Knifefish Stop and Shop

Northwestern researchers have recently worked on a study attempting to quantify two types of spatial volumes involved in an animal’s environment. One is the stopping motor volume, which is the amount of space it takes for an animal to come to a complete stop. The other is the sensory volume, which is the amount of space an animal sense around it. Using these volumes, researchers classified three possible modes that an animal could find itself in relation to another object: collision (collide every time), reactive (won’t collide if aware), and deliberative (perceived and processed much earlier than potential onset of collision).

The researchers applied these concepts to the black ghost knifefish, commonly found in the Amazon River. Instead of using the sense of sight or hearing to hunt for prey, the knifefish generates a weak electric field around its body. Any perturbations of the electric field will be sensed by specialized receptors and the knifefish will be informed of any potential prey items. Observations showed that not only could the knifefish swim both forward and backward, but it was also omnidirectional with respect to its three-dimensional volumes for movement and sensation. Video analysis of prey capture behavior showed that the two volumes (stopping motor and sensory) are roughly equal.

The researchers conclude that these conditions often place the knifefish in reactive mode, critical if the fish wants to eat and/or not collide with its prey. This reveals that the knifefish invest just enough energy into active sensing to be able to detect prey in the time required. It may be too expensive for the knifefish to evolve a greater sensory space, since the way in which it senses disturbances is not a passive process. Further studies have also shown that the knifefish prefers to keep its motor volume roughly equal to the sensory volume. When they are placed in water with increased conductivity, which decreases the fish’s sensing ability, they modify their behavior by swimming more slowly.

Posted by Mayur Patel (11)