News in Science

Eel genome unlocks mysteries of electric fish

Shocking secret Electric fish can unleash a wicked jolt, now scientists have used genetic studies to unravel how this remarkable ability evolved six times in the history of life.

A team of US scientists sequenced the genome of the electric eel — a fearsome denizen of South America that can zap you with an electric field of up to 600 volts — as well as studied genetic data on two other types of electric fish.

Even though six groups of electric fish have evolved independently in far-flung locales like the muddy waters of the Amazon and murky marine environments, they all seem to have reached into the same "genetic toolbox" to fashion their electricity-generating organ, say the researchers.

The new study, published in the journal Science, found that various electric fish rely on the same genes and biological pathways to build their electric organs from skeletal muscle despite the different appearance and body location of their organs.

Their electrical abilities stand as one of the wonders of nature alongside traits like bioluminescence in some insects and sea creatures and echolocation in bats and whales.

"It really is something truly unique in the animal kingdom," says study co-author Michigan State University zoology professor Jason Gallant.

"This only arose in fish because water is a conductor of electricity while air is not. Thus, birds or terrestrial animals could not come up with this," adds biochemistry professor Michael Sussman of the University of Wisconsin-Madison .

Electric power

There are hundreds of species of electric fish worldwide with varying degrees of electric power.

Fish with weak electric power use it to navigate in dim waters and communicate with one another. Those like the electric eel — a serpentine freshwater relative of the catfish that grows up to 2.4 metres long — possess a powerful jolt that is used to stun or kill prey and repel enemies.

Scientists have wondered about how these fish first acquired electric powers and how this characteristic emerged six times in groups not closely related to one another.

"Electric organs start out their lives as muscle precursor cells. Through a series of developmental steps, they become larger, more electrically excitable and lose their ability to contract," says Gallant.

All muscle cells have electrical potential because any muscle contraction releases a small amount of voltage. Certain fish exploited that by transforming ordinary muscle cells into a larger type of cell called an electrocyte that generates vastly higher voltages. The electric organ is made of these cells.

"Each electric organ cell makes only a small voltage, similar in magnitude to our own muscles. The secret of electric organs is that the cells are aligned in stacks and electrically insulated so that the voltages add like batteries in a series," says another member of the team Professor Harold Zakon of the University of Texas at Austin.

The six groups include: South American knife fishes, African electric catfish, African elephant fish, stargazers, some skates and some rays.

Scientists think the electric organ first appeared in a fish 150 million to 200 million years ago, Gallant adds