Ask anyone who keeps freshwater tropical fish to name the top five most exotic, bizarre fish available to hobbyists, and chances are the black ghost knife fish will be in there. Besides looking incredibly cool, these Amazon basin creatures have two rather unusual characteristics: they can sense all around themselves by generating a weak electrical field, and they can move in any direction, thanks to an undulating ribbon-like fin that runs along the length of their underside. In an effort to replicate that form of maneuverability for use in man-made submersibles, a team led by Northwestern University mechanical and biomedical engineer Dr. Malcolm MacIver has created the GhostBot – an underwater robot that moves via a knife fish-like fin.

MacIver has been a fan of black ghost knives for years, and marveled at how the fish could move forward or backward, or up or down simply by using the one fin. Along with graduate student Oscar Curet, he observed that while the horizontal movements were caused by just one wave traveling along the fin, forward or backward, the vertical movements involved two waves that started at either end of the fin and met in the middle. A computer model confirmed that such “inward counterpropagating waves” could indeed cancel horizontal thrust, replacing it with a downward jet that thrust the fish upward.

This discovery prompted MacIver to try building his own robotic knife fish, although after three attempts he still couldn’t get enough points of movement (or degrees of freedom) in its fin. He then contacted Kinea Design, a company founded by Northwestern faculty, that specializes in human interactive mechatronics. After seven months and US$200,000, MacIver and Kinea had created the present fourth version of the GhostBot. The device’s lycra-covered fin contains 32 artificial fin rays, each one controlled by a separate motor, to provide very fish-like movement.

The robot was tested in a water flow tunnel at Harvard University, where it performed flawlessly.

Like its real world counterpart, GhostBot is also equipped with an electrosensory system. MacIver’s next goal will be to coordinate that system with its single-fin propulsion system, so that it can sense objects and then move itself up beside them. Such maneuverability, he said, would have been particularly useful in submersibles used during the Gulf Oil Spill.

As an interesting side note, MacIver also recently used black ghost knives to create music ... of a sort.

Along with visual and conceptual artist Marlena Novak and composer and sound designer Jay Alan Yim, MacIver created an interactive exhibit entitled Scale. It consisted of 12 aquaria, each one containing a different species of Amazonian electric fish – such as the black ghost knife. Each fish’s electrical field was picked up by a sensor, then amplified as a tone. Because each fish was different, their fields likewise didn’t sound alike.

Participants would stand in front of the arc of fish tanks and act as a conductor, using a modified Wii controller to select or combine the tones of different fish, and utilizing a touchscreen to bring the volume up and down. The resulting tunes were ... well, let’s just say it sounded very experimental.

"Together we were looking for a way to develop something that combined science and art in an exciting way," MacIver told Gizmag, of his collaboration with Novak and Yim. "As I showed them my lab, and let them listen to the different tones of the different species, Novak became very excited by the possibility of making an installation art piece based on this."

Scale debuted at last November’s STRP Festival in The Netherlands, to what MacIver said was a tremendous response. He has posted a video of the exhibit on his lab’s website.

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