Electrical eels, a type of serpentine fish, hunt for naive frogs and other tiny prey along the Amazon River’s muddy bottom. Two 600-volt electrical pulses are released by the fish as one swims by in order to kill or stun it.
Although this high-voltage seeking technique is unique, a few other fish species also use electricity: They produce and feel lower voltages when traveling through muddy, slow-moving waters and when communicating with other members of their species using light shocks analogous to morse code.
Typically, when several species share a rare capacity like creating electricity, it’s because those species are closely linked. However, there are three separate marine lineages of electrical fish that came before them, and the electrical fish found in the rivers of South America and Africa belong to six different taxonomic groups.
Even Charles Darwin noted in On the Origin of Species that it is “impossible to conceive by what steps these wondrous organs have been produced”—not just once, but repeatedly—and wondered about both the novelty of their electrical abilities and the unusual taxonomic and geographic distribution of them.
A recent study published in Science Advances aids in solving this evolutionary mystery. As most biologists do, “we’re really simply following up on Darwin,” said Harold Zakon, an integrative biologist at the University of Texas at Austin and a senior author of the study.
His team in Texas and colleagues at Michigan State University discovered how several very similar electrical organs emerged in electrical fish lineages separated by around 120 million years of evolution and 1,600 miles of ocean by piecing together genomic hints.
Although it appears that there are numerous ways to generate an electrical organ, nature does have a few go-to strategies.
The fish that Zakon’s team studies in South America and Africa get their zap from specialized electrical organs that span along much of their body. Sodium ion gradients are produced within the organs by electrolytes, and modified muscle cells. A surge of current is created when sodium-gate proteins in the electrocytes’ membranes open. It’s about the simplest signal you can think of, Zakon said.
While in the electrical organs the voltage is directed outward, in muscle these electrical signals circulate in and between cells to help the cells contract for actions. The amount of electrolytes that immediately fire during each shock determines its energy. Electrical eels can unleash voltages powerful enough to kill small prey because they pack an extremely large number of electrical cells compared to most electrical fish, which only fire a few at a time.
In a recent study, Zakon and his former research assistant Sarah LaPotin—who is currently a doctorate candidate at the University of Utah—along with other researchers were able to reconstruct an important aspect of the evolution of those electrical organs by studying the fishes’ genomic background.
When the progenitor of all fish classified as teleosts survived a rare genetic mishap that reproduced its whole genome, it began between 320 million and 400 million years ago. For vertebrates, whole-genome duplications can be fatal.
However, because they make duplicate copies of everything in the genome, duplications can also reveal hitherto unexplored genetic possibilities. Gavin Conant, a molecular biologist at North Carolina State University who was not involved in the study, said: “Suddenly, you have the capacity to construct a completely new pathway, instead of just one new gene.”
One of the pioneers of the recent studies on the development of electrical fish was Harold Zakon, an integrative biologist at the University of Texas at Austin. As most biologists do, “we’re basically simply following up on Darwin,” he said. Photo credit: Quanta Magazine/Lynne McAnelly
