect of dopamine on the heart rate and swimming of Daphnia. It appears that dopamine free in solution at concentrations below 10 mM has no adverse effects on the organism, and all observed differences in Daphnia feeding behavior were due to environmental or metabolic factors. The feeding nature of daphnids in the presence or absence of food, and differences between the sexes is reported. Our results indicate that in the absence of food a Daphnia has a strict and repetitive feeding behavior with short delays between pumping actions. However, in the presence of food this behavior becomes complex, with increased delays between pumps, perhaps designed to maximize feeding efficiency. Our observations demonstrate that males have a higher appendage beat frequency than females under identical conditions. We hypothesize that the difference may be dictated by metabolic demand, as a male spends more time actively seeking a mate. The application of electrochemistry to the study of Daphnia feeding behavior is an improvement over current methods for its near real time quantification of behavioral response, its versatile application under varying environmental conditions and its extreme sensitivity to changes in the organism’s feeding behavior. This technique is a valuable addition to the current tools available for studying Daphnia feeding behavior and will allow us to learn more about the interactions of an organism with its environment. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera