These Fish Navigate with Electricity

Weakly electric fish use self-generated information and landmarks to navigate.

Posted Oct 30, 2019

Courtesy of Jacob Engelmann.
African elephant-nose fish.
Source: Courtesy of Jacob Engelmann.

The African elephant-nose fish (Gnathonemus petersii) is nocturnal and lives in murky lakes and rivers. It makes its way through its environment, avoiding obstacles and finding prey, using an unusual sensory system: These fish rely on their active electric sense to explore their surroundings.

African elephant-nose fish can produce electric fields in the range of a few millivolts. Within their tails is an electric organ that they can discharge to build up a weak electric field surrounding their bodies. Depending on the electric properties of the environment, this field is modulated and these modulations are picked up by electroreceptors distributed over the fish’s skin. This allows them to sense objects in their immediate surroundings (about a fish-length from the animal) and orient in the dark.

Recently, Sarah Nicola Jung of the Universität Bielefeld and colleagues took a closer look at the role of short-range electric sensing in spatial navigation in these fish. “We were particularly interested in the types of information fish used to solve a spatial navigation task in the dark,” says Jung. “Do they rely on idiothetic (self-generated) information or are they also integrating information from their surroundings such as local landmarks?”

Jung and her colleagues tested the fish in a circular tank that contained 15 potential targets around the periphery, only one of which contained a food reward. The tank also contained a metal landmark near the food target that the fish could detect with their electric sense, but only when they were close to it.

Courtesy of Jacob Engelmann.
An elephant-nose fish.
Source: Courtesy of Jacob Engelmann.

Once the animals had learned to solve the task, which took about two to four days, the researchers began trials in which they either changed the fish’s starting position or moved the food to a different target location, either leaving the metal landmark at the same place or moving it in conjunction with the food item.

In these tests, the fish relied primarily on self-generated information. When this strategy failed, they electrically attended to landmark information to complete the task.

“In test trials, fish widened their search area and sampled more vigorously around the landmark,” says Jung. “When the landmark was in the same place as during training, fish would persistently revisit the original rewarded location during the first few trials, but when the landmark was moved to a new place the fish could easily find the location of the food.”

The elephant-nose fish’s active electric sense is, in a lot of ways, similar to another near-range sensory system: whisking in rodents. Animals like rats use their whiskers to actively collect sensory information about their close surroundings. Rats and elephant-nose fish both increase their sampling rate near objects of interest. For instance, in this study, fish actively attended the landmark initially but reduced sensory probing of the landmark with increased familiarity of the task.

Overall, the results indicate that the fish’s active electric sense contributes to navigation in a context-dependent manner. These fish can incorporate highly localized sensory input in egocentric navigation, which is likely useful in the absence of vision.

“Our study provides a base from which the contribution of sensory information in the formation of spatial memories can be further investigated,” says Jung. “Future studies are needed to determine if fish are capable of generating a map-like representation of their environment based on the highly localized ‘electric samples’ they gather in our experiments.”

References

Jung SN, Kunzel S, and Engelmann J. (2019). Spatial learning through active electroreception in Gnathonemus petersii. Animal Behaviour 156: 1-10. doi: 10.1016/j.anbehav.2019.06.029.