Hydrodynamics and bio-locomotion of krill caridoid response maneuver

Krill are shrimp-like crustaceans with a high degree of mobility and variety of documented swimming behaviors. Whereas there is a richness of literature documenting and categorizing different swimming behaviors in krill, there are limited studies quantifying the hydrodynamics and locomotion. For the first time, the propulsion behavior and flow disturbance of a caridoid response performed by an Antarctic krill (Euphausia superba) has been quantified. A high-speed tomographic Particle Image Velocimetry (tomo-PIV) system quantifies the three-dimensional flow field around a free-swimming E. superba and in its wake. The specimen is roughly 2.5 - 3 cm in length. The caridoid escape response, a maneuver unique to crustaceans, occurs when the animal performs a series of rapid abdominal flexions resulting in powerful backward strokes. By performing caridoid response, the animal is able to accelerate backwards increasing its speed by 2 orders of magnitude in a 0.025 second interval and reaches a maximum speed of 25 cm/s. Antarctic krill typically swim in a low to intermediate Reynolds number (Re) regime where viscous forces are significant, but as shown by this analysis, its high maneuverability allows them to quickly change its body angle and swimming speed. The data from these flow fields are used to calculate the changes in the velocity and vorticity fields and the kinematics of its swimming behavior shedding light on both the flow behavior in this Re regime and intricacies of the bio-locomotion of zooplankton.