Fish recognize their prey by electric colors

November 13, 2018

The African elephantnose fish generates weak electrical pulses to navigate its environment. This localization sense apparently shows an astonishing similarity to vision, as a study by the University of Bonn now shows. The study demonstrates that different objects have different electrical "colors". Fish use these colors for instance to distinguish their favorite food - mosquito larvae - from other small animals or plants. The study is published in the renowned journal Current Biology.

Elephantnose fish are nocturnal, which means they cannot rely on their eyes when hunting for prey. But they don't need to: They carry a kind of "electric flashlight" in their tail, which they use to generate short electrical pulses up to 80 times per second. Their skin, especially their trunk-like chin, is covered with electroreceptors: small sensors with which they can measure how these pulses are reflected by the environment.

And in this they have become champions: With their electro-sense they can estimate distances, distinguish forms and materials, differentiate between dead and living objects. And more than that: Within fractions of a second, they can recognize whether mosquito larvae, their favorite food, are hiding in the gravel or sand at the bottom of their habitat. They can do this with considerable accuracy, largely ignoring the larvae of other insects.

How they do this was uncertain for a long time. Objects certainly change the intensity of the electrical signal in a characteristic way - some reduce it significantly, others reflect it better. "However, this is not enough to clearly identify prey animals," explains Martin Gottwald of the Institute of Zoology at the University of Bonn. "For example, the signal strength also decreases as the distance increases." But there is another characteristic of living organisms: They also modify the shape of the electric pulses. But even this signal change depends on distance, size and position.

The combination of the two signal characteristics could solve these problems. The human eye works in a similar fashion: Its retina contains receptors for red, green and blue light. Our brain then uses the "mixing ratio" to calculate the color of the object we see. And this remains largely constant, no matter how large or far away the object in question is.

Two different receptor types

However, until now there was no proof that a similar process occurs in elephantnose fish. Nevertheless, it is clear that the animals have two different types of electric receptors. One only measures the intensity of the signal, the other additionally measures its shape. "We have now been able to demonstrate that the fish uses the relation between these two measurements to identify their prey," explains Prof. Dr. Gerhard von der Emde, who led the study.

At first, the scientists determined how intensity and shape of the localization signal behave in relation to each other depending on the type of object. "We found that this ratio is always constant for the same objects," says von der Emde. "And this applies regardless of their distance or other environmental parameters." "A mosquito larva therefore actually has a constant 'electrical color', which is clearly different from that of other larvae, plant parts, members of the same species or other fish," adds Gottwald.

The researchers now examined the extent to which their laboratory animals used this information. They presented them with various electronic "mini chips" with a diameter of only one millimeter. Some chips produced different electrical colors; for example, they glowed like a mosquito larva or like other insect larvae. Other chips were electrically 'colorless', similar to a pebble.

Hungry for chips

The effect was astonishing: If the chips were colored like their favorite food, the elephantnose fish chomped down reflexively. They let themselves be fooled in this way in 70 percent of all cases, even though the fake meals did not smell at all like typical prey. Even after numerous experiments, the animals did not learn to avoid the chips. They largely spurned differently colored chips, and even completely ignored electrically colorless ones. "This may suggest that the prey color is hardwired in the brains of the fish," speculates von der Emde.

That would make sense: The electrical properties of living beings (and thus also their color) are decisively determined by their inner structure. And this cannot be changed easily. It is therefore barely feasible for a mosquito larva to simply add camouflage.
-end-
Publication: Martin Gottwald, Neha Singh, Andre Haubrich, Sophia Regett and Gerhard von der Emde: Electric Color Sensing in Weakly Electric Fish suggests Color Perception as a Sensory Concept beyond Vision; Current Biology; DOI: https://doi.org/10.1016/j.cub.2018.09.036

Contact:

Prof. Dr. Gerhard von der Emde
Institute of Zoology
University of Bonn
Tel. +49 (0)228/73-5555
E-mail: vonderemde@uni-bonn.de">vonderemde@uni-bonn.de

University of Bonn

Related Fish Articles from Brightsurf:

Fish banks
Society will require more food in the coming years to feed a growing population, and seafood will likely make up a significant portion of it.

More than 'just a fish' story
For recreational fishing enthusiasts, the thrill of snagging their next catch comes with discovering what's hooked on the end of the line.

Fish evolution in action: Land fish forced to adapt after leap out of water
Many blennies - a remarkable family of fishes - evolved from an aquatic 'jack of all trades' to a 'master of one' upon the invasion of land, a new study led by UNSW scientists has shown.

How fish got onto land, and stayed there
Research on blennies, a family of fish that have repeatedly left the sea for land, suggests that being a 'jack of all trades' allows species to make the dramatic transition onto land but adapting into a 'master of one' allows them to stay there.

Fish feed foresight
As the world increasingly turns to aqua farming to feed its growing population, there's no better time than now to design an aquaculture system that is sustainable and efficient.

Robo-turtles in fish farms reduce fish stress
Robotic turtles used for salmon farm surveillance could help prevent fish escapes.

Heatwaves risky for fish
A world-first study using sophisticated genetic analysis techniques have found that some fish are better than others at coping with heatwaves.

A new use for museum fish specimens
This paper suggests using museum specimens to estimate the length-weight relationships of fish that are hard to find alive in their natural environment.

Reef fish caring for their young are taken advantage of by other fish
Among birds, the practice of laying eggs in other birds' nests is surprisingly common.

Anemones are friends to fish
Any port in a storm, any anemone for a small fish trying to avoid being a predator's dinner.

Read More: Fish News and Fish Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.