Electroreception, such as that exhibited by South American knifefish, allows animals to navigate, sense prey, and even communicate by generating weak electric fields that reflect off objects in the environment. Scientists have drawn parallels between this unique sense and vision in people. But new research published in the September issue of Journal of Experimental Biology suggests that in contrast to vision, which paints a broad picture of the more distant environment, electroreception in weakly electric fish works over extremely short distances, providing information on the immediate environment much like the human sense of touch.
“[The researchers] combined the measurement of the physical stimuli and how distance and size of objects affect the electric image that’s cast on the surface of the fish,” said Rüdiger Krahe, who investigates the sensory processing of weakly electric fish at McGill University who did not participate in the research. Linking these characteristics to fish behavior, the authors demonstrated which changes in the electric field the fish could actually sense. “Vision is a true distance sense, and they show that [electrosensation] in these fish is a near sense.”
Gymnotus knifefish, a genus of weakly electric fish, use a specially adapted organ to produce electric fields for hunting, navigation, and mating. Some produce electromagnetic pulses to constantly ping their environment, while others generate a continuous electric field in a wave pattern. Regardless of the strategy, the electric fields produced by these fish are quite weak (only several millivolts); a person touching the fish wouldn’t feel a thing.
Though researchers have been studying varieties of Gymnotus for 60 years, it’s still unclear how much information the fish receive through their electric fields. A specialized organ, which evolved from muscle cells, produces an electric field. The fish sense perturbations to this electric field, as well as the presence of other organisms’ electric fields, through electroreceptors in their skin, most of which are located near the mouth.
“Electroreception is a non-intuitive sensory modality,” Angel Caputi, who led the research at the Instituto de Investigaciones Biológicas Clemente Estable in Uruguay, explained in an email—meaning that human researchers have no frame of reference within their five senses to understand how fish use it to “see” the world. It has previously been likened to vision, but modeling by Caputi and his colleagues hinted that electroreception acted at much shorter ranges, allowing the fish to detect features of their environment up close, but giving little information about things more than a few centimeters away.
In order to understand how an object’s size and location affect fish’s ability to sense it using electroreception, Caputi and his colleagues presented G. omarorum knifefish with metal objects. “The beauty of pulse-type fish ... is that they have a variable pulse rate,” noted Krahe. “When they’re doing nothing, [the electric pulses are] relatively stable.... If they detect something, they ramp up the pulse rate.” Thus, measuring the electric pulses produced by the fish, Caputi and his team could determine whether or not the animals had detected an object.
As predicted, the knifefish could only detect most objects at short distances—usually about 10 millimeters from their skin—a small distance for a fish that measures about 30 centimeters (1 foot) in length. And the fish could only precisely locate the objects when they were almost touching them. “Fish confused location of two objects 1 centimeter apart when they are beyond a distance of a half of centimeter from the skin,” Caputi said. Large objects could be detected at longer distances of up to 2 or 3 centimeters.
Caputi, trained as a physician, likens such limited electrosensation to the human sense of touch. Doctors are trained to detect different organs while pressing on a patient’s abdomen, but the deeper organs become more “blurred” to the touch as a doctor tries to sense them through more layers of viscera, Caputi explained—much as objects become more indistinct to the fish as they recede to the electrical field’s limits.
Interestingly, the fish’s own body appears to affect the strength of the electric field. An electric field’s strength decays quickly with distance, as do the electrical “images” reflected off nearby objects as they bounce back. But it turns out that the fish’s body helps prevent further reduction in field strength as the reflected pulses approach its body, helping to make a more robust electrical image on its skin.
Caputi and his group are investigating the deeper workings of the knifefish’s electrosensory system, implanting electrodes to record signals from individual neurons in freely swimming fish. “This will help us to find the way that the nervous system is organized for processing electrosensory images,” he said.
Another important next step will be to understand how the role movement could help the fish detect and locate objects, Krahe said. Without changes to an object’s location or electric field, the knifefish’s electroreceptors will “adapt” to the stimulus and no longer sense it, but movement may help the fish re-sensitize themselves to permanent stimuli. Krahe also pointed out that the metal objects used in the study will perturb the fish’s electric fields much more than, say, tiny daphnia crustaceans they might prey on in their natural habitat.
John Lewis, who studies sensory processing in weakly electric fish at the University of Ottawa, agreed. G. omarorum are primarily nocturnal and must navigate through tangled roots of lily plants. “We don’t know much about [how fish sense] complex, natural objects in more natural environments,” said Lewis, who did not participate in the research. “Detection is one thing, but figuring out where [an object is] is an important second level in prey capture.”
Check out other touch-inspired articles, including an exploration of the star-nosed mole’s bizarre namesake touch organ and a story about animals that communicate via sensing vibrations in the ground, in this month’s Special Issue.
A. C. Pereira et al., “The active electrosensory range of Gymnotus omarorum,” Journal of Experimental Biology, 215: 3266-3280, 2012.