E13 Flow Sensing

Topic

The lateral line is a mechanosensory system distributed along the fish's entire body that detects pressure and flow disturbances in the surrounding water, functioning as a hydrodynamic sensing organ capable of perceiving the environment close to the fish's body. Its functional units are neuromasts: clusters of hair cells encapsulated within a gelatinous cupula that deflects whenever a pressure gradient or flow velocity pushes against it. Superficial neuromasts respond primarily to local flow velocity, whereas canal neuromasts—located within the lateral line canal—respond to pressure gradients and are more sensitive to flow variations over time.

Overall, the lateral line functions as a dynamic pressure-field detector distributed along the fish's body; it records the pressure distribution across the body's surface, and from this distribution, information regarding the flow structure surrounding the fish can be retrieved—including the position, size, and speed of nearby objects, predators, and prey, and even the pressure wave left by another fish in the same school as it swims. Inferring the source of a disturbance from the pressure signals recorded by the lateral line is an inverse problem, analogous to the aperture synthesis method used by radar and sonar to reconstruct an object's position from received signals.

The sensitivity of each neuromast can be modeled as that of a damped oscillator: its cupula possesses an effective mass, a stiffness imposed by the fibers connecting it to the surrounding tissue, and damping imposed by the viscosity of the surrounding fluid. The way that neuromast responds to different frequencies is characteristic of a second-order system, defined by a natural frequency that depends on the square root of the ratio between stiffness and the effective mass of the cupula, and by a quality factor that depends on the square root of the product of that stiffness and mass, divided by the system's damping. Superficial neuromasts are critically damped and possess a low natural frequency—barely between one and ten hertz—whereas canal neuromasts exhibit a much flatter response across a frequency range extending up to two hundred hertz.

The lateral line is expected to have a flow-velocity detection threshold as low as one micrometer per second; thanks to this extreme sensitivity, it can detect motionless prey in total darkness at distances of one to two body lengths, extracting this information from the same distributed dynamic pressure field described at the beginning of this model.

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