Sense organs play a very important role in the life and behavior of fish. Fish, like other vertebrates, have a full set of five senses. But they have a significant difference - the lateral line. In fish, this sense organ is called the sixth. Land animals lost it in the process of evolution, but waterfowl still have it and it makes their life quite significantly easier, helps them survive and feed.
Anatomy of a fish. Sense organs
The senses of smell and taste are considered to be one of the most important in fish. With their help, they are able to detect even minor changes in the environment. The pike fish, for example, not only feeds with the help of its mouth, but also, sensing a touch to the ground, instantly reacts, changing direction. Sensitive cells located in the mouth transmit nerve impulses, signaling danger, obstacles or food.
Fish have a rather finely developed sense of temperature. Such high sensitivity to fluctuations in temperature and pressure is unusual for terrestrial animals.
The olfactory organs of fish are located on the sides of the head and resemble small cones. With their help, they can detect changes in the chemical composition of water. The sense of smell is especially sharply developed in those animals that hunt at night. For example, a pike fish can smell prey that swims several meters away from it.
Side line. Location
Many scientists believe that the lateral line in fish is the most important sensory organ that helps animals live more comfortably. The lateral line is a kind of single center that unites all the sensitive cells in the body, located in the head or body.
The organ is located throughout the body, starting at the head and ending at the tail. The anatomy of fish, their variety and subspecies determine the location of the lateral line and its color. In one species it may appear as a bright white streak, in others it may appear as a dark, almost black stripe.
In a larger number of fish, the lateral line is represented in a single copy. But there are some species that can boast five or more. The lateral line of a fish can be very noticeable visually, or it can be hidden in the scales and immediately invisible to the human eye. In some fish it is arched, in others it is in the form of small abrupt stripes on the head.
There are fish that lack a sixth sense organ. These include mullet, dahlia, and some fish of the carp-tooth family.
The lateral line consists of...
As we have already said, the lateral line in a fish is a kind of brain and nerve center that allows you to control what is happening around you. What does this center consist of?
The lateral line is a cluster of a number of receptors that are located among themselves at a certain interval. Receptors can be located in channels on the head or in depressions that are located on the sides of the body. Most of the receptors are hidden under the skin of the fish. Only a few come to the surface and are hidden in the scales. Resembles open pores on the skin.
Inside, the lateral line canal is filled with fluid. Nerve receptors (their sensitive hairs), detecting changes, send a signal to this very liquid. Any movement, change in pressure or temperature of the water can cause the receptors and, consequently, the water in the canal to move. The stronger the changes in the fish’s habitat, the more the receptor hairs will deviate, the faster the information will enter the central nervous system.
The importance of the lateral line in fish
The sixth sense, or lateral line, allows fish to sense the approach of other animals living in the water much earlier than the organs of vision or smell tell them about it. The lateral line is capable of detecting minute changes in pressure in the water. Scientists say that the distance at which it is able to detect approaching danger is six times the size (length) of the fish itself.
The importance of the lateral line in fish with poor vision is especially great. There are animals that are able to react exclusively to shadow or light, while completely not noticing movement in the water. The lateral line in this case allows you to compensate for the underdevelopment or absence of visual or olfactory skills.
The life of a fish often depends on the lateral line. If it is damaged, then external influences will not be perceived so clearly by the animal. It will stop reacting to danger from the outside, will not be able to fully hunt, get food, or hide from enemies. And soon he will die.
Lateral line and bite
Surely all experienced fishermen know the meaning of the lateral line of a fish. With its help, fish are able to detect the slightest noise and vibrations in the water. As experts say, a shot, an explosion, a normal conversation in a raised voice, a hit on the water will be immediately “sensed” by the side line. And the fish, therefore, will react, get scared and hide. It is for this reason that fishermen try never to make noise on a pond, not to speak too loudly, and not to throw anything into the water.
Movement, slight noise and vibrations should be created not by the fisherman, but by the bait in the water. Experienced fishermen say that the bait should not stand in the reservoir, it must certainly move, making vibrations in the liquid. Only in this case will the fish smell food with its lateral line and move in the direction of the hook.
Why do fish have a “lateral line”?
nature has rewarded fish with a special organ for perceiving vibrations and movement of water - the lateral line.
It is known that the acoustic pressure in water is 2 times greater than the acoustic pressure in air. Water is practically incompressible, its density is 800 times greater than the density of air. All this creates favorable conditions for the propagation in the aquatic environment of vibrations, vortices, and jets caused by the movement of various bodies. The lateral line organs of fish are designed to capture both mechanical displacements of water particles and sounds (mainly low frequencies). Any creature moving near a fish causes at least a small movement of the water and thereby reveals itself. The sensitivity of the lateral line of fish is amazing: in experiments, fish detect the movement of a glass hair 0.25 mm thick at a distance of 20 to 50 cm.
What are the lateral line organs and how do they function? On both sides of the fish’s body, dotted lines are visually detected, running from the head to the tail of the fish. If you look closely, you will find that each dotted line represents a channel or groove filled with mucus. Sensitive cells of the lateral line are collected in kidney-shaped groups and hidden in canals that are washed by water.
The bodies of the sensitive cells contain a hair, which, when water acts on the mucus in the canal, bends and sends a signal to the auditory center of the fish. These hair cells are called neuromasts. Neuromasts of the lateral line organs densely cover the head and lateral surface of slow-swimming bottom-dwelling fish. A juvenile bream, for example, has almost 2000 such cells. They allow the fry to perceive a detailed picture of jet currents, learn about the direction of waves on the surface of the water, navigate (without the help of vision) the bottom topography, the movements of prey or school neighbors, even become familiar with the shape of objects by fanning them from a distance of 3-4 cm with fins .
In fact, the lateral line serves as a distant sense of touch. For fish it is more necessary than vision. Avid fishermen rightfully claim that when fishing for pike, it doesn’t matter what the spoon looks like - it’s enough that it just sparkles in the water. Much more important is how it moves and vibrates during wiring. It has been established that the lateral line of both predators and peaceful fish perfectly captures infrasounds, which are formed as a result of the disruption of vortices from the surface of any streamlined bodies (fish, baits, boats, underwater hunters, etc.). Infrasonic noises are very “loud” during sudden changes in fish speeds (throws, turns, accelerations) and are most intense in fish with a poorly streamlined body shape.
A significant role in the behavior of fish is played by sensory organs - the lateral line, or seismosensory system. It unites all the sensitive displacement receptor cells that can be found in various areas of the body and head.
The lateral line runs in the form of a longitudinal canal, immersed in the skin and opening outwards with holes. Visually, the lateral line is visible as a dark or light stripe on both sides of the body from the head to the end of the caudal peduncle. Its structure, external shape and location on the body of the fish vary greatly among different species.
Most fish have one channel on each side, and some have up to 5 or more, for example, greenlings. In some fish it is arched, in others it has one or several tubercles; in some, it is hardly noticeable visually, in others, its branches are clearly visible on the head. In some fish, free neuromasts or canal organs are scattered throughout the body or individual parts of it, most often on the head. In sea ducks, for example, seismosensory canals are present only on the head; they are absent on the body and are replaced by openly seated seismosensory points. Fishes of the cetacean family have thick lateral line canals with huge round pores. At the same time, there are fish in which the lateral line is absent or incomplete. These fish include mullet, dallium, many carp-toothed fish, silversides and others.
Sensitive cells of the lateral line, free neuromasts and canal sensory organs end at the apex with papillae or hairs, and on the opposite side with a nerve branch. The displacement of the papilla or hair creates a generator potential that transmits information along the nerves to the acousticolateral center of the brain. The lateral line organs also contain ampullary and ampullary-like cells that perform electroreceptor functions.
Visual observations have established that a thunderstorm discharge causes panic among ruffes and rudd. Fish detect earthquakes before the most sensitive instruments. Some species of sharks sense even minor electrical impulses that accompany the muscular efforts of a swimming person. Using the lateral line, they can find fish in the dark that do not move, but only breathe on the seabed.
Sharks react differently to electrical impulses of varying strengths. If the source is weak, then they attack; if it is strong, they swim away. Taking into account this behavior, a method was developed and is used today to scare sharks away from sea beaches: exposure to the lateral line with electrical discharges that are harmless to humans.
The lateral line system analyzers are located differently on the fish’s body and functionally complement each other. This allows fish that have similar receptors to differentially perceive irritations coming from outside. Open neuromasts (genipores, buccal pores) receive vibrations of water, mainly from its contact with the surface of the body. Most fish species living in the coastal zone or near the bottom have predominantly or exclusively genipores on their heads. Receptors of closed channels of the lateral line are more or less isolated from surface stimuli. They perceive fluctuations in hydrodynamic fields, sound and infrasonic vibration. This type of structure of the lateral line organs is characteristic, first of all, of predator fish that live in open waters and can only occasionally approach the shores.
With the help of the lateral line and other receptors of the seismosensory system, fish detect the approach of an enemy or prey. Waves run in front of a swimming fish, reflecting from underwater objects, and, returning to the fish, are perceived by its lateral line.
Free neuromasts and canal organs of the lateral line are mechanoreceptors that perceive flows of water and sound as vibrations. With their help, the fish picks up tiny vibrations (from 6 vibrations per second or more), determining the direction of the flow of water and sound, the proximity of neighbors, obstacles, etc. By sensing water currents with their lateral line - strong or barely noticeable - fish can distinguish the size of an obstacle or objects moving in the water.
The lateral line organs, as displacement receptors, function effectively in the near acoustic field. Sources of mechanical stimuli are also determined by the lateral line organs at close range. Fish have two types of sound receptors: pressure receptors (hearing organs), which allow them to sense sound waves over long distances, and displacement receptors - lateral line organs, which allow them to subtly analyze the acoustic situation. Fish can use skin receptors for these purposes, which are also displacement receivers.
The topography of the displacement receptors of the seismosensory system is extremely important for determining the direction and distance from the source of mechanical, acoustic, and electromagnetic vibrations. Almost all fish that have a well-developed seismosensory system are perfectly oriented with its help when moving in schools, in feeding fields and spawning areas. Displacement receptors, directly related to the fish’s hearing, function simultaneously with vision. So, for example, when attacking prey, a pike is guided by vision and displacement receptors - the organs of the lateral line, which are well developed on its head, especially on the lower jaw and on the sides of the body. These are, of a kind, small primitive radars that determine the location of the victim target with great accuracy. It is thanks to this “guidance” that the pike does not make idle throws at the hunted victim.
The lateral line also functions well in sea eels. This voracious predator of the sea, like pike in fresh water, lies in wait for its prey in ambush, from where it rushes at the victim according to the indications of the displacement organs.
In monkfish, the lateral line organs are located in the grooves of the skin on the upper surface of the strongly flattened body, which allows it to perceive vibrations and currents of water coming mainly from above. This fish lies motionless on the ground, and the leathery brush of a separate dorsal ray moves above its head. This lazy predator “invites” its prey. As soon as the trusting fish saw the “worm-shaped tip” and approached it, it instantly finds itself in the huge toothy mouth of the monkfish.
The seismosensory system of cyprinid fish is well developed. For many of them, the sense of the lateral line, along with the sense of smell and touch, is leading in the search for food. Cod and many other cod fish have a well-developed lateral line on both sides of the body, and it is especially complexly branched on the head. On each side of the head, the lateral line forms many canals: preopercular-mandibular, infraorbital and supraorbital with a short commissure connecting the right and left canals. The interorbital commissure of the supraorbital canal is located in a special depression, the frontal - mucus fossa, the external shape of which varies greatly among different cod fish; in cod, haddock and pollock, the mucus pit is closed. In some cod fish it is open.
Along each of the channels of the lateral line system on the head there are multi-membered groups of nerve endings - genipores, or these channels open outwards with a number of pores. Cod, for example, has 26-27. Moreover, single genipores are also present in this case. The lateral line of some cod representatives is continuous (haddock, pollock), while in others it is discontinuous (cod). In some codfish, such as cod, the lateral line is continuous on the body and discontinuous on the caudal peduncle. Such a complex seismosensory system - displacement receptors - allows cod, cod and other cod fish to navigate in the complete darkness of the sea depths, find food, move in a coordinated manner in schools, and avoid enemies, including getting caught in trawl fishing gear. In conditions of poor visibility, cod uses the senses of the lateral line organs to find moving food (mainly small fish), and with the senses of smell and tactile senses (taste, touch) it looks for stationary, favorite food (mollusks, licks). Thus, in the Barents Sea, a blind cod was caught with a lot of food in its stomach - capelin. The fat content of the cod (the ratio of liver weight to body weight as a percentage) was quite high, which indicates good feeding conditions.
This example, as, by the way, other similar catches, indicates that cod, being blind, finds and obtains enough food for itself thanks to a well-developed sense of smell and touch, and the presence of a complex seismosensory system.
There are naturally blind cave fish - anopgichi, which, with the help of a seismosensory system, provide themselves with normal conditions of existence and reproduction. In underground karst waters live blind-eyes, which have highly developed lateral line organs and organs of touch on the head, body and caudal peduncle. They replace not only vision for these fish, but also other remote sensory organs.
The lateral line plays a significant role in spawning waters to attract a female or in competition between males over her. In some species of fish, the male, having built a nest house, sends acoustic-mechanical signals, which the female takes as an invitation to “enter the house” as a “young mistress.” In other species, the male, with an energetic movement of his tail, directs the flow of water towards his opponent and, thus, influences his lateral line, informing the enemy that the spawning area is occupied.
The functions of the lateral line and other displacement receptors, which allow fish to detect water vibrations in a certain frequency spectrum, have been poorly studied from the point of view of their significance in the schooling behavior of fish. Thus, the seismosensory system of fish is a unique invention of nature. It provides fish with the opportunity to adequately change their behavior depending on the biotic and abiotic environment, and in each specific case - and how, and is the most important sensory organ in the struggle for life.
Side line is the oldest sensory formation, which, even in evolutionarily young groups of fish, simultaneously performs several functions.
Taking into account the exceptional importance of this organ for fish, let us dwell in more detail on its morphofunctional characteristics.
Different ecological types of fish exhibit different variations of the lateral system. The location of the lateral line on the body of fish is often a species-specific feature. There are species of fish that have more than one lateral line. For example, the greenling has 4 lines on each side. This is where its second name comes from - eight-line chir.
In most bony fishes, the lateral line stretches along the body (without interruption or interruption in some places), extends onto the head, where it forms a complex system of canals. The lateral line channels are located either deep in the skin or open on the surface of the skin.
An example of the open surface arrangement of neuromasts - structural units of the lateral line - is the lateral line of the minnow.
Despite the obvious diversity in the morphology of the lateral system, it should be emphasized that the observed differences concern only the macrostructure of this sensory formation. The organ's receptor apparatus itself (the chain of neuromasts) is surprisingly the same in all fish, both morphologically and functionally.
The lateral line system responds to compression waves of the aquatic environment, to flow currents, to chemical stimuli and to electromagnetic fields with the help of neuromasts - structures that unite several hair cells. The neuromast consists of a mucous-gelatinous part - a capsule, into which the hairs of sensitive cells are immersed. Closed neuromasts communicate with the external environment through small holes that pierce the scales. Open neuromasts are characteristic of the canals of the lateral system extending onto the head of the fish.
Channel neuromasts stretch from head to tail along the sides of the body, usually in one row (fishes of the family Hexagramidae have six or more rows). The term “lateral line” in common usage refers specifically to canal neuromasts. However, in the world of fish, neuromasts are also described, separated from the canal part and looking like independent organs.
The labyrinth and canal and free neuromasts, located in different parts of the fish’s body, do not duplicate, but functionally complement each other.
It is believed that the sacculus and lagena of the inner ear are responsible for sound sensitivity from a great distance, and the lateral system allows you to localize the source of sound at close range. It has been experimentally proven that the lateral line perceives low-frequency vibrations, both sound and those arising from the movement of other fish, i.e. low-frequency vibrations arising from a fish hitting the water with its tail are perceived by other fish as low-frequency sounds.
Waves arising on the surface of the water have a noticeable influence on the activity of fish and the nature of their behavior. The causes of this physical phenomenon are many factors: the movement of large objects (large fish, birds, animals), wind, tides, earthquakes. Water disturbance serves as an important channel for informing aquatic animals about events both in the reservoir itself and beyond. Moreover, the disturbance of the reservoir is perceived by both pelagic fish and bottom fish. The reaction to surface waves on the part of fish is of two types: the fish sinks to greater depths or moves to another part of the reservoir.
The stimuli acting on the body of the fish during the period of disturbance of the reservoir is the movement of water relative to the body of the fish. The movement of water when it is agitated is sensed by the acoustic-lateral system. Moreover, the sensitivity of the lateral line to waves is extremely high. Thus, for afferentation to occur from the lateral line, a displacement of water relative to the cupula by 0.1 μm is sufficient. At the same time, the fish is able to very accurately localize both the source of wave formation and the direction of wave propagation.
As a rule, waves on the surface of a reservoir generate rolling motion. Therefore, when excited, not only the lateral line of the fish, but its labyrinth becomes excited. Experiments have shown that the semicircular canals of the labyrinth respond to rotational movements in which water currents involve the body of the fish. The utriculus receives the linear acceleration that occurs during the rolling process.
Observations of marine fish indicate that during a storm, fish, both solitary and schooling, change their behavior. During a weak storm, pelagic species in the coastal zone descend to the bottom layers. When the waves are strong, fish migrate to the open sea and go to greater depths, where the influence of waves is less noticeable. It is obvious that strong excitement is assessed by fish as an unfavorable or even dangerous factor. Disturbance suppresses feeding behavior and forces fish to migrate. Similar changes in feeding behavior are observed in fish species inhabiting inland waters. Anglers know this: when there is excitement, the fish stop biting.
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