On a hot Autumn morning at the National Zoo’s Amazonia exhibition, Smithsonian biologist Ed Smith is managing to explain a lot of animal biology to a squirmy audience. A gaggle of children, faces pressed up against a glass tank, are trying to get a closer look at a 5-foot-long electric eel. There’s not much in the tank apart from a large tube for the eel to hide in, some gravel and a shrimp dangling from a thread. It’s snack time.
The eel swims in loops in the tank, inching closer to the shrimp each time but never quite reaches it. “Why can’t he see the shrimp?” one of the children asks, and Smith explains that eels have really bad eyesight. Instead, they rely on another way of navigating the world: electric fields. They emit weak electric charges to navigate their surroundings and communicate with other eels.
At last, the eel gobbles up the shrimp, and the children gasp when an electric pulse crackles over the speakers above the tank. Smith explains that the tank has been rigged with sensors that convert the eel’s higher-frequency pulses into sound, a flashing LED light and waveforms on a screen. He points up at the high peak of energy on the screen. The waves are usually shorter and smaller, he says, but when electric eels are excited by things like food, they send out big pulses. “Would you like to see him do it again?” he asks, and the children nod their quick approval.
The National Zoo’s new Electric Fishes Demonstrations Lab integrates seamlessly into the rest of Amazonia. It’s housed on the first floor, right past a giant tank of arapaima, turtles and other river fish. There’s not just electric eels; there are at least a dozen tanks filled with different catfish, tetras, piranhas and the electric field-generating, ray-finned knifefish—black ghost, bluntnose and elephant nose.
This species’ electric abilities posed an evolutionary problem that scientists like Charles Darwin simply couldn’t understand. Darwin was baffled by their electricity-generating organs and found it “impossible to conceive by what steps these wondrous organs have been produced.” Now, scientists understand that electric eels are a kind of knifefish, Smith says, which also use low-level electric fields and have similar organs.
Amazonia represents a whole ecosystem, Smith says, designed to help visitors understand that animals don’t live or behave in isolation. There’s even a roseate spoonbill named Mike strutting freely around as if to remind his electric neighbors that he’s in charge. He eyes the eel swimming around, perhaps sizing him up as a snack, while Zoo volunteers chuckle at his antics.
The new demonstration lab is designed to help visitors understand how exactly these fishes use electricity. Visitors can touch the head and tail of a life-size metal model of an electric eel, which vibrates when the positively-charged head and negatively-charged tail complete a circuit. This illustrates how the fish’s organs have stacked cells that work like a battery, producing electric signals that travel through the water and shock their prey.
These electric fields aren’t just for zapping purposes, either. Electric fishes can also produce low-level pulses that communicate messages about mating, nearby predators or whether they’re male or female. These pulses also allow them to detect their whereabouts, similar to how bats use echolocation. If another fish or obstacle comes their way, it disrupts these fishes’ electric fields and tells them to get out of the way. This ability allows them to swim through the dark and murky depths of the Amazon River. “It’s able to put that stimulation together in a picture to know where that object is and a lot about that object,” Smith says.
Like other knifefish, eels glide through their environments with their long, undulating bodies. They’re remarkably sleek and don’t have any cumbersome dorsal fins, Smith points out. This helps them move freely in the water as they hunt for prey while swimming forward and backward without turning their whole bodies around. This kind of “ribbon fin” has even been appropriated by engineers to come up with designs for robots.
The Zoo’s eel hasn’t been given a name yet, but staffers have been coming up with a lot of good names, Smith says. (He can neither confirm nor deny that these would involve electricity puns, since he doesn’t want to get anyone’s hopes up before the name becomes official.)
In the meantime, the unnamed electric eel will still swim around its tank, eat and emit shocks in front of wide-eyed onlookers. One young visitor, Ava, presses her hands up against the glass and watches as Smith lowers down another shrimp for the eel to munch on. “He just can’t find it,” she observes as the eel once again swims circles around his snack. As soon as the eel devours it, her head immediately swivels up to the tank’s monitor. She shouts to her friends, “Whoa! Did you see that? Look at the screen!” There’s been a big jump in the waveform, and all the children around the tank yell “Whoa!” as they look up.
“How do you think the shrimp felt?” Smith asks as his attentive audience giggles. “That must have been really shocking.”