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. KEY WORDS: Lagrangian coherent structures · Medusae · Field measurements · Digital particle image velocimetry · Predator-prey interactions 435: 111-123, 2011 addition to providing thrust, these bell motions transport fluid to prey capture surfaces (i.e. tentacles and manubrium-oral arms), where entrained prey may be retained (Costello & Colin 1995, Colin et al. 2006, Peng & Dabiri 2009). The fluid interactions and patterns of flow around swimming-cruising medusae have been well described qualitatively (Dabiri et al. 2005, Colin et al. 2006. These studies have shown that as the bell expands, fluid adjacent to the bell margin is entrained and drawn through trailing tentacles into a stopping vortex ring (structure created at the end of a swimming cycle) that rotates inside the subumbrellar cavity. As the bell contracts, fluid continues to be entrained adjacent to the bell margin and, along with fluid from the stopping vortex, is drawn into a starting vortex ring (structure created at the start of a swimming cycle) that rotates through the trailing tentacles in the wake of the medusa. Consequently, fluid appears to be continuously entrained throughout the swimming cycle and interacts with capture surfaces (Dabiri et al. 2005). Resale or republication not permitted without written consent of the publisherMar Ecol Prog SerWhile these studies have provided an understanding of the general nature of fluid transport around the bell and in the medusan wake, they have not characterized the...

The widespread occurrence and frequent abundance of small hydromedusae suggests that they may play an important role in planktonic communities. However, rather than exhibiting dominant impacts on any specific planktonic group, field studies have demonstrated diverse dietary niches and only modest trophic impacts by small hydromedusae. To understand the functional bases for these patterns, we exposed 2 hydromedusae (Cladonema californicum and Leuckartiara sp.) to a variety of prey types (dinoflagellates, rotifers, barnacle nauplii, copepods and the hydromedusa Obelia sp.) while video-recording predation sequences (encounter, capture, ingestion). Both C. californicum and Leuckartiara sp. ambush prey and possess penetrating nematocysts (stenoteles and euryteles, respectively). Although similar prey selection patterns might be expected based on encounter models or nematocyst complements, the 2 species exhibited some markedly different ingestion patterns. For example, C. californicum positively selected copepod prey and negatively selected hydromedusae, whereas Leuckartiara sp. exhibited the opposite pattern. Quantification of predation sequences demonstrated that hydromedusan dietary variations resulted from speciesspecific differences in prey capture efficiencies as well as efficiencies in post-capture transfer to the gut. Species-specific prey selection patterns and limited ingestion capacities may explain the diverse prey selection patterns and limited trophic significance observed in field studies of ambush-foraging hydromedusae.