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The Shocking Truth

Scientists have learned how electric fish got their jolt

It is well known that a certain kind of fish swims the world’s waters protected, as it were, by its very own stun gun.

Unknown, until now, is how electric fish evolved such a defense. A team of researchers led by CALS biochemistry professor Michael Sussman has established the genetic basis for the electric organ, an anatomical feature found only in fish. It evolved independently half a dozen times in environments ranging from the flooded forests of the Amazon to murky marine environments.

“These fish have converted a muscle to an electric organ,” says UW Biotechnology Center director Sussman, who began this research almost a decade ago. The study, recently published in the journal Science, provides evidence to support the idea that the six electric fish lineages used essentially the same genes and developmental and cellular pathways to make the electric organ, which fish use to communi- cate with mates, navigate, stun prey, and as a shock- ing defense. The jolt from an electric organ can be several times more powerful than the current from a standard household electrical outlet.

Worldwide, there are hundreds of electric fish in six broad lineages. Their taxonomic diversity is so great that Darwin himself cited electric fishes as critical examples of convergent evolution, where unrelated animals independently evolve similar traits to adapt to a particular environment or ecological niche.

The new work includes the first draft assembly of the complete genome of the South American electric eel. “A six-foot eel is a top predator in the water and is in essence a frog with a built-in five-and-a- half-foot cattle prod,” says Sussman. “Since all of the visceral organs are near the face, the remaining 90 percent of the fish is almost all electric organ.”

Electric fish have long fascinated humans. The ancient Egyptians used the torpedo, an electric marine ray, in an early form of electrotherapy to treat epilepsy. Much of what Benjamin Franklin and other pioneering scientists learned about electric- ity came from studies of electric fish. In Victorian times, parties were organized where guests would form a chain to experi- ence the shock of an electric fish.

All muscle cells have electrical potential. Simple contraction of a muscle will release a small

amount of voltage. But at least 100 million years ago some fish began to amplify that potential by evolv- ing from muscle cells another type of cell called an electrocyte—larger cells, organized in sequence and capable of generating much higher voltages than those used to make muscles work.

The “in-series alignment” of the electrocytes and the unique polarity of each cell allows for the “sum- mation of voltages, much like batteries stacked in series in a flashlight,” says Sussman.

In addition to sequencing and assembling DNA from the electric eel genome, the team produced protein sequences from the cells of the electric organs and skeletal muscles of three other electric fish lin- eages using RNA sequencing and analysis.

“I consider exotic organisms such as the electric fish to be one of nature’s wonders and an important gift to humanity,” says Sussman. “Our study dem- onstrates nature’s creative powers and its parsimony, using the same genetic and developmental tools to invent an adaptive trait time and again in widely disparate environments.”

And the findings may be useful to humans. “By learning how nature does this, we may be able to manipulate the process with muscle in other organ- isms and, in the near future, perhaps use the tools of synthetic biology to create electrocytes for generating electrical power in bionic devices within the human body or for uses we have not thought of yet,” says Sussman.

Sussman’s collaborators include Harold Zakon of the University of Texas at Austin and Manoj Samanta of the Systemix Institute in Redmond, Washington. The study was funded by the National Science Foundation, the W. M. Keck Foundation and the National Institutes of Health.