The Electric Eel Holds an Evolutionary Mystery Even Charles Darwin Could Not Explain.
At the murky bottom of the Amazon River, electric eels (also known as electric catfish) are stealthily searching for unsuspecting prey. Upon detecting a hapless target swimming by, the eel discharges a jolt of 600 volts, stunning or even killing its victim.
This unique hunting strategy utilizing natural high-voltage electricity is peculiar, yet many different species of fish also employ electricity in similar ways: Throughout the muddy waters they navigate, they generate and sense electric fields, communicating with other species using slight electric currents, akin to Morse code.
When various species possess an unusual ability like electric discharge, they typically have a close biological relationship. However, the electric fish in South America and Africa belong to six separate branches and three different families.
Even Charles Darwin, the father of evolutionary theory, was perplexed by the geographic distribution and the unusual evolutionary branching of these electric species when he discussed their innate ability to generate electric shocks. In his book On the Origin of Species, he noted: “It is impossible to imagine how these marvelous organs were formed.”
In fact, this has occurred not just once in history, but repeatedly.
“It is impossible to imagine how these marvelous organs were formed.”
The Origin of Species – Charles Darwin
A new study published in Science Advances may help unravel this enigmatic evolutionary puzzle. Harold Zakon, an interdisciplinary biologist at the University of Texas, USA, and co-author of the study, stated: “We are merely adding to what Darwin theorized, much like other biologists often do.”
By comparing genes, Zakon’s research team at Michigan State University discovered how various organs and structures of electric fish evolved over approximately 120 million years along more than 2,500 kilometers of ocean. The results revealed that there is more than one way to develop electric organs.
It turns out that Mother Nature favors certain methods.
How do electric eels generate electricity? (Image: KUDLA/SHUTTERSTOCK/QUANTA MAGAZINE).
The species of fish in South America and Africa that Zakon’s team studied have developed specialized electric organs along their bodies. The muscle cells are modified to generate flows of sodium ions; these cells are called electrocytes. When the protein molecules responsible for opening and closing sodium channels in the cell membrane open, an electric pulse is generated.
In fact, even within the muscles of organisms, there is often electrical activity that runs through and between cells, allowing the organism to contract or relax muscles and create movement. However, with electric organs, this electrical activity is released into the environment. The strength of the electric shock also depends on how many electrocytes the organism uses during each discharge.
In most cases, electric fish only use a few electrocytes at a time, but electric eels, with their abundance of electric cells, naturally produce a strong current, enough to kill a small prey instantly.
Location of electric organs in the electric eel Electrophorus electricus. (Image: Britanica).
In the new study by Zakon’s team, they simulated a significant aspect of the evolution of these electric organs. They accomplished this by tracing the evolutionary history of the genes.
Specifically, around 320 to 400 million years ago, when the ancestors of modern teleost fish survived a remarkable genetic incident: their entire genome was duplicated. Normally, such a genetic alteration would lead to the extinction of vertebrates, but with gene duplication, latent traits had the opportunity to express themselves.
Commenting on this, system biologist Gavin Conant at North Carolina State University said: “Naturally, you can create an entirely new neural pathway instead of just generating a new [trait] gene.”
Sodium pumps are proteins that transport Na+ and K+ ions across cell membranes, controlling the concentrations of Na+ and K+ in cytosol and extracellular fluid.
The concentrations of Na+ and K+ play a crucial role in maintaining the electric-generating capacity of cells.
For the close ancestral species of today’s freshwater electric fish, gene duplication meant they had a copy of an important component: sodium pumps. As a result, one sodium pump continued to operate in muscle cells, while the new pump underwent modifications allowing electrocytes to acquire new characteristics.
Most importantly, before any electric-related organs can function, the gene copy must be inactive in muscle cells; otherwise, having such an organ would disrupt the organism’s mobility. When Zakon’s research team investigated how electric fish inactivated this gene, they were astounded by the results: Different electric fish species have different mechanisms.
In most species in South America, sodium pumps are not present in the muscle. (Image: The Living Planet).
In the muscle tissue of African electric fish, the sodium pump gene remains completely normal, not defective, but it is inactive—like a lock without a key. In contrast, in most species in South America, the sodium pump is absent in the muscle—often inactivated due to the lack of a regulatory element. In an unusual case in South America, the sodium pump gene remains functional, but it is inactivated during the juvenile stage; as it matures, a different set of genes takes over the electric organ.
Convergent evolution refers to the phenomenon where one or more different ancestral species develop similar traits. A common example is the wings of birds, bats, and flies.
In contrast to convergent evolution is divergent evolution, which is the evolution of new species that may serve similar functions but differ morphologically from their ancestors. An example would be domestic dog breeds.
In typical cases of convergent evolution, different fish species will “choose” various methods to modify muscle tissue to form electric organs. They may even be able to activate sodium pumps in certain tissues, but the ways they achieve this differ.
Molecular biologist Johann Eberhart at the University of Texas explains that often, when studying a case of convergent evolution, scientists find that biological traits tend to have a similar formation mechanism.
However, in the case of electric eels, “it’s quite different. It’s fascinating.”
System biologist Gavin Conant remarked that this new discovery somewhat “aligns with what we have observed” in his own research. Conant’s team found that several genes responsible for signaling between the nervous system and muscles in teleost fish were lost during earlier gene duplication, but some electric fish managed to retain them. Without these genes capable of regulating electric organs, electric eels would not be able to discharge their characteristic strong currents.
Researcher Harold Zakon and his colleagues are also intrigued by the potential significance of the regulatory region they discovered on the sodium pump gene, as it appears to allow precise control over specific tissues. In fact, this regulatory region is also present in the sodium pumps of humans and other vertebrates. It is quite possible that changes affecting the function of sodium pumps in human cells could have caused or contributed to certain pathologies, such as myotonia.
Zakon’s new study only reflects a few representative cases of convergent evolution and adaptive evolution that some electric fish possess. Several other fish species in South America may generate mild shock currents by altering neurons rather than modifying muscle tissue. Some saltwater electric fish have developed fascinating methods, such as the stargazer fish that can generate electric pulses to attack prey by altering the muscles around their eyes.
Stargazer fish can generate electric pulses from their eye region. (Image: Canvasman21 / Wikipedia).
However, for researcher Harold Zakon, convergent evolution serves as a compelling hint to solve a complex biological puzzle: When looking back, will evolution continue to develop as it once did?
Researcher Harold Zakon stated that making a new discovery is “incredibly exciting,” but whether this is the only path to developing that organ remains unanswered. The development of organs in various ways, both similar and different, across electric fish species has provided us with additional insights into how biology can be surprisingly intricate.
