How Would It Be to See with the Chin?

Generally, a long nose does not mean a better smelling sense. The African elephantnose fish (Gnathonemus petersii) has a "trunk" with a totally different goal. A team at the University of Bonn has shown how this works.

In complete darkness, these fishes can see objects and make the difference between dead and live organisms. The fish, which is no longer than one foot (35 cm), hovers with its head tilted, close to the water bed. While slowly swimming forward, its trunk (the elongated chin) moves steadily from right to left, always at just a few mm from the bottom, like a treasure hunter using a metal detector.

Only that the fish does not look for gold, but buried insect larvae and worms. But the strange trunk does not smell the prey; instead, it harbors over 500 electric sensors taking an image of the surroundings; this is how this fish is active during the night. During daytime, it remains hidden.

The electrical impulses that make the trunk work are generated by special muscle cells from the tail zone, that produce regular electrical pulses of a few volts. The frequency of the current is of 100-2500 Hz. "At the same time it measures the electrical field which builds up around it via sensors in the skin," said co-author Professor Gerhard von der Emde.

Nearby objects change the field's shape, and this way, the fish gets a complex image of its surroundings.

The research team set up a small cube and a pyramid in an aquarium. Whenever the fish approached the pyramid, they received a mosquito larva. The experiments took place in total darkness, and the researchers employed special infrared cameras to see.

In complete darkness, in 90 % of the cases, the fish swam straight towards the pyramid. Even when the team replaced solid objects with wire models, the fish did not miss. They could even handle discontinuous contours.

"For example, we removed the vertical edges of a cube, i.e. we embedded two wire squares on top of each other in a gel that was permeable for electrical fields. The fish still perceived it as a cube, so they supplied the contours very much like humans would. Furthermore, they seem to calculate the volume of objects in water. A cube has a larger volume than a pyramid of the same height. If we decreased the size of the cube so much that its volume became smaller than that of the pyramid, the fish often changed their minds and swam to the cube. So, the Peters' elephantnose fish can also internalize abstract concepts: Always swim to the less voluminous of two objects, irrespective of their absolute size.", said von der Emde.

The elephantnose fish can easily differentiate between live and dead organisms without touching them. "With its electric sense, it measures their capacitative properties, i.e. their ability to store charges. Dead plants or animals cannot do that.", said von der Emde.

The electrical field imaging can even expose the material's composition. A metal appears very bright, while non-conductors (like wood) weaken the fish's electrical field. The electrical chin can even assess distances to a precision of several mm: the degree of fuzziness tells the distance. This amazing ability is possible also due to a special brain: the cerebellum of elephantnose fish is huge. Compared to their body length, these fish have a larger brain than Homo sapiens. "They are really intelligent, that's why it is so much fun working with them." said von der Emde.

With electric fish from South America, there is another story. "That was a flop, the fish are beautiful, but definitely too stupid for complex tasks."