The hydrodynamics of hovering in Antarctic krill

Abstract

Lay Abstract Antarctic krill, one of the most important species of the Southern Ocean ecosystem, are denser than water and must swim continuously to avoid sinking. They swim by beating their five pairs of swimming legs in a sequential pattern from back to front. Hovering by continuous swimming is costly in energy, and we hypothesize that the observed sequential stroking pattern provides an efficient means for krill to remain in the water column. Our goal was to measure the flow around a swimming Antarctic krill to understand the swimming mode and the induced water motion. We used four high‐speed cameras filming at 400 frames per second to measure the three‐dimensional flow produced by a krill hovering in an aquarium, allowing us to estimate the energy required. An additional estimate was made based on a theoretical model that is usually applied to helicopter hovering. The detailed velocity measurements provided information about the flow induced by the motion of the individual legs and the added benefit of the leg moving into water that was already flowing due to the motion of the previous leg. The water motion underneath the krill appeared as a time‐varying jet consistent with that observed for other multilegged swimming animals that use the sequential stroking pattern. This suggested that Antarctic krill are operating in a similar regime of high energy efficiency.

Underwater flight by the planktonic sea butterfly

“…Tomographic particle image velocimetry (PIV) measurements of freely swimming animals were collected at 500 frames s −1 within 1 week using the four-camera system described by Murphy et al (2012Murphy et al ( , 2013. The flow was seeded with 11.7 µm mean diameter hollow glass spheres and illumination was provided by near-infrared lasers (808 nm).…”

Internal physiology of live krill revealed using new aquaria techniques and mixed optical microscopy and optical coherence tomography (OCT) imaging techniques

“…Imaging of food digestion and clearance rate in the stomach and gut will further help improve understanding and estimation of gut evacuation and turnover rate which are important parameters to estimate carbon and nutrients recycling and transport in the marine system (Kawaguchi and Nicol 2014). Imaging shell movement, particularly in response to varying water flow rates through the water bath and krill trap could aid understanding of krill hydrodynamics, and energy budgets (Murphy et al, 2013). …”

Quantitative wake analysis of a freely swimming fish using 3D synthetic aperture PIV

“…The mask reveals the general shape of the caudal fin, with some elongation in Z that tapers to a point as the overlap between transformed camera views decreases. The elongation is an artifact created by the partial convergence of masks outside the actual plane of focus and is referred to in Murphy et al (2013) as the "shadow" of the visual hull. Scharfman et al (2013) eliminate the effects of this shadowing for spherical droplets using knowledge of the in-plane geometry.…”

“…Despite some elongation in the Z direction, the approximate size and orientation of the fish body is clear, and the mask is adequately large to block any regions within the body from biasing PIV results. While similar body reconstructions can be performed using fewer cameras (e.g., Adhikari and Longmire 2012;Murphy et al 2012Murphy et al , 2013, the additional cameras help smooth the mask edges and reduce the overestimation of body volume. The reduction in visual hull volume as a result of using all nine cameras instead of only the four outermost cameras is between 5 % (caudal fin mask only) and 20 % (full body mask).…”