Quantification of flows generated by the hydromedusa Aequorea victoria: a Lagrangian coherent structure analysis

Most oblate medusae use flow generated during swimming to capture prey. Quantification of their interactions with surrounding fluid is necessary to understand their feeding mechanics and to develop models to predict their predatory impact. In the present study, we quantified how the hydromedusa Aequorea victoria interacts with both its surrounding fluid and prey. The fluid interactions were examined in the laboratory and in natural field settings using digital particle image velocimetry (DPIV) measurements. The laboratory DPIV data were used to compute finite-time Lyapunov exponent (FTLE) fields, and Lagrangian coherent structures (LCS) were extracted from the FTLE fields. The laboratory LCS analysis demonstrated that swimming A. victoria only encounter discrete packets of fluid originating upstream of the medusan bell. Based on the size of these packets, we estimated that the A. victoria examined have the potential to clear 11.4 l h(-1). Used in conjunction with measured prey capture efficiencies, we estimated potential clearance rates on different prey types. These hydrodynamically based clearance rate estimates are consistent with previously measured empirical clearance rate estimates. Velocity vector and shear fields also suggested that the feeding current created by A. victoria may be more suitable for encountering copepods than previously thought. Although still preliminary, in situ DPIV data indicate that natural background flows may alter the encounter process from what is observed in still-water laboratory conditions.