Form, function and flow in the plankton : jet propulsion and filtration by pelagic tunicates

Abstract

Trade-offs between filtration rate and swimming performance among several salp species with distinct morphologies and swimming styles were compared. Small-scale particle encounter at the salp filtering apparatus was also explored. Observations and experiments were conducted at the Liquid Jungle Lab, off the pacific coast of Panama in January 2006 through 2009. First, time-varying body volume was calculated by digitizing salp outlines from in situ video sequences. The resulting volume flow rates were higher than previous measurements, setting an upper limit on filtration capacity. Though each species possessed a unique combination of body kinematics, normalized filtration rates were comparable across species, with the exception of significantly higher rates in Weelia cylindrica aggregates, suggesting a tendency towards a flow optimum. Secondly, a combination of in situ dye visualization and particle image velocimetry (PIV) measurements were used to describe properties of the jet wake and swimming performance variables including thrust, drag and propulsive efficiency. All species investigated swam via vortex ring propulsion. Though Weelia cylindrica was the fastest swimmer, Pegea confoederata was the most efficient, producing the highest weight-specific thrust and whole-cycle propulsive efficiency. Weak swimming performance parameters in Cyclosalpa affinis, including low weight-specific thrust and low propulsive efficiency, may be compensated by comparatively low energetic requirements. Finally, a low Reynolds number mathematical model using accurately measured parameters and realistic oceanic particle size concentrations showed that submicron particles are encountered at higher rates than larger particles. Results from feeding experiments with 0.5, 1 and 3 μm polystyrene microspheres corroborated model predictions. Though 1 to 10 μm-sized particles (e.g. flagellates, small diatoms) are predicted to provide four times as much carbon as 0.1 to 1 μm- sized particles (e.g. bacteria, Prochlorococcus), particles smaller than the mesh size (1.4 μm) can still fully satisfy salp energetic needs.