Fish Senses: Lateral Line, Hearing, Smell, Taste and Electroreception

How neuromasts, the inner ear and Weberian apparatus, olfaction, taste barbels and electroreceptors work in fish, and what zebrafish noise studies mean for aquarium design.

The lateral line and neuromasts

The lateral line is a system of sense organs that detects movement, vibration and pressure gradients in the water around a fish. On most fish it shows as a faint row of pores along each flank. Its working units are neuromasts: small mechanoreceptive organs built from hair cells, which are modified epithelial cells carrying bundles of roughly 40–50 microvilli arranged in a staircase from short to long. The tips of these bundles sit inside a flexible, jelly-like cupula. When water movement bends the cupula, the hair cells change their firing rate: deflection toward the longest hairs raises it, deflection the other way lowers it, so the signal carries both strength and direction.

Superficial neuromasts sit on the body surface and give coarse but wide-ranging detection of water flow.
Canal neuromasts lie in fluid-filled canals under the skin; water passing over the pores sets the canal fluid moving, which allows finer sensing of pressure differences.
When the fish swims, inhibitory nerve signals damp the hair cells, so the fish's own movements do not mask external stimuli.

The lateral line works at low frequencies, roughly below 100 Hz, picking up the near-field water movement around swimming animals, while the inner ear handles higher-frequency pressure waves. Experiments summarised on Wikipedia show its role in behaviour: blinded predatory fish can still hunt, but not once the lateral line is chemically blocked, and blinded saithe (Pollachius virens) could still join a school, whereas fish with severed lateral lines could not. In the blind cave form of Astyanax mexicanus, neuromasts around the eye are larger and about twice as sensitive as in surface fish of the same species.

Hearing, the swim bladder and the Weberian apparatus

Fish have an inner ear but no external or middle ear, and they detect sound mainly through the otoliths of the inner ear. In some groups the gas-filled swim bladder is coupled to the ear and boosts hearing (the swim bladder itself is covered in a separate guide). The clearest example is the Weberian apparatus of the superorder Ostariophysi, which includes carps and minnows, characins and catfishes. It is a chain of small bones, the Weberian ossicles, derived from the first few vertebrae, that links the front of the swim bladder to the inner ear. The bladder acts as a resonance chamber and the ossicles pass its vibrations on, amplifying sounds that the ear alone would barely register.

For assessing sensitivity to underwater sound, fish are grouped by whether the swim bladder takes part in hearing. Goldfish and zebrafish belong to the group in which the bladder is mechanically connected to the ear, so their hearing responds to sound pressure. Fish such as sturgeon and salmon have a swim bladder that is not involved in hearing, and sharks, skates and bladderless bony fish respond to particle motion rather than sound pressure.

Smell

Fish nostrils (nares) are, in almost all species, pits that do not open into the mouth; signals from them run through the olfactory nerves to the olfactory lobes at the front of the brain. These lobes are very large in fish that hunt mainly by smell, such as sharks and catfish. Smell guides salmon back to their natal rivers, the odour of which is imprinted when the young fish become smolts. Fish can also smell aspects of the MHC immune genes of potential mates and tend to prefer partners whose MHC genes differ from their own.

Many ostariophysans react to a chemical alarm cue, first reported by Karl von Frisch in minnows in 1938 and called Schreckstoff (fright substance). It is released only when the skin of a fish is damaged, for example by a predator, and nearby fish detect it through olfaction and show antipredator behaviour. Suggested components include hypoxanthine-3N-oxide and fragments of chondroitin sulfate from skin mucus.

Taste and barbels

Underwater, smelling and tasting overlap: both involve sensing chemicals in the water. Many larger catfish carry chemoreceptors across the whole body, so they effectively taste whatever they touch. Barbels are slender, whisker-like organs near the mouth of catfish, carp, sturgeon, zebrafish and others. Their skin carries taste buds on small ridges (dermal papillae) that increase surface area; the bullhead catfish has about 25 taste buds per square millimetre of barbel skin. Barbels mainly help fish find food where light is poor or water is murky.

Electroreception: knifefish and elephantfish

Electroreception is the ability to sense electric fields. In passive electrolocation, a fish detects the weak fields that other animals produce through nerve and muscle activity; this uses ampullary receptors sensitive below about 50 Hz, such as the ampullae of Lorenzini of sharks and rays, which evolved from lateral line organs. In active electrolocation, a fish produces its own weak field with an electric organ made of modified muscle and senses how nearby objects distort it. This is done by the Neotropical knifefishes (Gymnotiformes), the African elephantfishes (Mormyridae) and Gymnarchus. The field is usually under one volt and its useful range is about one body length.

Elephantfish emit short pulses and sense them with tuberous receptors called knollenorgans and mormyromasts.
Knifefishes such as glass knifefishes emit a continuous, nearly sine-shaped wave; two glass knifefishes near each other shift frequency to avoid jamming.

Peters's elephantnose fish (Gnathonemus petersii) has a trunk-like extension of the mouth, the Schnauzenorgan, covered in electroreceptors, which it uses to find worms and insects in soft substrate and to navigate in dark or murky water. In aquaria it is described as timid, preferring dense planting, subdued light, a pipe or hollow log for shelter and a soft sandy substrate it can sift with its extended lip.

What this means for tank design

Research on zebrafish shows that sound in the water affects fish behaviour. In one study, 24 hours of continuous white noise raised anxiety-like responses in a novel-tank test, and these responses were weaker when fish were tested in a group. The authors held their quiet control tanks without filters or pumps to avoid added noise and noted that housing facilities carry elevated chronic noise. In another study, seven days of noise in the 100–1000 Hz range led to more bottom-dwelling and less exploration, with stronger effects when the noise was played at night, and to raised plasma cortisol in females (stress physiology has its own guide).

Pumps, filters and air pumps add sound and vibration; ostariophysans such as zebrafish, goldfish and other cyprinids hear sound pressure through the Weberian apparatus.
Night-time noise produced stronger effects than daytime noise in the zebrafish study above.
Fish that rely on non-visual senses, such as elephantnose fish, are described as needing cover, subdued light and a soft substrate.