The vast majority of the animals, including us, walk forward because they sense what's in front of them. Predators focus on the prey located in front of them. But a small electric fish from the Amazon, the black ghost knifefish (Apteronotus albifrons), just 15 cm (6 in) long, can move also backwards to catch its prey. This is possible because while most predators use passive sensing, waiting for information to come to them, the knifefish actively generates a weak electric field surrounding its body.

The team led by James Snyder at Northwestern University that published the results of the research in the open-access on-line journal PLoS Biology, revealed the energetic trade off of the active sensing system belonging to the fish, involving far more energy consumption than passive sensing and the habitat in which the fish is hunting.

The team determined the sensory volume (SV) (the size and shape of the volume within which things can be tracked down by an individual) and compared it to the motor volume (MV), the distance reached by a fish in a determined time period. The energetic constraints of the active sensing system caused a restricted SV compared to passive-sensing animals.

Video analysis of prey (water fleas, Daphnia) behavior, patterns of the electric receptors located along the body of the fish and its behavior during capture of the water fleas enabled the researchers to determine the 3D volumes for SV and MV for the first time in any animal species.

The foraging knifefish swims forward at a fast speed of 10 cm/s, with the head downward at a 30° angle. When the prey is detected, the fish speeds up and changes direction to grab its prey.

SV and MV roughly match. The prey detection distance was on average 3.5 cm from the body of the fish.

In waters with higher conductivity, SV was bigger and the knifefish lowered its foraging speed decreasing the size of the MV, thus tuning behaviorally MV to SV, avoiding collision with objects located outside of its SV, as the fish cannot see much in the murky waters of the Amazon Basin.