Volumetric flow imaging reveals the importance of vortex ring formation in squid swimming tail-first and arms-first

Abstract

Squids use a pulsed jet and fin movements to swim both arms-first (forward) and tail-first (backward). Given the complexity of the squid multi-propulsor system, 3D velocimetry techniques are required for the comprehensive study of wake dynamics. Defocusing digital particle tracking velocimetry, a volumetric velocimetry technique, and high-speed videography were used to study arms-first and tail-first swimming of brief squid Lolliguncula brevis over a broad range of speeds (0-10 dorsal mantle lengths (DML) s−1) in a swim tunnel. Although there was considerable complexity in the wakes of these multi-propulsor swimmers, 3D vortex rings and their derivatives were prominent reoccurring features during both tail-first and arms-first swimming, with the greatest jet and fin flow complexity occuring at intermediate speeds (1.5 – 3.0 DML s−1). The jet generally produced the majority of thrust during rectilinear swimming, increasing in relative importance with speed, and the fins provided no thrust at speeds >4.5 DML s−1. For both swimming orientations, the fins sometimes acted as stabilizers, producing negative thrust (drag), and consistently provided lift at low/intermediate speeds (<2.0 DML s−1) to counteract negative buoyancy. Propulsive efficiency (η) increased with speed irrespective of swimming orientation, and η for swimming sequences with clear isolated jet vortex rings was significantly greater (η = 78.6 ± 7.6% (s.d.)) than swimming sequences with clear elongated regions of concentrated jet vorticity (η = 67.9 ± 19.2% (s.d.)). This study reveals the complexity of 3D vortex wake flows produced by nekton with hydrodynamically distinct propulsors.