A new study led by Prof Michael Sussman from the University of Wisconsin-Madison demonstrates that six electric fish lineages – Torpediniformes, Rajiformes, Mormyridae, Euteleostei, Siluriformes and Gymnotiformes (all of which evolved independently) – used essentially the same genes and developmental and cellular pathways to make an electric organ for defense, predation, navigation and communication.
Electric eel (Electrophorus electricus) at the New England Aquarium, Boston. Image credit: Steven G. Johnson / CC BY-SA 3.0.
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 electricity came from studies of electric fish. In Victorian times, parties were organized where guests would form a chain to experience the shock of an electric fish.
Worldwide, there are hundreds of species of electric fish in six broad lineages. Charles Darwin himself cited electric fish as prime examples of convergent evolution, where unrelated animals independently evolve similar traits to adapt to a particular environment or ecological niche.
The electric organ is used by fish to communicate with mates, navigate, stun prey and as a shocking defense, probably a reason the muddy Amazon and its tributaries teem with electric fish, including the electric eel (Electrophorus electricus).
Not really an eel but a fish more closely related to the catfish, the electric eel produces a jolting electric field of up to 600 volts, about 100 volts per foot of fish.
All muscle cells have electrical potential. Simple contraction of a muscle will release a small amount of current. But at least 100 million years ago some fish began to amplify that potential by evolving 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 study, reported in the journal Science, sheds light on the genetic blueprint used to evolve such complex electric organs.
“The surprising result of our study is that electric fish seem to use the same genetic toolbox to build their electric organ, despite the fact that they evolved independently,” said study first author Dr Jason Gallant of Michigan State University.
To reach their conclusions, Prof Sussman, Dr Gallant along with their colleagues assembled the complete genome of the electric eel and the genetic sequences involved in constructing electric organs and skeletal muscles from three lineages of electric fish.
“A 6-foot eel is a top predator in the water and is in essence a frog with a built-in 5.5-foot cattle prod. Since all of the visceral organs are near the face, the remaining 90 percent of the fish is almost all electric organ,” Prof Sussman said.
“The in-series alignment of the electrocytes and unique polarity of each cell allow for the summation of voltages, much like batteries stacked in series in a flashlight. An electric eel body contains many millions of such ‘flashlights’ working together and firing their electrical discharge simultaneously.”
Lindsay Traeger, a co-author on the study and a graduate student at the University of Wisconsin-Madison, added: “if you remove the ability of the muscle cell to contract and change the distribution of proteins in the cell membrane, now all they do is push ions across a membrane to create a massive flow of positive charge.”
“Only vertebrates have evolved this and only among fishes. You need water as a conductor,” said co-author Prof James Albert from the University of Louisiana.
“Our study demonstrates 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. By learning how nature does this, we may be able to manipulate the process with muscle in other organisms, 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,” Prof Susmann concluded.
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Jason R. Gallant et al. 2014. Genomic basis for the convergent evolution of electric organs. Science, vol. 344, no. 6191, pp. 1522-1525; doi: 10.1126/science.1254432
