The euphemistically named elephant-nose fish (Gnathonemus petersii) both perceives its environment through electric fields and produces electrical signals itself. The combination could create conflict, like trying to listen to other people while making noise yourself, but new research reveals Gnathonemus' enviable self-awareness. Rather than being dazzled by its own signals, its brain subtracts them from the signal sent by its electro-sensors, allowing it to see only externally generated electric activity.
Many species depend on electric or magnetic fields to find prey or navigate long journeys. Just in the last few weeks, we have learned spiders use electric fields to travel, one Australian moth depends on the Earth's magnetic field to migrate, and that animals' sensory capacity in this regard probably dates back more than 400 million years.
Although Gnathonemus has become somewhat popular in the aquarium trade, its natural habitat is the rivers of west and central Africa. These tend to be very muddy, making vision of little use to find prey. Like many other inhabitants of such conditions, Gnathonemus has turned to the detection of electrical signals produced by their small invertebrate prey instead.
This doesn't come without a cost – the fish uses 60 percent of the oxygen it consumes to power its unusually large brain, three times more than us humans and some 10 times as much as other fish. It's likely a lot of that brain capacity is devoted to processing the electric images it perceives.
Gnathonemus has a particular challenge because it generates electric signals itself, although not powerful enough to shock its enemies like an electric eel. Dr Nathaniel Sawtell of Columbia University said in a statement: "We needed to determine whether being able to predict its own electrical signals would help the fish better detect environmental cues."
The fish's own electricity is emitted as electrical pulses lasting 100-200 milliseconds, used for navigation and to send messages to other fish. Researchers had previously speculated Ganthonemus produces signals within its brain that exactly balance the electric detection signals it produces itself, a process called “negative images” – a little like having an internal set of noise-canceling headphones.
To test the theory, Sawtell and colleagues injected fish with a drug that prevents the neural plasticity required for negative images. They report in Neuron that the drug left Gnathonemus effectively blind, dazzled by its own emissions, while undrugged counterparts honed in on electric fields, mimicking those produced by Gnathonemus' favored prey.
Although the use of negative images makes intuitive sense, the paper notes that the noisiness and non-linear responses of actual neurons makes it challenging, perhaps why Gnathonemus needs such an active brain.
The work could shed light on tinnitus, thought to be a failure of equivalent mechanisms in humans.