Even in murky waters hardly penetrated by light, pike and pickerel can feel out their prey before making contact. The blind Mexican cave fish can perceive structures in its surroundings and can effortlessly avoid obstacles. Catfish on the hunt follow invisible tracks that lead directly to their prey. The organ that makes this possible is the lateral-line system, which registers changes in currents and even smaller disturbances, providing backup support for the sense of sight particularly in dark or muddy waters.

This remote sensing system, at first glance mysterious, rests on measurement of the pressure distribution and velocity field in the surrounding water. The lateral-line organs responsible for this are aligned along the left and right sides of the fish's body and also surround the eyes and mouth. They consist of gelatinous, flexible, flag-like units about a tenth of a millimeter long. These so-called neuromasts – which sit either directly on the animal's skin or just underneath, in channels that water can permeate through pores – are sensitive to the slightest motion of the water. Coupled to them are hair cells similar to the acoustic pressure sensors in the human inner ear. Nerves deliver signals from the hair cells for processing in the brain, which localizes and identifies possible sources of the changes detected in the water's motion.

These changes can arise from various sources: A fish swimming by produces vibrations or waves that are directly conveyed to the lateral-line organ. Schooling fishes can recognize a nearby attacker and synchronize their swimming motion so that they resemble a single large animal. The Mexican cave fish pushes a bow wave ahead of itself, which is reflected from obstacles. The catfish takes advantage of the fact that a swimming fish that beats its tail fin leaves a trail of eddies behind. This so-called "vortex street" persists for more than a minute and can betray the prey.