An African freshwater fish species called the Peters’ elephantnose fish (Gnathonemus petersii) generates weak electrical pulses to safely navigate its environment. Animals, plants, or the fish’s favorite food — mosquito larvae — provide specific electric colors during this electrolocation, according to a new study published in the journal Current Biology.
The Peters’ elephantnose fish (Gnathonemus petersii). Image credit: Maik Dobiey / University of Bonn.
Elephantnose fish belong to Mormyridae, a diverse family of freshwater fish in the order Osteoglossiformes native to Africa.
They inhabit muddy, slowly moving rivers and pools with cover such as submerged branches.
“Elephantnose fish are nocturnal, which means they cannot rely on their eyes when hunting for prey,” explained study lead author Dr. Martin Gottwald of the University of Bonn and colleagues.
“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.”
“With their electrosense 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 elephantnose fish do this was uncertain for a long time.
The striking ‘trunk’ of the Peters’ elephantnose fish (Gnathonemus petersii) is actually its chin that has numerous electroreceptors. Image credit: Maik Dobiey / University of Bonn.
“Objects certainly change the intensity of the electrical signal in a characteristic way — some reduce it significantly, others reflect it better,” Dr. Gottwald said.
“However, this is not enough to clearly identify prey animals. 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.
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,” said senior author Professor Gerhard von der Emde, also from the University of Bonn.
In the study, the team 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,” Professor von der Emde said.
“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,” Dr. Gottwald said.
The researchers then examined the extent to which Peters’ elephantnose fish in their lab 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.
“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% 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,” Professor von der Emde said.
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Martin Gottwald et al. Electric-Color Sensing in Weakly Electric Fish Suggests Color Perception as a Sensory Concept beyond Vision. Current Biology, published online November 8, 2018; doi: 10.1016/j.cub.2018.09.036
