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How do fishes use lateral lines to sense pressure and water movement?

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A porcupine fish in the Andaman Sea — fish *Diodon hystrix* is a species of porcupine fish. These fishes are short and broad-bodied, with large eyes and spines covering their skin. Like puffer fishes, they inflate their bodies when threatened.

Fishes utilize a specialized sensory network known as the lateral line system to detect water movement and pressure changes. This system consists of mechanoreceptors called neuromasts, which are arranged in rows along the head and body.12

Each neuromast contains sensory hair cells encapsulated in a jellylike sheath called the cupula. When water displacement occurs, the cupula bends, deforming the sensory hairs and triggering neural impulses. In many fishes, these neuromasts are housed within lateral line canals—fluid-filled structures beneath the skin that protect the receptors from constant, intense stimulation caused by the fish's own swimming or external currents.12

By sensing these minute displacements, fishes can detect and localize nearby prey, predators, or other members of their species before physical contact occurs. This system effectively functions as a form of "touch at a distance," allowing the fish to perceive its environment and navigate through water by interpreting the direction and rate of water movement.21