Coloration of fish and its biological significance. Cave fish and fish coloration. Poisonous and poisonous fish

The color of the fish is very diverse. AT Far Eastern waters inhabited by small (8-10 centimeters *), smelt-like noodle fish with a colorless, completely transparent body: the insides are visible through the thin skin. Near sea ​​shore, where the water foams so often, the herds of this fish are invisible. Seagulls manage to eat "noodles" only when the fish jump out and appear above the water. But the same whitish coastal waves that protect the fish from birds often destroy them: on the shores you can sometimes see whole shafts of fish noodles thrown out by the sea. It is believed that after the first spawning, this fish dies. This phenomenon is characteristic of some fish. So cruel nature! The sea throws out both living and natural death "noodles".

* (In the text and below the figures, largest dimensions fish)

Since fish noodles are usually found in large herds, they should have been used; in part, it is still mined.

There are other fish with a transparent body, for example, the deep-sea Baikal golomyanka, which we will discuss in more detail below.

At the far eastern tip of Asia, in the lakes of the Chukchi Peninsula, there is a black dallium fish.

Its length is up to 20 centimeters. The black coloration makes the fish unobtrusive. Dallium lives in peaty dark-water rivers, lakes and swamps, buries itself in wet moss and grass for the winter. Outwardly, dahlia looks like common fish, but it differs from them in that its bones are tender, thin, and some are completely absent (there are no infraorbital bones). But this fish has highly developed pectoral fins. Do not fins such as shoulder blades help fish burrow into the soft bottom of the reservoir in order to survive in the winter cold?

Brook trout are colored with black, blue and red spots of various sizes. If you look closely, you can see that the trout changes its clothes: during the spawning period, it is dressed in a particularly flowery "dress", at other times - in more modest clothes.

A small minnow fish, which can be found in almost every cool stream and lake, has an unusually variegated color: the back is greenish, the sides are yellow with gold and silver reflections, the abdomen is red, yellowish fins are with a dark rim. In a word, the minnow is small in stature, but he has a lot of force. Apparently, for this he was nicknamed "buffoon", and this name is perhaps more just than "minnow", since the minnow is not at all naked, but has scales.

The most brightly colored sea fish, especially tropical waters. Many of them can successfully compete with birds of paradise. Look at table 1. There are no flowers here! Red, ruby, turquoise, black velvet ... They are surprisingly harmoniously combined with each other. Curly, as if honed by skilled craftsmen, the fins and body of some fish are decorated with geometrically regular stripes.

In nature, among corals and sea lilies, these colorful fish are a fabulous picture. Here is what the famous Swiss scientist Keller writes about tropical fish in his book "Life of the Sea": "The coral reef fish represent the most elegant sight. Their colors are not inferior in brightness and brilliance tropical butterflies and birds. Azure, yellowish green, velvety black and striped fish flicker and curl in whole crowds. You involuntarily take up the net to catch them, but .., one blink of an eye - and they all disappear. With a laterally compressed body, they can easily penetrate the cracks and crevices of coral reefs."

The well-known pikes and perches have greenish stripes on their bodies, which mask these predators in the grassy thickets of rivers and lakes and help them approach their prey unnoticed. But the pursued fish (bleak, roach, etc.) also have a protective coloration: the white belly makes them almost invisible when viewed from below, the dark back is not striking when viewed from above.

Fish living in the upper layers of the water have a more silvery color. Deeper than 100-500 meters there are red fish ( sea ​​bass), pink (liparis) and dark brown (pinagore) flowers. At depths exceeding 1000 meters, the fish are predominantly dark in color (anglerfish). In area ocean depths, more than 1700 meters, the color of the fish is black, blue, purple.

The color of the fish largely depends on the color of the water and the bottom.

In transparent WATERS, the bersh, which is usually gray in color, is distinguished by whiteness. Against this background, dark transverse stripes stand out especially sharply. In shallow swampy lakes, perch is black, and in rivers flowing from peat bogs, blue and yellow perch are found.

Volkhov whitefish, which was once in large numbers lived in the Volkhov Bay and the Volkhov River, which flows through limestone, differs from all Ladoga whitefish in light scales. According to it, this whitefish is easy to find in the total catch of Ladoga whitefish. Among the whitefish of the northern half of Lake Ladoga, black whitefish are distinguished (in Finnish it is called "musta siyka", which means black whitefish in translation).

The black color of the northern Ladoga whitefish, like the light Volkhov one, remains quite stable: the black whitefish, finding itself in southern Ladoga, does not lose its color. But over time, after many generations, the descendants of this whitefish, who remained to live in southern Ladoga, will lose their black color. Therefore, this feature may vary depending on the color of the water.

After low tide, the flounder remaining in the coastal gray mud is almost completely invisible: grey colour her back merges with the color of silt. The flounder did not acquire such a protective coloration at the moment when it found itself on a dirty shore, but received it by inheritance from its neighbors; and distant ancestors. But fish are capable of changing color very quickly. Put a minnow or other brightly colored fish in a black-bottomed tank and after a while you will see that the color of the fish has faded.

There are many surprising things in the coloring of fish. Among the fish that live at depths where even a weak ray of the sun does not penetrate, there are brightly colored ones.

It also happens like this: in a flock of fish with a color common to a given species, individuals of white or black color come across; in the first case, so-called albinism is observed, in the second - melanism.

Fish coloring

The color of the fish is very diverse. Small (8–10 centimeters), smelt-like noodle fish with a colorless, completely transparent body lives in the Far Eastern waters: the insides shine through the thin skin. Near the seashore, where the water so often foams, the herds of this fish are invisible. Seagulls manage to eat "noodles" only when the fish jump out and appear above the water. But the same whitish coastal waves that protect the fish from birds often destroy them: on the shores you can sometimes see whole shafts of fish noodles thrown out by the sea. It is believed that after the first spawning, this fish dies. This phenomenon is characteristic of some fish. So cruel nature! The sea throws out both living and “noodles” that died of natural death.

Since fish noodles are usually found in large herds, they should have been used; in part, it is still mined.

There are other fish with a transparent body, for example, the deep-sea Baikal golomyanka, which we will discuss in more detail below.

At the far eastern tip of Asia, in the lakes of the Chukchi Peninsula, there is a black dallium fish.

Its length is up to 20 centimeters. The black coloration makes the fish unobtrusive. Dallium lives in peaty dark-water rivers, lakes and swamps, buries itself in wet moss and grass for the winter. Outwardly, dallium is similar to ordinary fish, but it differs from them in that its bones are delicate, thin, and some are completely absent (there are no infraorbital bones). But this fish has strongly developed pectoral fins. Do not fins such as shoulder blades help fish burrow into the soft bottom of the reservoir in order to survive in the winter cold?

Brook trout are colored with black, blue and red spots of various sizes. If you look closely, you can see that the trout changes its clothes: during the spawning period, it is dressed in a particularly flowery “dress”, at other times - in more modest clothes.

The small minnow fish, which can be found in almost every cool stream and lake, has an unusually variegated color: the back is greenish, the sides are yellow with gold and silver reflections, the abdomen is red, yellowish fins are with a dark rim. In a word, the minnow is small in stature, but he has a lot of force. Apparently, for this he was nicknamed "buffoon", and such a name is perhaps more just than "minnow", since the minnow is not at all naked, but has scales.

The most brightly colored fish are marine, especially tropical waters. Many of them can successfully compete with birds of paradise. Look at table 1. There are no flowers here! Red, ruby, turquoise, black velvet... They are surprisingly harmoniously combined with each other. Curly, as if honed by skilled craftsmen, the fins and body of some fish are decorated with geometrically regular stripes.

In nature, among corals and sea lilies, these colorful fish are a fabulous picture. Here is what the famous Swiss scientist Keller writes about tropical fish in his book Life of the Sea: “The fish of the coral reefs are the most elegant sight. Their colors are not inferior in brightness and brilliance to the color of tropical butterflies and birds. Azure, yellowish green, velvety black and striped fish flicker and curl in crowds. You involuntarily take hold of the net to catch them, but... one blink of an eye - and they all disappear. With a laterally compressed body, they can easily penetrate the cracks and crevices of coral reefs.

The well-known pikes and perches have greenish stripes on their bodies, which mask these predators in the grassy thickets of rivers and lakes and help them approach their prey unnoticed. But the pursued fish (bleak, roach, etc.) also have a protective coloration: the white belly makes them almost invisible when viewed from below, the dark back is not striking when viewed from above.

Fish living in the upper layers of the water have a more silvery color. Deeper than 100–500 meters, there are fish of red (sea bass), pink (liparis) and dark brown (pinagora) colors. At depths exceeding 1000 meters, the fish are predominantly dark in color (anglerfish). In the area of ​​ocean depths, more than 1700 meters, the color of fish is black, blue, purple.

Table 1. tropical water fish

The color of the fish largely depends on the color of the water and the bottom.

AT clear waters bersh, which is usually gray in color, is distinguished by whiteness. Against this background, dark transverse stripes stand out especially sharply. In shallow swampy lakes, perch is black, and in rivers flowing from peat bogs, blue and yellow perch are found.

Volkhov whitefish, which once lived in large numbers in the Volkhov Bay and the Volkhov River, which flows through limestone, differs from all Ladoga whitefish in light scales. According to it, this whitefish is easy to find in the total catch of Ladoga whitefish. Among the whitefish of the northern half of Lake Ladoga, black whitefish are distinguished (in Finnish it is called “musta siyka”, which means black whitefish in translation).

The black color of the northern Ladoga whitefish, like the light Volkhov one, remains quite stable: the black whitefish, finding itself in southern Ladoga, does not lose its color. But over time, after many generations, the descendants of this whitefish, who remained to live in southern Ladoga, will lose their black color. Therefore, this feature may vary depending on the color of the water.

After low tide, the flounder remaining in the coastal gray mud is almost completely invisible: the gray color of its back merges with the color of the silt. The flounder did not acquire such a protective coloration at the moment when it found itself on a dirty shore, but inherited it from its near and distant ancestors. But fish are capable of changing color very quickly. Put a minnow or other brightly colored fish in a black-bottomed tank and after a while you will see that the color of the fish has faded.

There are many surprising things in the coloring of fish. Among the fish that live at depths where even a weak ray of the sun does not penetrate, there are brightly colored ones.

It also happens like this: in a flock of fish with a color common to a given species, individuals of white or black color come across; in the first case, the so-called albinism is observed, in the second - melanism.

Why do fish need bright colors? What is the origin of the varied pigmentation of fish? What is mimicry? Who sees the bright colors of fish at a depth where eternal darkness reigns? About how the color of fish correlates with their behavioral reactions and what social functions it has - biologists Alexander Mikulin and Gerard Chernyaev.

Topic Overview

Coloring is important environmental significance for fish. There are protective and warning colors. Protective coloration designed to mask the fish against the background environment. Warning, or sematic, coloration usually consists of conspicuous large, contrasting spots or bands that have clear boundaries. It is intended, for example, in poisonous and poisonous fish, to prevent a predator from attacking them, and in this case it is called a deterrent. Identification coloration is used to warn territorial fish of rivals, or to attract females to males, warning them that males are ready to spawn. The last type of warning coloration is commonly referred to as the mating dress of fish. Often the identification coloration unmasks the fish. It is for this reason that in many fish guarding the territory or their offspring, the identification coloration in the form of a bright red spot is located on the belly, shown to the opponent if necessary, and does not interfere with the disguise of the fish when it is located belly to the bottom.

There is also a pseudosematic coloration that mimics the warning coloration of another species. It is also called mimicry. It allows harmless species of fish to avoid the attack of a predator that takes them for a dangerous species.

There are other color classifications. For example, types of fish coloration are distinguished, reflecting the characteristics of the ecological confinement of this species. Pelagic coloration is characteristic of near-surface inhabitants of fresh and marine waters. It is characterized by a black, blue or green back and silvery sides and belly. The dark back makes the fish less visible against the bottom. river fish have a black and dark brown back color, so they are less noticeable against the background of a dark bottom. In lake fish, the back is colored in bluish and greenish tones, since this color of their back is less noticeable against the background of greenish water. The blue and green back is characteristic of most marine pelagic fish, which hides them against the background of blue sea ​​depths. The silvery sides and light belly of the fish are poorly visible from below against the background of a mirror surface. The presence of a keel on the belly in pelagic fish minimizes the shadow formed from the ventral side and unmasks the fish. When looking at the fish from the side, the light falling on the dark back, and the shadow of the lower part of the fish, hidden by the sheen of the scales, give the fish a gray, inconspicuous appearance.

The bottom coloration is characterized by a dark back and sides, sometimes with darker stains, and a light belly. In bottom fish living above the pebbly soil of rivers with clear water, usually on the sides of the body there are light, black and other colored spots, sometimes slightly elongated in the dorsal-abdominal direction, sometimes located in the form of a longitudinal strip (the so-called channel coloration). This coloration makes the fish hardly noticeable against the background of pebbly soil in clear flowing water. Bottom fish of stagnant freshwater reservoirs do not have bright dark spots on the sides of the body or they have blurred outlines.

The overgrown coloration of fish is characterized by a brownish, greenish or yellowish back and usually transverse or longitudinal stripes and stains on the sides. This coloration is characteristic of fish that live among underwater vegetation and coral reefs. Transverse stripes are characteristic of ambush predators hunting from an ambush of coastal thickets (pike, perch), or fish swimming slowly among them (barbs). Fish living near the surface, among the algae lying on the surface, are characterized by longitudinal stripes (zebrafish). The stripes not only mask the fish among the algae, but also dissect the appearance of the fish. Dissecting coloration, often very bright against a background unusual for fish, is characteristic of coral fish, where they are invisible against the background of bright corals.

Schooling fish are characterized by schooling coloration. This coloration facilitates the orientation of individuals in the flock to each other. It usually appears on the background of other forms of coloration and is expressed either in the form of one or more spots on the sides of the body or on dorsal fin, or as a dark strip along the body or at the base of the caudal peduncle.

Many peaceful fish in the back of the body there is a "deceptive eye", which disorients the predator in the direction of the prey's throw.

The whole variety of fish colors is due to special cells - chromatophores, which occur in the skin of fish and contain pigments. The following chromatophores are distinguished: melanophores containing black pigment grains (melanin); red erythrophores and yellow xanthophores, called lipophores, since the pigments (carotenoids) in them are dissolved in lipids; guanophores or iridocytes containing guanine crystals in their structure, which give the fish a metallic sheen and silvery scales. Melanophores and erythrophores are stellate, xanthophores are rounded.

Chemically, the pigments of different pigment cells differ significantly. Melanins are polymers with a relatively high molecular weight black, brown, red or yellow.

Melanins are very stable compounds. They are insoluble in any of the polar or non-polar solvents, nor in acids. However, melanins can discolor in bright sunlight, prolonged exposure to air, or, especially effectively, prolonged oxidation with hydrogen peroxide.

Melanophores are capable of synthesizing melanins. The formation of melanin occurs in several stages due to the sequential oxidation of tyrosine to dihydroxyphenylalanine (DOPA) and then until the polymerization of the melanin macromolecule occurs. Melanins can also be synthesized from tryptophan and even from adrenaline.

In xanthophores and erythrophores, the predominant pigments are carotenoids dissolved in fats. In addition to them, these cells can contain pterins, either without carotenoids or in combination with them. The pterins in these cells are localized in specialized small organelles called pterinosomes, which are located throughout the cytoplasm. Even in species that are colored primarily by carotenoids, pterins are first synthesized and become visible in developing xanthophores and erythrophores, while carotenoids, which must be obtained from food, are detected only later.

Pterins provide yellow, orange, or red coloration in a number of fish groups, as well as in amphibians and reptiles. Pterins are amphoteric molecules with weak acidic and basic properties. They are poorly soluble in water. Synthesis of pterins occurs through purine (guanine) intermediates.

Guanophores (iridophores) are very diverse in shape and size. Guanophores are composed of guanine crystals. Guanine is a purine base. Hexagonal crystals of guanine are located in the plasma of guanophores and, due to plasma currents, can be concentrated or distributed throughout the cell. This circumstance, taking into account the angle of incidence of light, leads to a change in the color of the integument of fish from silver-white to bluish-violet and blue-green or even yellow-red. So, a brilliant blue-green stripe neon fish Under the influence electric current acquires a red luster, like erythrosonus. Guanophores, located in the skin below the rest of the pigment cells, in combination with xanthophores and erythrophores give green, and with these cells and melanophores - blue.

Another method of acquiring the bluish-green color of their integuments by fish has been discovered. It has been noted that not all oocytes are spawned by female lumpfish during spawning. Some of them remain in the gonads and acquire a bluish-green color in the process of resorption. In the post-spawning period, the blood plasma of lumpfish females acquires bright green color. A similar blue-green pigment was found in the fins and skin of females, which, apparently, has an adaptive value during their post-spawning fattening in the coastal zone of the sea among algae.

According to some researchers, only melanophores are suitable for nerve endings, and melanophores have dual innervation: sympathetic and parasympathetic, while xanthophores, erythrophores and guanophores do not have innervation. The experimental data of other authors point to the nervous regulation of erythrophores as well. All types of pigment cells are subject to humoral regulation.

Changes in the color of fish occur in two ways: due to the accumulation, synthesis or destruction of the pigment in the cell and due to a change in the physiological state of the chromatophore itself without changing the pigment content in it. An example of the first method of color change is its enhancement during the pre-spawning period in many fish due to the accumulation of carotenoid pigments in xanthophores and erythrophores when they enter these cells from other organs and tissues. Another example: the habitation of fish on a light background causes an increase in the formation of guanine in guanophores and at the same time the decay of melanin in melanophores and, conversely, the formation of melanin occurring on a dark background is accompanied by the disappearance of guanine.

With a physiological change in the state of the melanophore under the action of a nerve impulse, the pigment grains located in the moving part of the plasma - in the kinoplasm, together with it are collected in the central part of the cell. This process is called contraction (aggregation) of the melanophore. Due to contraction, the vast majority of the pigment cell is freed from pigment grains, resulting in a decrease in color brightness. At the same time, the form of the melanophore, supported by the cell surface membrane and skeletal fibrils, remains unchanged. The process of distribution of pigment grains throughout the cell is called expansion.

Melanophores located in the epidermis of lungfish and you and me are not capable of changing color due to the movement of pigment grains in them. In humans, darkening of the skin in the sun occurs due to the synthesis of pigment in melanophores, and enlightenment due to desquamation of the epidermis along with pigment cells.

Under the influence of hormonal regulation, the color of xanthophores, erythrophores and guanophores changes due to a change in the shape of the cell itself, and in xanthophores and erythrophores, and due to a change in the concentration of pigments in the cell itself.

The processes of contraction and expansion of pigment granules of melanophores are associated with changes in the processes of wettability of the kinoplasm and ectoplasm of the cell, leading to a change in the surface tension at the boundary of these two plasma layers. This is a purely physical process and can be carried out artificially even in dead fish.

Under hormonal regulation, melatonin and adrenaline cause contraction of melanophores, in turn, hormones of the posterior pituitary gland - expansion: pituitrin - melanophores, and prolactin causes expansion of xanthophores and erythrophores. Guanophores are also subject to hormonal influences. Thus, adrenaline increases the dispersion of platelets in guanophores, while an increase in the intracellular level of cAMP enhances platelet aggregation. Melanophores regulate the movement of the pigment by changing the intracellular content of cAMP and Ca ++, while in erythrophores, regulation is carried out only on the basis of calcium. A sharp increase in the level of extracellular calcium or its microinjection into the cell is accompanied by aggregation of pigment granules in erythrophores, but not in melanophores.

The above data show that both intracellular and extracellular calcium play an important role in the regulation of expansion and contraction of both melanophores and erythrophores.

The coloration of fish in their evolution could not have arisen specifically for behavioral responses and must have some prior physiological function. In other words, the set of skin pigments, the structure of pigment cells and their location in the skin of fish, apparently, are not random and should reflect evolutionary path changes in the functions of these structures, during which modern organization pigment complex of the skin of living fish.

Presumably, initially the pigment system participated in the physiological processes of the body as part of excretory system skin. Subsequently, the pigment complex of fish skin began to participate in the regulation of photochemical processes occurring in the corium, and at the later stages of evolutionary development, it began to perform the function of the actual coloration of fish in behavioral reactions.

For primitive organisms, the excretory system of the skin plays an important role in their life. Naturally, one of the tasks of reducing harmful action end products of metabolism is to reduce their solubility in water by polymerization. This, on the one hand, makes it possible to neutralize their toxic effect and simultaneously accumulate metabolites in specialized cells without them. significant costs with further removal of these polymer structures from the body. On the other hand, the polymerization process itself is often associated with elongation of structures that absorb light, which can lead to the appearance of colored compounds.

Apparently, purines, in the form of guanine crystals, and pterins ended up in the skin as products of nitrogen metabolism and were removed or accumulated, for example, in the ancient inhabitants of the marshes during periods of drought, when they fell into hibernation. It is interesting to note that purines and especially pterins are widely represented in the integument of the body not only of fish, but also of amphibians and reptiles, as well as arthropods, in particular insects, which may be due to the difficulty of their removal due to the emergence of these groups of animals on land. .

It is more difficult to explain the accumulation of melanin and carotenoids in the skin of fish. As mentioned above, melanin biosynthesis is carried out due to the polymerization of indole molecules, which are products of the enzymatic oxidation of tyrosine. Indole is toxic to the body. Melanin turns out to be an ideal option for the preservation of harmful indole derivatives.

Carotenoid pigments, unlike those discussed above, are not end products of metabolism and are highly reactive. They are of food origin and, therefore, to clarify their role, it is more convenient to consider their participation in metabolism in closed system, for example, in fish roe.

Over the past century, more than two dozen opinions have been expressed about the functional significance of carotenoids in the body of animals, including fish and their caviar. Particularly heated debate was about the role of carotenoids in respiration and other redox processes. Thus, it was assumed that carotenoids are capable of transmembranely transporting oxygen, or storing it along the central double bond of the pigment. In the seventies of the last century, Viktor Vladimirovich Petrunyaka suggested the possible participation of carotenoids in calcium metabolism. He discovered the concentration of carotenoids in certain areas of the mitochondria, called calcospherules. An interaction of carotenoids with calcium during the embryonic development of fish, due to which a change in the color of these pigments occurs, has been found.

It has been established that the main functions of carotenoids in fish roe are: their antioxidant role in relation to lipids, as well as participation in the regulation of calcium metabolism. They are not directly involved in the processes of respiration, but purely physically contribute to the dissolution, and, consequently, the storage of oxygen in fatty inclusions.

The views on the functions of carotenoids have fundamentally changed in connection with the structural organization of their molecules. Carotenoids consist of ionic rings, including oxygen-containing groups - xanthophylls, or without them - carotenes and a carbon chain, including a system of double conjugated bonds. Previously, great importance in the functions of carotenoids was given to changes in the groups in the ionone rings of their molecules, that is, the transformation of some carotenoids into others. We have shown that qualitative composition in the work of carotenoids of great importance does not, and the functionality of carotenoids is associated with the presence of a conjugation chain. It determines the spectral properties of these pigments, as well as the spatial structure of their molecules. This structure quenches the energy of radicals in the processes of lipid peroxidation, performing the function of antioxidants. It provides or interferes with the transmembrane transport of calcium.

There are other pigments in fish caviar. Thus, a pigment close in light absorption spectrum to bile pigments and its protein complex in scorpion fish determine the diversity of color of eggs of these fish, ensuring the detection of native clutch. A unique hemoprotein in the yolk of whitefish eggs contributes to its survival during development in the pagon state, that is, when it freezes into ice. It contributes to the idle burning of part of the yolk. It was found that its content in caviar is higher in those species of whitefish, the development of which occurs in more severe conditions. temperature conditions winters.

Carotenoids and their derivatives - retinoids, such as vitamin A, are able to accumulate or transmembrane transfer salts of divalent metals. This property, apparently, is very important for marine invertebrates, which remove calcium from the body, which is used later in the construction of the external skeleton. Perhaps this is the reason for the presence of an external rather than an internal skeleton in the vast majority of invertebrates. It is well known that external calcium-containing structures are widely represented in sponges, hydroids, corals, and worms. They contain significant concentrations of carotenoids. In mollusks, the main mass of carotenoids is concentrated in motile mantle cells - amoebocytes, which transport and secrete CaCO 3 into the shell. In crustaceans and echinoderms, carotenoids in combination with calcium and protein are part of their shell.

It remains unclear how these pigments are delivered to the skin. It is possible that phagocytes were the original cells delivering pigments to the skin. Macrophages that phagocytize melanin have been found in fish. The similarity of melanophores with phagocytes is indicated by the presence of processes in their cells and the amoeboid movement of both phagocytes and melanophore precursors to their permanent locations in the skin. When the epidermis is destroyed, macrophages also appear in it, consuming melanin, lipofuscin and guanine.

The place of formation of chromatophores in all classes of vertebrates is the accumulation of cells of the so-called neural crest, which arises above the neural tube at the site of separation of the neural tube from the ectoderm during neurulation. This detachment is carried out by phagocytes. Chromatophores in the form of unpigmented chromatoblasts at the embryonic stages of fish development are able to move to genetically predetermined areas of the body. More mature chromatophores are not capable of amoeboid movements and do not change their shape. Further, a pigment corresponding to this chromatophore is formed in them. AT embryonic development bony fish chromatophores different types appear in a certain sequence. Dermal melanophores differentiate first, followed by xanthophores and guanophores. In the process of ontogenesis, erythrophores originate from xanthophores. Thus, the early processes of phagocytosis in embryogenesis coincide in time and space with the appearance of unpigmented chromatoblasts, precursors of melanophores.

Thus, a comparative analysis of the structure and functions of melanophores and melanomacrophages gives reason to believe that at the early stages of animal phylogenesis, the pigment system, apparently, was part of the excretory system of the skin.

Having appeared in the surface layers of the body, pigment cells began to perform a different function, not related to excretory processes. In the dermal layer of the skin of bony fish, chromatophores are localized in a special way. Xanthophores and erythrophores are usually located in the middle layer of the dermis. Below them lie guanophores. Melanophores are found in the lower dermis below the guanophores and in the upper dermis just below the epidermis. Such an arrangement of pigment cells is not accidental and, possibly, due to the fact that photoinduced processes of synthesis of a number of substances important for metabolic processes, in particular, vitamins of group D, are concentrated in the skin. To perform this function, melanophores regulate the intensity of light penetration into the skin, and guanophores perform the function of a reflector, passing light twice through the dermis when it is lacking. It is interesting to note that direct exposure to light on skin areas leads to a change in the response of melanophores.

There are two types of melanophores, differing in appearance, localization in the skin, reactions to nervous and humoral influences.

In higher vertebrates, including mammals and birds, mainly epidermal melanophores, more commonly referred to as melanocytes, are found. In amphibians and reptiles, they are thin elongated cells that play a minor role in the rapid color change. There are epidermal melanophores in primitive fish, in particular lungfish. They do not have innervation, do not contain microtubules, and are not capable of contraction and expansion. To a greater extent, the change in the color of these cells is associated with their ability to synthesize their own melanin pigment, especially when exposed to light, and the weakening of the color occurs in the process of desquamation of the epidermis. Epidermal melanophores are characteristic of organisms living either in drying up water bodies and falling into anabiosis (lungfish), or living out of water (terrestrial vertebrates).

Almost all poikilothermic animals, including fish, have dendro-shaped dermal melanophores that quickly respond to nervous and humoral influences. Given that melanin is not reactive, it cannot perform any other physiological functions, except to shield or dose light into the skin. It is interesting to note that the process of tyrosine oxidation from a certain moment goes in two directions: towards the formation of melanin and towards the formation of adrenaline. In evolutionary terms, in ancient chordates, such oxidation of tyrosine could occur only in the skin, where oxygen was available. At the same time, adrenaline itself in modern fish acts through nervous system on melanophores, and in the past, possibly produced in the skin, directly led to their contraction. Considering that the excretory function was originally performed by the skin, and, later, the kidneys, which are intensively supplied with blood oxygen, specialized in performing this function, chromaffin cells in modern fish that produce adrenaline are located in the adrenal glands.

Let us consider the formation of the pigment system in the skin during the phylogenetic development of primitive chordates, pisciformes, and fish.

The lancelet has no pigment cells in the skin. However, the lancelet has an unpaired photosensitive pigment spot on the anterior wall of the neural tube. Also, along the entire neural tube, along the edges of the neurocoel, there are light-sensitive formations - Hesse's eyes. Each of them is a combination of two cells: photosensitive and pigment.

In tunicates, the body is dressed in a single-layer cellular epidermis, which highlights on its surface a special thick gelatinous membrane - a tunic. Vessels pass through the thickness of the tunic, through which blood circulates. There are no specialized pigment cells in the skin. There are no tunicates and specialized excretory organs. However, they have special cells - nephrocytes, in which metabolic products accumulate, giving them and the body a reddish-brown color.

Primitive cyclostomes have two layers of melanophores in their skin. In the upper layer of the skin - the corium, under the epidermis there are rare cells, and in the lower part of the corium there is a powerful layer of cells containing melanin or guanine, which shields the light from entering the underlying organs and tissues. As mentioned above, lungfish have non-innervated stellate epidermal and dermal melanophores. In phylogenetically more advanced fish, melanophores, capable of changing their light transmission due to nervous and humoral regulation, are located in the upper layers under the epidermis, and guanophores - in the lower layers of the dermis. In bony ganoids and teleosts, xanthophores and erythrophores appear in the dermis between the layers of melanophores and guanophores.

In the process of phylogenetic development of lower vertebrates, in parallel with the complication of the pigment system of the skin, the organs of vision improved. It is the photosensitivity nerve cells in combination with the regulation of light transmission by melanophores formed the basis for the emergence of visual organs in vertebrates.

Thus, the neurons of many animals respond to illumination by a change in electrical activity, as well as an increase in the rate of neurotransmitter release from nerve endings. Nonspecific photosensitivity of nervous tissue containing carotenoids was found.

All parts of the brain are photosensitive, but the middle part of the brain, located between the eyes, and the pineal gland are the most photosensitive. In the cells of the pineal gland there is an enzyme whose function is the conversion of serotonin to melatonin. The latter causes contraction of skin melanophores and retardation of the growth of gonads of producers. When the pineal gland is illuminated, the concentration of melatonin in it decreases.

It is known that sighted fish darken on a dark background, and brighten on a light background. However, bright light causes darkening of the fish due to a decrease in the production of melatonin by the pineal gland, and low or no light causes brightening. Similarly, fish react to light after removing their eyes, that is, they brighten in the dark and darken in the light. It was noted that in a blind cave fish, residual melanophores of the scalp and middle part of the body react to light. In many fish, when they mature, due to the hormones of the pineal gland, the color of the skin intensifies.

A light-induced color change in reflection by guanophores was found in fundulus, red neon, and blue neon. This indicates that the change in the color of the luster, which determines day and night coloration, depends not only on the visual perception of light by the fish, but also on the direct effect of light on the skin.

In embryos, larvae and fry of fish developing in the upper, well-lit layers of water, melanophores, with dorsal side, cover the central nervous system from exposure to light and it seems that all five parts of the brain are visible. Those developing at the bottom have no such adaptation. Exposure to light on eggs and larvae of the Sevan whitefish causes an increased synthesis of melanin in the skin of embryos during the embryonic development of this species.

The melanophore-guanophore system of light regulation in fish skin, however, has a disadvantage. To perform photochemical processes, a light sensor is needed, which would determine how much light actually passed into the skin, and would transmit this information to melanophores, which should either enhance or weaken the light flux. Consequently, the structures of such a sensor must, on the one hand, absorb light, i.e., contain pigments, and, on the other hand, report information about the magnitude of the flux of light falling on them. To do this, they must be highly reactive, be fat-soluble, and also change the structure of membranes under the action of light and change its permeability to various substances. Such pigment sensors should be located in the skin below the melanophores, but above the guanophores. It is in this place that erythrophores and xanthophores containing carotenoids are located.

As is known, carotenoids are involved in light perception in primitive organisms. Carotenoids are present in the eyes of unicellular organisms capable of phototaxis, in the structures of fungi, the hyphae of which react to light, in the eyes of a number of invertebrates and fish.

Later, in more highly developed organisms, carotenoids in the organs of vision are replaced by vitamin A, which does not absorb light in the visible part of the spectrum, but, being part of rhodopsin, is also a pigment. The advantage of such a system is obvious, since colored rhodopsin, having absorbed light, decomposes into opsin and vitamin A, which, unlike carotenoids, do not absorb visible light.

The division of the lipophores themselves into erythrophores, which are capable of changing light transmission under the action of hormones, and xanthophores, which, in fact, apparently, are light detectors, allowed this system to regulate photosynthetic processes in the skin, not only when light is simultaneously exposed to the body from the outside, but also to correlate it is with the physiological state and the body's needs for these substances, hormonally regulating light transmission through both melanophores and erythrophores.

Thus, the color itself, apparently, was a transformed consequence of the performance by pigments of other physiological functions associated with the surface of the body and, picked up by evolutionary selection, acquired independent function in mimicry and for signal purposes.

emergence various types originally had colors physiological causes. So, for the inhabitants of surface waters, influenced significant insolation, powerful melanin pigmentation is required on the dorsal part of the body in the form of melanophores of the upper dermis (to regulate the transmission of light into the skin) and in the lower layer of the dermis (to shield the body from excess light). On the sides and especially the belly, where the intensity of light penetration into the skin is less, it is necessary to reduce the concentration of melanophores in the skin with an increase in the number of guanophores. The appearance of such coloration in pelagic fish simultaneously contributed to a decrease in the visibility of these fish in the water column.

Juvenile fish react to the intensity of illumination to a greater extent than to a change in the background, that is, in complete darkness they brighten, and darken in the light. This indicates the protective role of melanophores against excessive exposure to light on the body. In this case, fish fry, due to their smaller size than adults, are more susceptible to harmful effects Sveta. This is confirmed by the significantly greater death of fry less pigmented with melanophores when exposed to direct rays of sunlight. On the other hand, darker fry are eaten more intensively by predators. The impact of these two factors: light and predators leads to the occurrence of diurnal vertical migrations in most fish.

In juveniles of many species of fish that lead a schooling life at the very surface of the water, in order to protect the body from excessive exposure to light, a powerful layer of guanophores develops on the back under the melanophores, giving the back a bluish or greenish tint, and in the fry of some fish, such as mullets, the back is behind guanine literally glows with reflected light, protecting from excessive insolation, but also making fry visible to fish-eating birds.

In many tropical fish that live in small streams, shaded by the forest canopy from sunlight, a layer of guanophores is enhanced in the skin under the melanophores, for the secondary transmission of light through the skin. In such fish, species are often found that additionally use guanine luster in the form of “luminous” stripes, like neons, or spots as a guide when creating flocks or in spawning behavior to detect individuals of the opposite sex of their species in the twilight.

Marine bottom fish, often flattened in the dorso-ventral direction and leading sedentary image life, must have, in order to regulate photochemical processes in the skin, rapid changes in individual groups of pigment cells on their surface in accordance with the local focusing of light on their skin surface, which occurs in the process of its refraction by the water surface during waves and ripples. This phenomenon could be picked up by selection and lead to the emergence of mimicry, expressed in a rapid change in the tone or pattern of the body to match the color of the bottom. It is interesting to note that sea bottom inhabitants or fish whose ancestors were bottom usually have a high ability to change their color. AT fresh waters the phenomenon of "sunbeams" at the bottom, as a rule, does not occur, and there are no fish with a rapid change in color.

With depth, the light intensity decreases, which, in our opinion, leads to the need to increase light transmission through the integument, and, consequently, to a decrease in the number of melanophores with a simultaneous increase in the regulation of light penetration with the help of lipophores. It is with this, apparently, that it becomes red in many semi-deep-water fish. Red pigments at a depth where the red rays of sunlight do not reach appear black. At great depths, fish are either colorless or, at glowing fish, are black in color. In this they differ from cave fish, where in the absence of light there is no need at all for a light-regulating system in the skin, in connection with which melanophores and guanophores disappear in them, and last of all, in many, lipophores.

The development of protective and warning coloration in different systematic groups of fish, in our opinion, could proceed only on the basis of the level of organization of the pigment complex of the skin of a particular group of fish that had already arisen in the process of evolutionary development.

Thus, such a complex organization of the skin pigment system, which allows many fish to change color and adapt to different living conditions, had its own prehistory with a change in functions, such as participation in excretory processes, in skin photoprocesses, and, finally, in the actual color of the body of fish.

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Why in the animal world is the color of males brighter and more attractive than that of females?

The bright colors of birds arise in evolution due to sexual selection.
Sexual selection is natural selection for reproductive success. Traits that reduce the viability of their carriers can emerge and spread if the advantages they provide in breeding success are significantly greater than their disadvantages for survival. A male that lives a short time but is liked by females and therefore produces many offspring has a much higher cumulative fitness than one that lives long but leaves few offspring. In each generation, fierce competition for females arises between males. In cases where females choose males, male competition manifests itself in demonstrating their bright appearance or complex behavior courtship. Females choose those males that they like the most. As a rule, these are the brightest males.

But why do females like bright males?
The fitness of the female depends on how objectively she is able to assess the potential fitness of the future father of her children. She must choose a male whose sons will be highly adaptable and attractive to females.

According to the “attractive sons” hypothesis, the logic of female selection is somewhat different. If bright males, for whatever reason, are attractive to females, then it is worth choosing a bright father for your future sons, because his sons will inherit the bright color genes and will be attractive to females in the next generation. Thus, there is a positive Feedback, which leads to the fact that from generation to generation the brightness of the plumage of males is more and more enhanced. The process goes on increasing until it reaches the limit of viability.

In fact, in choosing males, females are no more and no less logical than in all other behaviors. When an animal feels thirsty, it does not reason that it should drink water in order to restore the water-salt balance in the body - it goes to the watering place because it feels thirsty. When a worker bee stings a predator attacking a hive, she does not calculate how much by this self-sacrifice she increases the cumulative fitness of her sisters - she follows instinct. Similarly, females, choosing bright males, follow their instincts - they like bright tails. All those who instinctively prompted a different behavior, all of them left no offspring.