The Fish that Swims Like a Paddle-Wheel Boat

Anup A. Shirgaonkar (
Mechanical Engineering, Northwestern University 

Oscar M. Curet,
Mechanical Engineering, Northwestern University

Neelesh A. Patankar,
Mechanical Engineering, Northwestern University

Malcolm A. MacIver,
Mechanical Engineering and Biomedical Engineering,Northwestern University

The vortex structure around a sinusoidally undulating ribbon fin of a weakly electric fish. A series of organized vortex rings creates a jet that propels the fish.

The Fish that Swims Like a Paddle-Wheel Boat

The fish that swims like a paddle wheel boat. A simulation of the undulating fin of a weakly electric fish, a fresh water fish that hunts in the rivers of the Amazon Basin. These fish have the unique ability to "see" with a self-generated electric field, enabling it to hunt at night and in the muddy water common to these rivers. Because their field goes out in all directions, they require the ability to move in all directions to quickly reach prey they have sensed with their field. Key to this astonishing agility is their long ribbon-like fin, running along most of the bottom edge of their body, a simulation of which is shown in the left portion of the image.

Using these simulations, we have discovered that the fish pushes itself through water much as a paddle wheel boat does, where each "paddle" is one undulation of the fin. The fish can quickly change from swimming forward to swimming backward to reach a prey it has detected, simply by reversing the direction of paddle movement along the fin. The right panel shows a front view of the fin, indicating rings coming away from the fin much like smoke rings are created.

This fish is a key laboratory animal for investigations into how the nervous system processes sensory information and controls movement. This work is leading to exciting new robotic applications for maneuverable underwater robots and systems that can perceive with electric fields.

A video of this work is available at

This work was supported by NSF.

The images have been published in the journal article "The hydrodynamics of ribbon- fin propulsion during impulsive motion" by A. A. Shirgaonkar, O. M. Curet, N. A. Patankar, and M. A. MacIver. J. Expt. Biol., 211:3490-3503, 2008.