Lateral line

Early in the evolution of fish, some of the sensory organs of the lateral line were modified to function as the electroreceptors called ampullae of Lorenzini.

The lateral line system is ancient and basal to the vertebrate clade, as it is found in fishes that diverged over 400 million years ago.

The lateral line system allows the detection of movement, vibration, and pressure gradients in the water surrounding an animal.

It plays an essential role in orientation, predation, and fish schooling by providing spatial awareness and the ability to navigate in the environment.

[5] Blinded predatory fishes remain able to hunt, but not when lateral line function is inhibited by cobalt ions.

This makes it difficult for predatory fishes to identify individual prey through lateral line perception.

[1] The functional units of the lateral line are the neuromasts, discrete mechanoreceptive organs that sense movement in water.

The deflection allows cations to enter through a mechanically gated channel, causing depolarization or hyperpolarization of the hair cell.

[15] The mechanoreceptive hair cells of the lateral line structure are integrated into more complex circuits through their afferent and efferent connections.

The synapses that directly participate in the transduction of mechanical information are excitatory afferent connections that utilize glutamate.

For instance, the superficial neuromasts of the midshipman fish, Porichthys notatus, are sensitive to specific stimulation frequencies.

The area where these signals most often terminate is the medial octavolateralis nucleus (MON), which probably processes and integrates mechanoreceptive information.

[19] The deep MON contains distinct layers of basilar and non-basilar crest cells, suggesting computational pathways analogous to the electrosensory lateral line lobe of electric fish.

The auditory system detects pressure fluctuations with frequencies above 100 Hz that propagate to the far field as waves.

Oblique view of a goldfish ( Carassius auratus ), showing pored scales of the lateral line system
The lateral line system consists of canals that run along a fish's body, connecting rows of openings (pores) through the skin and scales to the water outside the body. Small sense organs, neuromasts (inset), are positioned both at intervals along the canals, and on the surface of the body. Each hair cell contains a bundle of sensory hairs.
A three-spined stickleback with stained neuromasts [ 10 ]
Lateral line rate coding indicates direction and sometimes strength of stimulus. One neuromast shown.
The motion detection system in fish works despite "noise" created by the fish itself. The brain copies the efferent commands it gives to the swimming muscles to the lateral line, effectively suppressing swimming noise and revealing small signals from the environment, such as from prey. [ 18 ]
The electroreceptive organs called ampullae of Lorenzini (red dots), illustrated here on the head of a shark , evolved from the mechanosensory lateral line organs (gray lines) of the last common ancestor of vertebrates. [ 21 ] [ 22 ]