Figure 1.

Illustration of the boxfish swimming paradox. (a) The capacity of boxfish to perform 180° turns with near-zero turning radii is illustrated (adapted from [7]) showing body outlines at 0.32 s intervals for Ostracion meleagris. (b,c) The automatic course-stabilization hypothesis [8–11] is illustrated for Rhinesomus triqueter. During nose-up pitch angles of attack (b), a vortex is formed resulting in a zone of negative pressure just above the left and right ventral keels (shown in oblique latero-frontal view). The dorsal component of the pressure force at this zone of the carapace causes a moment that will reorient the boxfish in line with the flow (i.e. a stabilizing, nose-down pitch moment about the centre of mass (COM)). During yaw angles of attack (c), a vortex is formed resulting in a zone of negative pressure on the far-field side of the dorsal keel (shown in dorsal view). The lateral component of the pressure force at this zone causes a stabilizing moment about the centre of mass forcing the head to turn right.