Most animals rely on two eyes for accurate depth perception. Not the African elephant-nosed fish, which uses electrical pulses to navigate at night. Scientists report in this week's Nature that these nocturnal animals can gauge the distance to an object, even though they only get one glimpse of the electrical image it creates.
In addition to good eyes, Gnathonemus petersii come equipped with an advanced electrical system for tracking down insect larvae after nightfall. Hunting in rivers and lakes, the fish emit weak pulses from an electric organ in their tails that zip through the water and bounce back to sensors that cover the animal's body. "It's like turning on a flashlight so you can see the environment for a moment," says Gerhard von der Emde, a neuroscientist at the University of Bonn, Germany. Because the fish only get one look at the world per pulse, it wasn't clear how--or if--they could tell a large distant object from a small one nearby.
To find out, the team switched off the lights and watched the fish with infrared cameras. They rewarded a fish with a worm when it swam toward the closer of two objects presented to it. At distances of less than about 12 centimeters, the fish correctly picked the closer object about 95% of the time--even when the difference was only a few millimeters. "It was very surprising," says von der Emde, "this was thought to be impossible."
After studying the electrical pulses received by the fish, the team could find only one solution--the fish must be taking into account sharpness and intensity of the electrical image. Like a shadow cast by the object, the image grows and becomes fuzzy the further away the object is. If so, the model predicted that a sphere should act as a kind of optical illusion, appearing further away than it was. This turned out to be the case. When the researchers tested the fish with spheres, their depth perception was thrown off by about 15 millimeters, about what the model had predicted.
The work "resolves or suggests an answer to this old problem" of how the elephant-nosed fish could measure distances with electrical pulses, says Curtis Bell, an neuroscientist at Oregon Health Sciences University in Portland. The question now, he says, is why no fish in North America or Asia have evolved this clever trick. About a third of freshwater fish in South America and Africa use the technique, he says.