Electric Elephantfish: How this brilliant bottom-feeder finds its way without light

When it comes to electric animals, the go-to  example is the electric eel dramatically stunning its prey with a powerful shock. And while electric eels do have some fascinating abilities, they’re just one of the many species of fish that have evolved to sense and produce electricity. In addition to shocks, these fish use electricity to communicate, locate prey, and possibly even navigate their surroundings. Navigation with electricity would be useful for fish living in environments where light is scarce, like the turbid, cloudy, and dark water at the bottom of slow-moving, muddy rivers. Members of the elephantfish family bottom-feed in murky African rivers like this, and typically have reduced eyes and an electric organ, making them an obvious subject for electro-navigation research. With an electric organ, an elephantfish could surround itself with an electric field, and then, as it swam, sense its surroundings as they distort the electric field—a process called electro-location. Research has shown that elephantfish do have the ability to sense those distortions and electro-locate, but only for objects very close to the fish’s body. So how can we know for sure if these bottom-feeders are really using electricity to find their way in the dark? That’s exactly the question German scientists Jung, Künzel, and Engelmenn set out to answer when they brought 15 elephantfish to their lab at Bielefeld University.

The elephantfish were housed entirely in the dark. Each one lived in a small tank with a sliding door that opened into a bigger circular tank with feeders lining the wall. The first thing the researchers did was train the elephantfish to exit the sliding door and navigate to a “target” feeder that had food in it. The food was in the same feeder for every trial, so eventually the fish learned exactly where it was and swam directly to it every time. Here’s the catch: to get to the target feeder, the fish always had to turn right and pass a metal object located along the route. Why a metal object? Because of the conductive properties of metal, a metal object should stand out very powerfully to an animal using electro-location. This means that the fish should have sensed that it was there each time they passed it to reach the food. Could the elephantfish be using the object as a landmark to help them find the food in complete darkness?

To answer that question the researchers split the fish into two treatment groups: the stable landmark treatment and the nonstable landmark treatment. In both treatments, the food was moved to a new feeder on the opposite side of the tank. In the nonstable landmark group, the metal object went with the food, and sat near the new target feeder. However, in the stable landmark group, the metal object was left behind close to the old target feeder, now empty. Which group would learn where to find the new food faster? If the elephantfish used landmarks to navigate, then the fish in the nonstable treatment should learn faster, since the metal object would guide them to the new food. In the stable treatment, the metal object points them to an empty feeder, so finding the food would be much more difficult.

So what did the fish do? As expected, the fish in the stable condition went back to the old food location much more often than the fish in the unstable condition and took much longer to learn where the new food was. Fish in the unstable condition did not return to the food’s original location very much after the first few trials and learned where the new food was very quickly. This difference in behavior shows that the elephantfish had learned to associate the metal object with food. When the metal object no longer brought the fish to food, they became lost, unable to trust their landmark. But when the metal object was near the food, even if the food was in a different position, the elephant fish learned to search that new position much more quickly. And if that wasn’t enough evidence, throughout the trials the researchers used electric diodes in the tank to measure how often the fish used their electric fields to “feel” the environment around them.  Once the initial training was over and the food was relocated, the fish used their electric fields a lot more frequently, probably aiming to relearn the changed environment. Once the fish has gone through enough trials to learn the food’s location, they used their electric sense less, suggesting that the sense assists in the formation of an internal map of the environment, but once that layout is learned, the electric sense becomes less necessary.

The results of this experiment support the idea that these fish really do use electric senses to help them navigate. In dark and turbulent situations when vision won’t cut it, electric fishes can still get a sense for what’s around them and where they are. This work is still preliminary, however. To further solidify the argument that elephantfish use their electric abilities to navigate, researchers need to show that these abilities come in handy in natural situations. The bottom of a river is very different from a dark tank, and natural objects will not stand out as clearly as a metal object. Because the sense does not reach far from the fish’s body, it’s probably most useful in close-quarters situations, while an internal map directs longer-distance navigation. Perhaps a future study could test the fish in a series of more complex mazes including objects of different shapes, sizes, and textures. It may seem strange to think that fish might demonstrate such high intelligence, but it should come as no surprise when animals continue to prove us wrong.

Jung, S.N., Künzel, S., & Engelmann, J. (2019) Spatial learning through active electroreception in Gnathonemus petersii. Animal Behaviour  156:1-10.

Doi:10.1016/j.anbehav.2019.06.029

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