Biomechanics: hydrodynamic function of the shark's tail.

The tail of most sharks has an elongated upper lobe that differs from the externally symmetrical tail structure common among bony fishes, but the hydrodynamic purpose of this asymmetric tail shape is unclear. Here we quantify water flow patterns in the wakes of freely swimming dogfish sharks and find that they have a ring-within-a-ring vortex structure, in contrast to the single rings shed by symmetrical fish tails. The branched-ring vortex is generated by the inclined angle of the tail's trailing edge and by its motion at an angle to the horizontal body axis; the vortex directs water backwards and downwards, which may increase the shark's vertical manoeuvrability.