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The octavo-lateralis system in fish


The octavo-lateralis system is an ichthyological term for the sensory system present in fish and amphibians and is made up of the lateral line and the inner-ear apparatus.

The lateral line was first been observed in the 19th century. In 1850, Leydig was the first to describe it as a sense organ. However, it is only from the 60’s that the actual functions of this system have been studied and published by various researchers.

As described by its name, the octavo-lateralis system consists of the combined functions of the lateral line system and the inner-ear. The lateral line responds to changes in the water pressure and displacement whereas the inner-ear reports on sound and gravity.

The basic sensory unit of the octavo-lateralis system in fish is called the neuromast.

The neuromasts

Neuromasts can detect changes in water pressure. They consist of a base of cells with sensory hair (cilia) which are also to be found in the inner-ear. These hairs are covered with a dome shaped gel-like cupula, typically 0.1 to 0.2 mm long.




A swimming fish generates a pressure wave as it displaces a bow wave in front of itself. The pressure is higher in front of the fish than it is on its sides, difference that is taken into consideration by the fish’s own neuromasts. The presence of a fish nearby displacing water itself or an obstacle reflecting the vibration will cause the neuromasts to be pushed laterally and cause a nervous impulse to be sent to the fish’s brain. The hair cells inside the cupula have a directional sensitivity and instruct by a higher or lower frequency signal transmitted to the brain whether they are being bent in one direction or the other, thus helping the fish analyse his constantly changing environment.

Neuromasts can be on their own on the fish’s body or within canals (the lateral line system).

There are 2 different groups of neuromasts : the superficial ones which are positioned on the skin of the animal and the canal neuromasts which are located deeper under the skin, as part of the lateral line system. The superficial ones tend to be smaller than the canal neuromasts.

The distribution and size of the lateral line organs differ widely among different species. The frog, for example, has around 180 superficial lateral lines all over its body but does not have any canal lateral line. Fish emanating from strong water currents have narrow and parallel lateral line canals whereas deep water fish have wider canals.

The lateral line system

The lateral line system consists of canals along the sides of the fish and is present on both sides of the fish, from the base of its tail to the snout. Depending on the species, these lines can also give rise to a few or many branch lines.

These canals are buried in the fish’s skin and communicate with the environment through a series of ducts that are open at the surface (pores or pit organs). Variations in water pressure between different pores will cause a directional movement of water through the canal. Water moving through the canal disturbs the cupula which in turn stimulates the hair cells.

Depending on how the fish live, its habits and physical capabilities, the lateral line system has evolved in several ways.

Shoaling fish like the pollock have a very developed lateral line. Sensing the movements of its shoal-mates through variations of water pressure, each fish can synchronise its movements with the rest of the group.

Active swimming fish tend to have a higher proportion of canal neuromasts to surface ones and the line itself will be farther away from the pectoral fins so that the motion of their own fins doesn’t interfere as much with the surrounding currents.

The lateral line system also extends to the head of the fish. These cephalic lateral lines enable fish to locate prey. Killifish have highly developed cephalic lines which enable them to perceive vibrations caused by insects resting on the water surface. Similarly, Blind Cave fish have additional neuromasts on their head which allow them to locate food with precision as they cannot see.



Cephalic Lateral Lines


  • temporal (t),

  • supra-temporal (st),

  • supra-orbital (so),

  • infra-orbital (io) and

  • operlo-mandibular (om)

Another evolution of the pressure-sensitive system is seen in the Ampullae of Lorenzini which can be found in sharks for example. The neuromasts have changed to operate as electro-receptors and are able to detect electrical charges or fields in the water. Most animals, including humans, emit a DC field when in the seawater (electrical potential differences between body fluids and seawater). An AC field is also emitted with muscular contractions. A wound or even a scratch would alter significantly these fields and would enable the shark, for example, to locate its prey rapidly.

The lateral line system is also used by the fish as a tool to supplement their hearing. Sound waves are also waves of pressure. As a consequence, the neuromasts are able to detect very low frequency sounds of 100 Hz or less.

The inner-ear

In addition to sensing currents around itself via the lateral line, getting information about its surroundings and perceiving very low frequency sounds, the fish’s inner-ear is another very important part of the octavo-lateralis system.


The Inner-ear of the Minnow


  • B = Brain,

  • O = Otolith,

  • S = Saccule,

  • L= Lagena




The inner-ear

Lateral View


  • SC = Semi-Circular canals,

  • U = Utriculus,

  • UO = Utriculus otolith or Lapillus,

  • S = Sacculus,

  • SO = Sacculus otolith or Sagitta,

  • L = Lagena,

  • LO = Lagena otolith or Astericus,

  • M = Macula


Others (not mentioned in this article)

SU = Sulcus


The inner-ear of the fish consists of 4 sections :

These 3 latter sections of the inner-ear are essentially a sac-like structure inside of which the otoliths can be found (ear-bones).

Fish have approximately the same density as water, therefore, sound waves cause the entire fish to move with the water and sound passes right through their bodies. However, the density of the otoliths is much denser than water and their movement is much slower in response to sound waves than the rest of the fish’s body. This difference between the motion of the fish and the otoliths stimulate the cilia (sensory hair cells – also present in the neuromasts) of the Macula, located on inside of the sac-like structure. This difference is then transmitted by the means of nervous impulses to the brain of the fish.

If the sensory hair cells were damaged, they can repair themselves, something human sensory hair cells cannot do. Some studies however have found that if fish were exposed to extremely loud deflagrations like the ones caused a seismic air-gun, a tool commonly used to search for underwater oil deposits, they experience a hearing loss and their hair cells do not necessarily grow back. It might explain to some extent a reduction of some species’ population as hearing is also an attribute necessary to their reproduction.

Another hearing system element : the swim-bladder

The swim-bladder can also a significant contributor to the fish’s hearing system as it links up to the inner-ear. As far as hearing is concerned, there are 2 categories of fish :



The octavo-lateralis system is vital for the fish and give its brain sufficient information to avoid predators, capture prey, evolve in an ever changing space and perceive mate attraction.

The neuromasts are the key element and are present throughout the whole system ; their movements in the currents are registered by fish’s brain via nervous impulses and allow the fish to decide on what to do next.

The lateral line system registers water movements whereas the inner-ear’s bones capture the surrounding sounds. Some species complete their hearing apparatus with a link between their swim-bladder and their inner-ear which renders them “hearing specialists”.

If the hair cells were to get damaged in “normal conditions”, they repair themselves which perhaps underscores how vital this mechanism has become for fish.

Valérie Rousseau – 31/01/2008

A semi-related tidbit

Another fact, that is quite interesting, though unrelated to the octavo-lateralis system and could open a door for another article, is that otoliths are also used to age fish. As the fish grows, the otoliths accumulate layers of mineral depositions which result in the appearance of rings that resemble tree rings. By counting them, it is possible to determine the age of the fish in terms of days.