Print PageChemical cues from plants and animals can impact the behavior of Daphnia. Daphnia dentifera is expected to alter its behavior when exposed to kairomones from planktivorous fish, based on observation of daily migration behavior in Sites Lake, OH (40¢ªN, 82¢ªW). Initial bioassays of lab-grown D. magna and D. pulex indicated no chemical or physical recognition of a natural predator, bluegill sunfish (Lepomis macrochirus). Therefore, we must use “wild” Daphnia, from an environment that selects for traits that help individuals cope with visual predators. However, the small size (<1.2mm) and relative lack of color of D. dentifera impedes our ability to effectively monitor their behavior in experimental tanks. To enhance their visibility, Daphnia can be fed transgenic Escherichia coli labeled with the Green Fluorescent Protein (GFP). GFP-labeled E. coli fluoresce yellow-green when exposed to UV light. When clumped together, fluorescence is observable using an epifluorescence microscope or macroscopically using a black light. It is crucial to be able to quantify and manipulate cell concentrations in a variety of feeding experiments to optimize gut fullness and determine a standard protocol.
Results/Conclusions
Optical density of E. coli culture dilutions used for experiments (determined with a NanoDrop spectrophotometer at 595nm) varied over two orders of magnitude. These readings were compared to direct counts of GFP-E.coli at 1000X magnification (Olympus IX71). Dilutions with optical density readings below 0.005 could not be assessed directly; those cell densities must be extrapolated from measurement of higher-order dilutions. The standard curve allowed the relatively easy assessment of cell density and helped to speed up and facilitate feeding and evacuation experiments. Preliminary experiments suggested that optical densities of the upper one-third of measurements negatively correlated with maximum gut fullness of D. dentifera. This may represent feeding interference at higher cell densities. Optimized fluorescence levels may eventually allow effective monitoring of the behavior of Daphnia or other small species.
