Researchers studying a weakly electric fish reveal that despite its small brain, the elephantnose fish explores its surroundings using either its vision or its electric sense – sometimes both at the same time. Until now, this level of sensory complexity was thought to be something only mammals could accomplish. The findings were published in Proceedings of the National Academy of Sciences earlier this month.
Most animals use multiple sensory modes to gather information about objects in our surroundings. Some animals are able to use information about an object acquired from one sensory system to recognize the object with another. This is called cross-modal object recognition, and the ability to do this spontaneously has only been described in humans, apes, monkeys, dolphins, and rats.
The elephantnose fish (Gnathonemus petersii) is a weakly electric fish that lives in west Africa where it hunts small insect larvae at dawn and dusk using “electrolocation”. It’s similar to echolocation in bats. An electrical organ in its tail emits electrical impulses, and its skin contains several sensor organs that can perceive objects in the water based on changes in the electrical field.
To study how they process information from different sensory channels, a team led by Sarah Schumacher of the University of Bonn placed 10 elephantnose fish in individual tanks that have two additional chambers. The fish were trained to swim through the gate that had a positive object associated with a food reward (insect larvae) and avoid the other gate that had a negative object associated with mild punishment (being chased back to the start position). The fish had to use their vision or electric sense to tell the difference between the two objects. One experiment was conducted in absolute darkness, so the fish couldn’t see, and in another experiment, the conductivity of the objects were changed to that of the water, rendering them electrically invisible.
"The animals normally use both senses,” Schumacher explained in a statement. But if one of the two senses doesn’t provide useful information, or if the info from the two vary greatly, “the fish can switch back and forth between their visual and electrical senses.” When they became visually familiar with an object, for example, they could recognize it again using the electrical sense, even if they hadn’t perceived it electrically before.
If the conflicting information is about an object that’s within 20 millimeters, the fish puts more weight on the electrical information and is essentially “blind” to the object. But when the object is farther away, the fish relied on its eyes more. They’re able to transfer between different senses as successfully as mammals, despite having no cerebral cortex.