Lateral line

The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial cells, known as hair cells, which respond to displacement caused by motion and transduce these signals into electrical impulses via excitatory synapses. Lateral lines play an important role in schooling behavior, predation, and orientation.

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. Analysis has shown that the lateral line system should be an effective passive sensing system able to discriminate between submerged obstacles by their shape. The lateral line allows fish to navigate and hunt in water with poor visibility.

The lateral line system enables predatory fishes to detect vibrations made by their prey, and to orient towards the source to begin predatory action. Blinded predatory fishes remain able to hunt, but not when lateral line function is inhibited by cobalt ions.

The lateral line plays a role in fish schooling. Blinded Pollachius virens were able to integrate into a school, whereas fish with severed lateral lines could not. It may have evolved further to allow fish to forage in dark caves. In Mexican blind cave fish, Astyanax mexicanus, neuromasts in and around the orbit of the eye are bigger and around twice as sensitive as those of surface-living fish.

One function of schooling may be to confuse the lateral line of predatory fishes. A single prey fish creates a simple particle velocity pattern, whereas the pressure gradients of many closely swimming (schooling) prey fish overlap, creating a complex pattern. This makes it difficult for predatory fishes to identify individual prey through lateral line perception.

Lateral lines are usually visible as faint lines of pores running along each side of a fish's body. The functional units of the lateral line are the neuromasts, discrete mechanoreceptive organs that sense movement in water. There are two main varieties: canal neuromasts and superficial neuromasts. Superficial neuromasts are on the surface of the body, while canal neuromasts are along the lateral lines in subdermal, fluid-filled canals. Each neuromast consists of receptive hair cells whose tips are covered by a flexible jellylike cupula. Hair cells typically possess both glutamatergic afferent connections and cholinergic efferent connections. The receptive hair cells are modified epithelial cells; they typically possess bundles of 40–50 microvilli "hairs" which function as the mechanoreceptors. Within each bundle, the hairs are organized in a rough "staircase" from shortest to longest.

The hair cells are stimulated by the deflection of their hair bundles in the direction of the tallest "hairs" or stereocilia. The deflection allows cations to enter through a mechanically gated channel, causing depolarization or hyperpolarization of the hair cell. Depolarization opens Ca_v1.3 calcium channels in the basolateral membrane.