How fish eggs appear. How fish reproduce: features and methods
Reproduction methods. Fish reproduce sexually. In rare cases, fish have:
1. Parthenogenesis (development of eggs without fertilization), the development of eggs reaches only the crushing stage (herring, sturgeon, salmon cyprinids) and only in exceptional cases to the larvae that survive until the yolk sac is reabsorbed (burbot, herring). In most cases, such development does not lead to the production of viable juveniles, but in the Issyk-Kul chebak, normal offspring are observed during the parthenogenetic development of eggs. In salmon, unfertilized eggs, finding themselves in the spawning mound in a place with fertilized ones, often develop parthenogenetically. As a result, they do not rot and the entire clutch of eggs does not die.
2. Gynogenesis (birth of females), sperm of related fish species penetrate the egg and stimulate its development, but fertilization does not occur. As a result of such reproduction, only females are observed in the offspring. In Central Asia, Western Siberia and Europe, populations of silver crucian carp are found, and in the reservoirs of Mexico - mollies (order Cyprinodontiformes), consisting almost entirely of females.
Fish are usually unisexual, but there are also hermaphrodites among them. Among bony fish, hermaphrodites include the rock perch, in which eggs and sperm develop in the gonads, but their maturation usually occurs alternately, and the red pagella, in which a change (reversion) of sex occurs during life: in young individuals, the gonads function as ovaries, in older ones are like testes. Occasionally, hermaphroditism occurs in herring, salmon, carp, and perch fish.
In fish, fertilization occurs:
1) external (in most fish);
2) internal (in cartilaginous fish, in some bony fish - sea bass, eelpout; in many carp-toothed fish - gambusia, guppies, swordtails, etc.).
In fish there are:
1) oviparous, laying eggs in the external environment (most species);
2) ovoviviparous, giving birth to fry. Fertilized eggs are retained in the posterior sections of the oviducts and develop there until the young are hatched (most cartilaginous fish - katran, white shark, fox shark, sawfish); in some species, for example, in the stingray, the walls of the posterior sections of the oviducts (“uterus”) even have special outgrowths through which nutrient fluid enters the oral cavity of the embryos through the squirters;
3) viviparous - in fish, in the posterior sections of the oviducts (“uterus”), something similar to the placenta of mammals is formed, and the embryo receives nutrients from the mother’s blood (blue shark, mustel shark, etc.).
The adaptive significance of viviparity and ovoviviparity in fish lies in the fact that during intrauterine development, greater survival of the juveniles is ensured.
Depending on the nature of reproduction, fish are divided into:
1) monocyclic - fish die after a single spawn (river eel, Pacific salmon, river lamprey, Baikal golomyanka);
2) polycyclic - fish reproduce several times during their lives (most fish).
The age at which fish reach sexual maturity varies significantly - from 1 - 2 months (gambusia) to 15 - 30 years (sturgeon). Fish with a short life cycle mature earlier (sprat, smelt and some gobies - at the age of 1 year), fish with a long life cycle become sexually mature much later (Atlantic cod - at 7-10 years, sea bass - at 12-15 years, etc. .d.).
The age of sexual maturation of fish depends on the species, the living conditions of the fish, primarily on the feeding conditions. The onset of sexual maturity in fish occurs when they reach a certain length. As a rule, the better a fish is fed, the faster it grows, and therefore the faster it matures. Males usually mature earlier than females.
The rate of ripening is also influenced by climatic conditions. Thus, bream reaches sexual maturity in the Aral Sea at 3-4 years, in the Northern Caspian Sea at 3-6 years, in the Middle Volga at 6-7 years, in Lake Ladoga at 8-9 years.
Age at sexual maturity is important for determining the size of fish catches and assessing raw material resources. For short-lived fish that mature in the 2-3rd year of life (sprat, sprats, sardines, etc.), the permissible commercial removal from the population can be 40-60%; for long-lived fish, this removal should be significantly less (5-20% ).
Sexual dimorphism. In most fish, sexual dimorphism (secondary sexual characteristics) is not manifested; females and males are externally difficult to distinguish. In some species, secondary sexual characteristics are clearly expressed: females are larger than males, males are characterized by brighter colors, elongated fins, etc. Males of polar flounder have ctenoid scales, females have cycloid scales. Males of cartilaginous fish have copulatory organs (pterygopodia), while females do not; in male tench, unlike females, the first ray of the pelvic fins is thickened, etc.
In some fish, during the pre-spawning period, under the influence of sex hormones, a nuptial plumage appears, which disappears after spawning. Many carp and whitefish fish develop white horny formations on the head and body of males - “pearl rash”; male round gobies become completely black by the time of spawning; The abdomen of the male stickleback turns from silvery to bright red. In Pacific and Atlantic salmon, during spawning, the silver color darkens, black and crimson spots appear on the body, significant morphological changes are observed (the jaws lengthen and bend, changes in the cranial skeleton are observed, pink salmon grow a hump).
Sex ratio. The sex ratio is an adaptive property of fish and is aimed at ensuring successful reproduction. For most fish it is close to 1:1.
The sex ratio of fish can change under the influence of various factors. In guppies, significant development of saprolegnia is sometimes accompanied by the transformation of most of the surviving females into males. Exposure of green swordtail fry to high temperatures results in male predominance.
Hormonal drugs can also help change normal sex ratios in fish. In fish farming, when breeding commercial fish, targeted sex changes can occur by introducing steroid hormones into the feed (rainbow trout).
Among the fish there are:
1) monogamous - usually one male (salmon) spawns with one female;
2) polygamous - there are 3-4 or more males per female (carp) or one male ensures the fertilization of eggs of several females (stickleback).
Timing of reproduction and spawning characteristics. Depending on the timing of reproduction, fish are distinguished:
1) spring spawning (pike, perch, grayling);
2) summer spawning (carp, sturgeon, anchovy);
3) autumn-winter spawning (salmon, Pacific salmon, whitefish, burbot, navaga).
The timing of reproduction of each species, as well as the timing of hatching of larvae and development of juveniles, is related to the availability of their food. Thus, pike reproduces immediately after the ice melts - much earlier than carp fish, which allows its juveniles to reach a length of 5-6 cm and completely switch to feeding on carp fish larvae.
The timing of reproduction of the same species may be different. Thus, capelin near Finnmarken and western Murman spawn from March to May, near eastern Murman - in June-July, in the eastern part of the Barents Sea - in August-September. Inhabitants of mid-latitudes usually spawn once a year, but some of them lay eggs not annually, but at intervals of 2 to 6 years (sturgeon); many tropical fish reproduce several times during the year.
Based on the duration of the spawning period, there are 2 groups of fish:
1) with one-time spawning - all eggs mature at the same time (pike, whitefish, salmon);
2) with portioned spawning - the eggs ripen and are spawned in portions over a long period of time (anchovy, bleak, Caspian herring, catfish, etc.).
In stickleback, the spawning process is measured in a few seconds, in roach and perch - in hours, in carp and bream - in days. Cod, which spawn 3-4 portions of eggs during the spawning season, spends 1.5-2 months on the spawning ground, cod - 3 months.
Often, one and the same species exhibits simultaneous spawning in one body of water and portioned spawning in another. Thus, bream in the Aral Sea exhibit batch spawning; in northern reservoirs (Lake Onega, etc.) it spawns eggs at once.
Portioned spawning is an adaptation of the species to the influence of environmental factors and contributes to increased fertility, greater survival of eggs and larvae, and better nutrition of the young due to the uniform use of the food supply.
Scale, coefficient and maturity index. To assess the degree of maturity of reproductive products in fish, maturity scales are used, of which the most common for polycyclic fish with one-time spawning is a six-point scale.
Stage I - juvenile (juvenales), immature fish. The reproductive cells of the ovaries are indistinguishable to the naked eye, and sex is not visually determined. The ovaries and testes look like thin transparent cords of yellowish or pinkish color.
Stage II - maturing individuals or individuals with developing reproductive products after spawning. The eggs are very small and are visible only under a magnifying glass. The ovaries are transparent and colorless, with a large blood vessel running along them. The testes increase in size, lose transparency and look like rounded strands of grayish or pale pink color.
Stage III - individuals whose gonads are far from mature, but relatively well developed. The ovaries fill from 1/3 to 3 of the entire abdominal cavity, filled with opaque eggs, clearly visible to the naked eye. The testes are dense and elastic. When pressed, liquid milk cannot be extracted from the testes. Their color ranges from pinkish-gray to yellowish-white.
Stage IV - individuals in which the gonads have almost reached full development. The ovaries and testes are the largest and fill up to 2/3 of the entire abdominal cavity. The eggs are round, transparent and flow out when pressed. The testes are soft, white, filled with liquid milk, and flow out when pressed.
Stage V - current individuals. The caviar and milt are so mature that they flow freely with light pressure on the abdomen.
Stage VI - spawned individuals (slaughter). The reproductive products are completely swept out. Gonads in the form of collapsing sacs. Remaining eggs may be observed in the ovaries, and remains of sperm in the testes. The ovaries and testes are inflamed and dark red in color. Some time after reproduction, the ovaries and testes enter stage II of maturity.
In fish with portioned spawning, the stage of maturity is determined by the state of the portion that is most developed and will be spawned first. After spawning the first portion, the ovaries do not move into stage VI, as in fish with simultaneous spawning, but into stage IV or III, and these stages of maturity are designated VI-IV or VI-III. Then, after the completion of the entire spawning period, the state of the ovary is assessed as being in stage VI, and then in stage II. If the remaining oocytes (reserve for the next year) begin to grow already at stage VI, then the ovary from stage VI passes into stage III and is designated VI-III.
When assessing the degree of maturity of fish gonads, the maturity coefficient and index are used.
Maturity coefficient is the ratio of the mass of the gonads to the body mass of the fish (in%). In fish with spring-summer spawning, the maturity coefficient is highest in the spring, decreases in the summer, and begins to increase again in the fall (carp, pike perch, roach, etc.). Fish with autumn-winter spawning have the highest maturity coefficient in autumn (salmon). Maturity index is the percentage ratio of the gonad maturity coefficient, calculated in individual periods of gonad maturation, to the maximum maturity coefficient.
The structure of reproductive products. Fish eggs differ in shape, size, color, the presence of fat drops, and the structure of the shell. In fish, eggs are usually spherical in shape, but other shapes are also found. Representatives of garfish have spherical eggs with thread-like outgrowths; in gobies, pear-shaped eggs at the lower end are equipped with a rosette of threads for attachment to the substrate; in anchovies - ellipsoidal eggs, etc.
The size of the eggs, like other morphological characteristics, is a stable characteristic of the species. Large fish lay eggs of larger diameter. The size of the eggs depends on the nutrient content (yolk) in them and varies significantly. Among bony fish, the smallest eggs are found in flounder, the largest - in salmon (chum salmon). Cartilaginous fish have the largest eggs, so in heterotoothed sharks 1.5 m long, the length of the egg capsule is about 10 cm. The development of embryos in some of them lasts a very long time - 18-22 months (katran).
The color of the eggs is specific to each species. Eggs that develop in less favorable oxygen conditions are usually more intensely colored. The caviar of vendace is yellow, that of salmon is orange, that of pike is dark gray, that of carp is greenish, and that of greenlings is emerald green, blue, pink and purple. The yellowish and reddish color of caviar is due to the presence of respiratory pigments (carotenoids). Pelagic eggs, which develop with sufficient oxygen, are poorly pigmented.
The eggs of many fish contain one or more fat droplets, which provide buoyancy to the eggs.
The eggs are covered on the outside with shells:
1. The primary - vitelline (radiate) membrane, formed by the egg itself, is penetrated by numerous pores through which nutrients enter the egg during its development in the ovary. In some species this shell is two-layered (sturgeon).
2. Secondary - gelatinous, sticky (develops above the primary shell), with various outgrowths for attaching eggs to the substrate.
At the animal pole of both membranes there is a special channel - the micropyle, through which the sperm penetrates into the egg during fertilization. Teleosts have one canal; sturgeons may have several.
3. Tertiary - horny (in cartilaginous fishes and hagfishes) and protein (only in cartilaginous fishes).
In lampreys, like bony fish, the eggs are small; in hagfish, they are elliptical in shape, 2-3 cm long. The hagfish's cornea has hook-shaped processes, with the help of which the eggs are attached to each other and to underwater objects. The cornea of cartilaginous fish is much larger than the egg itself; often horny filaments extend from it, with the help of which the egg is attached to aquatic plants.
Sperm differ significantly among different fish species. The sperm has a head, middle part and tail. The shape of the head is different: spherical (in most bony fishes), rod-shaped (in sturgeons and some teleosts), spear-shaped (in lungfishes), cylindrical (in sharks, lobe fins). The head houses the core. An acrosome is located in front of the nucleus in sharks, sturgeons and some other fish. Teleosts do not have it. The sperm secreted by the male consists of sperm immersed in sperm fluid, similar in composition to saline. In sperm fluid, sperm are immobile. When they come into contact with water, their activity increases sharply. Having met the eggs, they penetrate into them through the micropyle, after which fertilization occurs. The duration of sperm activity depends on the salinity and temperature of the water. In salt water it is much longer - up to several days (Pacific herring), in fresh water - no more than 1-3 minutes (for most fish - carp, salmon, perch).
In one and the same male, sperm are not qualitatively the same and differ in size; upon centrifugation, they are distinguished: small (light), medium (intermediate) and large (heavy). Among large spermatozoa, X-gametes are found in large numbers, and among small spermatozoa, Y-gametes are found. Thus, from eggs fertilized by large spermatozoa, predominantly females are born, and from small spermatozoa, males are born. This is important when artificially breeding valuable fish species.
Laying eggs. The beginning of fish spawning depends on various factors (readiness of reproductive products, temperature and salinity of water, presence of spawning substrate, etc.). Among the abiotic factors, water temperature is important. Each species is characterized by optimal and extreme water temperatures during the breeding season. At negative temperatures, cod, saffron cod, Arctic and Antarctic fish breed. The minimum water temperature at which spawning is possible is -2.3°C for navaga, +3.6°C for cod, +4.5°C for Atlantic herring, and 13°C for carp. For many carp fish, the most intense spawning is observed at temperatures of +18-20° C and above.
Fish lay eggs in different conditions, some species spawn in the tidal zone (lumpfish), others in the oceanic pelagic zone at depths of more than 1000 m (eel). The vast majority of marine fish spawn in relatively warm areas of the coastal zone at depths less than 500 m, where there is a high concentration of food organisms and the larvae are provided with food. If there are no conditions for reproduction and spawning does not occur or the eggs are not completely spawned, they are absorbed.
Caviar is distinguished:
Pelagic (floating);
Bottom (demersal), deposited on the ground and bottom vegetation.
Depending on the place of spawning, fish are divided into the following groups:
1) lithophiles - lay eggs on rocky and pebble soil (sturgeon, salmon, kutum, shemaya, chub, podust);
2) phytophiles - lay eggs on plants and algae (roach, bream, carp, crucian carp, perch, Pacific herring);
3) psammophiles - lay eggs on the sand (minnow);
4) pelagophiles - lay floating eggs in the water column (sprat, anchovy, Atlantic cod, sabrefish, grass carp, silver carp);
5) ostracophiles - lay eggs in the shells of bivalve mollusks (gorchaki).
Caring for offspring. Most fish do not care about their offspring. However, there are a number of species that build various nests and protect eggs and larvae.
Pacific and Atlantic salmon use their tails to dig out nests up to 2-3 m long and 1.5-2 m wide in the ground, lay eggs in them, fertilize them and cover them with gravel. The male stickleback builds a nest from plant debris in the form of a muff and guards the eggs. The male pike perch clears a place at the bottom for the future laying of eggs, then guards it, cleans it of silt, washing it away with strong movements of the pectoral fins. If the clutch is left without a guard male, then another one continues guarding. Labyrinth fish build a nest of air bubbles, enveloping them with sticky secretions from the mouth. The male lumpfish guards the clutch of eggs laid in the littoral zone and, when dry, pours water on the eggs from his mouth.
Some fish carry fertilized eggs, for example, the female tilapia holds them in her mouth. The most perfect form of caring for offspring can be considered viviparity in fish.
Fertility and reproductive ability of fish. In fish, there are absolute (individual), relative and working fecundity.
Absolute (individual) fertility is the number of eggs laid by a female during one spawning period.
The fertility of fish is an adaptive property of the species and varies significantly. Cartilaginous fish have the lowest fertility. A manta ray gives birth to one calf. In sharks, fertility ranges from 2 to 100 eggs or fry, and only the Arctic shark lays about 500 large eggs 8 cm long (without the cornea). Among bony fishes, the highest fertility is found in fish that lay pelagic eggs (sunfish - up to 300 million eggs, moth - about 60 million, cod - up to 10 million eggs).
Fish that care about their offspring are characterized by lower fertility. Thus, the viviparous eelpout lays from 10 to 400 larvae, the stickleback lays 60-550 eggs.
The quantity and quality of caviar depend on body weight, age, fat content and environmental factors. As the fish grows and its body weight increases, absolute fecundity increases.
Fish are able to regulate fertility depending on changing environmental conditions. Greater fertility is produced in species under conditions of more intense mortality. Changes in absolute fecundity are regulated through changes in food supply. Improved feeding conditions lead to faster growth rates and, consequently, higher fertility of similar-sized fish. In this regard, the fertility of one species in different reservoirs is different, reflects the conditions of existence of fish and is aimed at ensuring a certain amount of replenishment.
Relative fecundity is the number of eggs per 1 g of female body weight.
Working fertility is the number of eggs obtained from one female for fish farming purposes. In peled it accounts for about 70% of absolute (individual) fertility.
In some cases, species absolute and population fecundity is calculated.
The reproductive ability of fish is strongly influenced by their age, since the quality of reproductive products varies throughout life. In most species, the highest quality offspring come from middle-aged fish. Young and very old individuals produce less resilient offspring.
Duration of the incubation period, survival of eggs and larvae. In fish, the duration of the incubation period ranges from several hours (zebrafish) to 22 months (spiny shark). Incubation of eggs requires a certain amount of heat, expressed in degree days. This value changes depending on the water temperature. When the water temperature increases (within the limits characteristic of this species), the development of eggs proceeds faster. In carp fish, caviar develops within 3-6 days, in saffron cod - 3-4 months, in salmon - up to 5-6 months
The population size largely depends on the survival of embryos and the food supply of larvae at the stage of transition to active feeding. These periods account for the highest mortality compared to all other periods of fish life. The main factors determining the survival of embryos and prelarvae are water temperature, salinity, gas regime, wind, and disturbances. The high fertility of some fish cannot indicate their high abundance, since the survival rate of eggs and larvae is very low.
Metamorphosis. In some fish, the development of larvae occurs with metamorphosis (flounder, river eel, sunfish, etc.). Flounder has symmetrical larvae that swim in the upper layers of water with their backs up; over time, they gradually descend into deeper layers of water and lose bilateral symmetry, one eye moves to the other side of the body, and after metamorphosis is completed, the young flounder begins to lead a bottom-dwelling lifestyle . River eel larvae (leptocephali), hatching from eggs in the Sargasso Sea, have a leaf-like shape. Over the course of 2-3 years, they drift with the Gulf Stream, transform into transparent eel-like fish, which enter the rivers of Europe, where they grow, lose transparency and turn into adult eels.
It is well known that fish lay eggs during the breeding season. However, not all fish are hatched from eggs; there are also viviparous ones, but these are in the minority, and moreover, these are mainly inhabitants of great depths - some sea fish, but in our case we are talking about inhabitants of fresh waters *. And they, as a rule, spawn.
* (Some freshwater fish from tropical waters used for breeding in aquariums are also viviparous. - Approx. ed.)
The reproduction of fish has characteristics that depend on the conditions of their existence - on life in water. First of all, in most fish, fertilization occurs outside the body. And not only eggs, but also sperm, remain in the water for some time before fertilization occurs.
Each type of fish has its own characteristics of spawning and the caviar itself. They have different numbers of eggs, their appearance, size, color, and weight. Even among the same species in different places, and therefore under different conditions, differences in this regard are observed.
The number of eggs in fish varies within enormous limits - from several tens of eggs to millions and even hundreds of millions. Fish in general are much more fertile than terrestrial vertebrates. Those fish that lay pelagic eggs spawn the most; They are followed by those who spawn in more reliable places - on plants or various underwater objects. There are even significantly fewer eggs in fish that hide their eggs or protect them.
The amount of caviar also varies from year to year. In some fish it is greater in years rich in food, and less in leaner years. In fish that spawn not all at once, but in portions, most often the largest portions are spawned at a time when the young that emerge from it are best protected from harmful influences and unfavorable conditions.
Most freshwater fish lay eggs in the warm season - usually in the spring. And a number of fish spawn at lower temperatures in autumn and winter (salmon, whitefish, burbot). In general, the timing of egg laying is largely influenced by air and water temperature.
The duration of spawning itself also varies: for some it lasts a month or even less, for others it lasts for the whole summer.
The duration of the spawning process in individual fish also varies. Some fish spawn all their eggs within an hour or several hours, others do this within several days.
Most freshwater fish spawn on aquatic vegetation, branches in the water, on solid objects, and on the bottom. Their eggs are usually heavy and sink in the water. In a number of fish, it swells in water, and a sticky substance is released from it, attaching it to the substrate (carp, pike perch, roach, etc.).
The ability to spawn, as well as to fertilize eggs, occurs in different fish at different ages, and here too there are very significant fluctuations: some fish become mature in the first year of life (for example, smelt), others - only after living about a quarter century (beluga).
With poorer nutrition and lower temperatures, fish mature later.
When the time of reproduction approaches, the fish begin to show special mobility. The so-called resident fish that constantly live in one or another body of water spawn there, while others travel to the spawning site - spawning migrations. These migrations represent one of the protective devices that provides more reliable and safe conditions for eggs and juveniles, that is, for the preservation of the species as a whole.
The timing of spawning migrations, as well as the time of spawning, varies greatly among fish. Even in the same species, two varieties or, as they are called, races are sometimes noted - winter and spring, depending on the timing of their spawning.
Almost always, the movement of fish to spawn occurs just before spawning. But for some salmon, carp and sturgeon, the spawning migration coincides with the wintering migration: the fish appear in the area where they are supposed to spawn and remain here for the winter, and the spawning itself occurs right there, but after the end of wintering.
There is a very significant difference not only in the time of migration to spawning for different fish, but also in its range. Some fish go from the sea to rivers only a few kilometers, others - thousands of kilometers. Again, differences in this respect are observed even in the same species.
Currently, due to the construction of reservoirs and dams, the path of a number of fish to spawning sites is blocked, and therefore, for some species, the very route of their journey has changed.
Not without reason, the writer Prishvin called the journey of fish to spawn “a selfless journey.” Indeed, many fish (salmon, trout) on their way to spawning grounds have to face difficult obstacles - shallow riffles, waterfalls, etc. And in itself, traveling against the flow of water is associated with a very large expenditure of effort, especially with fast currents. The journey of fish becomes even more difficult during flood periods, when they overcome particularly strong currents.
During the spawning migration, fish usually feed much less or even not feed at all. Meanwhile, they have to expend a lot of energy at this time. Therefore, they arrive at the spawning grounds very exhausted. For example, in white fish the amount of fat decreases from 21 to 2 percent. In some fish, the fat on the internal organs completely disappears.
In general, by the time of spawning, significant changes occur in the body of many fish. Thus, due to the cessation or weakening of nutrition, the intestinal walls become thin. The action of enzymes secreted by the glands also changes. In migratory fish, blood pressure in the vessels changes, which is associated with changes in the salinity of the water. The content of vitamins in their body also changes: instead of some vitamins, others are produced.
In some fish, the appearance also changes noticeably, for example, in eels, the eyes are greatly enlarged (3-4 times), the head becomes sharper.
Many fish develop a so-called nuptial plumage, which manifests itself in changes in color, body proportions, the appearance of various formations on the body and head, etc. As a rule, during the spawning period, males (salmon, many cyprinids) look the most “decorated”.
In freshwater fish, spawning migrations are of a different nature than in anadromous ones: they migrate within fresh waters - from lakes to rivers or to flooded meadows. Freshwater fish usually do not stop feeding during migration for spawning, although it becomes less intense.
Spawning takes place differently: for some fish it is quiet, imperceptible (pike perch), for others it is vigorous, accompanied by splashes, water movement, noise (carp, catfish).
Some fish are monogamous, others are polygamous, that is, in some fish one female and only one male takes part in spawning, in others - several males. In catfish, salmon and some other fish, males fight over females.
There are fish that, before laying eggs, make nests and then guard the eggs and young; Guard is carried out either by a male together with a female, or by only one male (pike perch, stickleback).
Fish often arrive at the spawning site in a certain order: the larger females appear here first.
There are other differences between the sexes, mainly in the structure and size of the fins, which are larger in males, and sometimes there are swellings between them. The larger size of the fins in males contributes to the fertilization of the eggs (the turbulence of the water near them, due to which sperm more easily reach the eggs).
To improve the conditions for spawning and development of juvenile semi-anadromous and river fish, reclamation * measures are carried out on natural spawning grounds.
* (Reclamation means improvement.)
The regime of spawning grounds in different basins differs in its characteristics, and therefore the nature of reclamation measures is also different. For example, in the deltas of the Volga, Ural, and Danube, semi-anadromous fish spawn on ilmen and hollows. These places are constantly clogged with sediment that comes with flood waters. In low-water years, fish cannot spawn here.
In the delta of the Kuban and a number of other rivers, fresh and slightly salty estuaries serve as spawning and feeding grounds for commercial juveniles (pike perch, ram, carp, bream). They warm up well, are rich in food organisms, and are bordered by vegetation. Therefore, estuaries are excellent spawning grounds for semi-anadromous fish. But often the estuaries become silted and overgrown (the river fills the channels with silt); they are poorly connected to the sea, since the mouths are clogged with drifts; they have insufficient depths, weak flow, and high salinity. There are a lot of weeds and predatory fish in the estuaries.
Due to the systematic deterioration of spawning conditions for semi-anadromous and river fish, it is necessary to carry out reclamation of natural spawning grounds for valuable commercial fish.
It is necessary to clear and deepen the channels and eriks on the ilmen and hollows in the deltas of the Volga, Ural, and Danube. In order to combat waterlogging in water bodies, it is necessary to systematically mow down tough vegetation and clear riffles in channels. To regulate the water regime of the fields during the flood period, sections of the delta and channels are embanked, and a system of sluices is installed.
Reclamation should aim to desalinate the estuaries, protect them from being overgrown with tough vegetation, and ensure a normal connection between the estuaries and the sea. The lack of connection with the river leads to salinization of the estuary spawning grounds, and the loss of connection with the sea leads to complete drying out, which is often accompanied by salt precipitation.
To improve conditions in the spawning estuaries, it is also necessary to build powerful desalination systems consisting of a whole network of canals.
The mating plumage serves to recognize the sexes, stimulate the ripening and release of reproductive products. As a rule, fish lay eggs in the evening or early in the morning. The duration of spawning varies from 30 minutes to several hours and even days with rest intervals - brachydanios, silversides, princess of Burundi, etc. An increase in productivity can be achieved by keeping the spawners separate and properly feeding them for 10-15 days before spawning. Key stimuli stimulating may be individuals of the other sex, composition (pH, dH, EH), movement, substitution, water level, temperature changes, duration and intensity of lighting (lunar phases in Indian cyprinid fish, catfish), substrate (soil, vegetation , and with a certain leaf configuration and bushiness, etc.), the composition of microorganisms, the presence of satellite fish, water-soluble, species-specific metabolites responsible for (copulin), etc. Thus, exposing aquariums to direct sunlight stimulates the coloration, growth and reproduction of young females. The cyclicity of spawning also depends on the length of daylight hours. Adding calf thymus gland to the feed leads to an increase in the number and size of offspring, feeding the fry the thyroid gland accelerates metamorphosis, but slows down growth.
Before breeding fish, you should check the food supply for future offspring. The best food for juveniles is “live dust”, so all breedings are planned for the period from April to September (in the absence of cultivated organisms). The monogamous (pair family) includes four-toothed fish: cichlids, rheas, snakeheads, etc., the polygamous (gregarious family) includes iris, gobies, eleotraces, sticklebacks, etc. School spawning (in cyprinids, characids, etc.) is not always justified, especially if the fish destroy their eggs. In such cases, the amount of substrate should be increased, and spawning grates should be laid on the bottom. Fish with high intraspecific aggressiveness (distichods, leporines, tropheus, etc.) should be gradually accustomed to each other or the spawning area and the number of shelters should be increased. The size of breeding aquariums can vary greatly. For nanostomes, 200 ml is enough, and for a pair of discus fish, at least 100 liters. Fish that take care of their offspring (loricariid catfish, anappass, cichlids, etc.) are able to raise young animals in a general aquarium. But usually the development of eggs occurs either in expansion tanks, or in special incubators, modifications of the Weiss apparatus, etc. For disinfection, add methylene blue, rivanol (1-2 mg/l), malachite green, violet K, bright green oxalate (0.5-2 mg/l) to the water.
During the development of eggs, as a rule, 6 phases are distinguished:
1. Dead, unfertilized eggs without signs of crushing (quick removal after spawning).
2. Not yet developed, but fertilized (in opaque eggs, division occurs at the sharp end).
3. Germinal disc.
4. Transparent, young embryo.
5. Pigmented embryo.
6. Embryo with pigmented eyes.
Overripening of eggs does not reduce their ability to fertilize, but greatly increases the percentage of waste during development. The moment of pigmentation of the eyes of the embryo (ocelli stage) signals that the critical periods of development are left behind. In this state, eggs can be subjected to transportation and other manipulations without significant harm to the embryo (minor death is also observed in the early stages, immediately after spawning). The formation of eggs and embryos is accelerated by temperature, increased content of oxygen, iron (up to 1 mg/l), vitamin B 2 in water, and sometimes increased salinity (0.5 - 3%o). The hatching process is regulated by a special enzyme - chorionase. Mass hatching can be stimulated by adding water from the aquarium where the fry have just hatched or freshly shed egg shells to the eggs. The sum of temperatures multiplied by the number of hours or days of incubation equals a roughly constant value called degree days or degree hours. For trout, development lasts 205 days (410 degree days) at 2°C, 82 days (410) at 5°C, and 41 days (410 degree days) at 10°C. After hatching of the embryos, the dose of the drug is gradually reduced to zero (by changing the water, filtration with activated carbon), and the water mineralization (for soft-water fish) is slowly increased. A free embryo has a yolk sac with a supply of energy substances that nourish it in the first hours or days of life. At this time, it leads a passive existence, lying on the ground, suspended from plants, snags and stones using a cement organ or adhesive threads. In polygons, the attachment organ arises due to a protrusion of the intestinal wall; in African and American bipulmonates, it is a transverse groove in the pharynx; in most bony fish, it is formed on the upper side of the snout or in the brain part of the head. The opercula and pseudobranchia of the larvae supply oxygen to the brain and eyes, and the capillary networks of blood vessels, the yolk bladder, pectoral, dorsal, anal fins and external gills supply the rest of the body. Resorption of the sac is a signal for the immediate supply of food to the ear of the actively moving larval embryo. Petrochromes, cyphotilapias, etc. To stimulate the resorption of the sac and proper differentiation of the gastrointestinal tract, microdoses of plankton are given 3-5 days earlier than the expected date. The larva becomes a fry only by acquiring the scaly cover and structure of an adult fish.
Well-grown, even in size (undergrown and overgrown ones are discarded), healthy juveniles with desirable characteristics (brightness and purity of color, proportions of the body and fins, mobility, etc.) are selected for the tribe, from which spawning pairs or groups are subsequently formed. This is the so-called mass selection. With targeted work and keeping diaries, you can also take into account internal, no less important features: growth rate, disease resistance, speed of maturation, etc. Here, individual selection comes into force, which consists of assessing each specific sire by the quality of the offspring or comparing quality indicators several families (family selection). A skillful combination of both selection methods will ensure the best final results.
Free crossing in nature of all individuals (usually at the subspecies level) is called panmixia. Numerous experiments on sexual selection have shown that the female chooses the most brightly colored male during copulation. Thus, the golden principle when finalizing pairs will be - best to best.
Without knowledge of the basics of genetics, today it is impossible to competently deal with issues of reproduction of any animals, and in particular fish. Fish selection is based mainly on selection for quantitative traits, determined by changes in both internal (genotype) and external (phenotype) factors. The hereditary inclinations (genes) of germ cells (gametes) are closely related to the behavior of fibrous, spirally twisted structures - chromosomes, enclosed in cell nuclei. Genes are sections of a continuous molecular chain of DNA (deoxyribonucleic acid). The commands transmitted by chromosomal DNA using transfer RNA to numerous ribosome protein synthesizers scattered throughout the cell are based on a genetic code that determines the order and ratio of amino acids (there are 20 of them) in newly formed protein molecules. Transformations of any part of the DNA molecule lead to changes in the protein composition of the organism and ensure the adaptation of the species through natural selection to new living conditions. Each species usually has a constant number of pairs of chromosomes from 16 to 240. During the development of a new organism, any pair of its homologous (externally similar) chromosomes (diploid set) includes a chromosome (haploid set) from each parent. The chromosomes of each pair are thus different from the chromosomes of all other pairs. With two pairs of chromosomes, as a result of maturation divisions, four types of gametes are formed (in females and males). Adding one pair of chromosomes doubles the number of chromosome combinations each time. As a result, the number of gamete types is equal to In, and the number of different zygotes (after fertilization) is equal to 4n, where n is the number of pairs of chromosomes. With 24 pairs of chromosomes, a quantity characteristic of many fish species, the number of individual gametes will exceed 16 million, and zygotes (in one cross) - 250 trillion.
The problem of closely related breeding (inbreeding) and its extreme manifestation - inbreeding (mating of producers with their offspring) is that the offspring inherit the same properties from their parents (harmful and beneficial), which become stronger with further breeding (homogeneity). Genetic material obtained from non-identical parents ensures diversity (heterogeneity) of future generations, i.e. the effect of heterosis - hybrid vigor - will constantly manifest itself. As can be seen from previous mathematical calculations, for fish the danger of inbreeding is greatly exaggerated. However, outbreeding (periodic, once every 3-5 years, crossing with unrelated producers) and optimal conditions of keeping and breeding can completely eliminate it.
Dominant (strong) traits always prevail over recessive (subordinate) ones. When crossing in the first generation, all individuals will have a dominant type. Only with further breeding do we get 25% purebred dominants - 50% crossbreds, but with a dominant trait, and 25% purebred recessive individuals, which visually looks like a 3:1 split. Selection for two traits gives a distribution of 9:3:3:1.
The culling of individuals that deviate from the desired breeding type must be strict. Fry that have a worse exterior compared to their parents are removed from the aquarium. A hybrid is the end product of crossing at least different species, while a crossbreed is the result of a fusion of breeds. In practice, the following forms of crossing are used:
1. Industrial (mass) - ensures an increase in diversity, but does not consolidate the breed.
2. Synthetic crossing, which allows you to combine the desired characteristics of the original breeds and at the same time increases heterogeneity.
3. Introductory crossing - helps to improve the breed by strengthening it with new improving genetic material.
4. Absorption - after the initial crossing of two breeds, the crossbreeds are combined with individuals of the improver breed.
5. Alternative crossing - alternating mating of crossbreeds after the first crossing with individuals of 1, two original breeds, in the fourth or fifth generations is replaced by reproductive crossing, stabilizing the necessary characteristics as a result of breeding in itself.
An additional and very significant source of variability in fish are mutations - changes in chromosomes and genes. Mutant genes that cause steel coloration and albinism reduce the viability of pure lines. By exposure to hard radiation and chemical compounds (nitrosmethylurea - 0.97 - 9.7 mmol (millimoles), dimethyl sulfate - 0.11-0.13 mmol, etc.), artificial mutations can be caused. They are classified into point (gene) rearrangements, chromosomal rearrangements (inversions, translocations, etc.) and polyploidy (the presence of one or more additional gene sets). Poeciliops (P. turrubarensis), for example, has a triploid set of chromosomes. Cold and exposure to cytochalasin lead to polyploidy larvae.
When determining the sex of fish, it turned out that guppies, pecilia sphenops, medaka, etc. belong to the XX-XY type (male heterogamety). A Xiphophorus maculatus have both male (XY) and female heterogamety (WY) and even three types of sex chromosomes (WY, WX, XX).
Different lines of Mozambican tilapia have heterogametic females and males. The crossing of these lines led to the formation of only males in the offspring. In green swordtails (X. helleri) and black macropods (M. orercularis concolor), sex differentiation depends on male and female hereditary factors located in autosomes (there are no sex chromosomes).
It has long been noticed that female fish, under the influence of certain conditions (environment, hormonal drugs, etc.) turn into males (gambusiaceae, melanochromis, etc.). Female sex hormone (estrol) and male sex hormone (methylgestosterone) when added to water or food redefine gender. Japanese geneticist Yamamoto turned female goldfish into males. Upon further breeding, all offspring turned out to be female. In guppies, females with male fins are sterile.
Aquarists have bred about 200 breeds of aquarium fish (angelfish, goldfish, cockerels, viviparous fish, barbs, etc.). Very often, very spectacular offspring are obtained from the hybridization of egg-marking cyprinids, cichlids, gambusias, cyprinids, etc. As a result of “reprehensible” inbreeding (constant crossing of littermate fry obtained from a single pair of sires), in the 4th-6th generations many albino ones appear (labeo, swordtails, minor, neon, pristella, Aripiranga nannostom, platies, Gambian barbs, oligolepis and sumatranus, speckled catfish, plecostome, guppies, pseudo-tropheus zebra, etc.), veil forms (rasbora heteromorph, hassemania, ternezia, cardinal, zebrafish, fire barb, angelfish) and chromium forms (cichlazomas, barbus- "mutant", parrot cichlid, etroplus, tilapia aurea, labeotropheus, etc.).
The growth rate of fish can be significantly accelerated using intramuscular injections (mg/kg): bovine growth hormone or bovine insulin (10), 4-chlorotestosterone acetate (0.5), thyroidin (10), testosterone propionate (10), methidandrostenediol (4 mg/kg). kg every 4 days); or when added to feed (mg/kg): ethyl estrol (2.5), 17-methyl-testosterone (1-2), dimethazine (5), 17-ethynyltestosterone (2.5-3.5), methylandrosterone (15), androstedione (500), testosterone propionate (560), androsterone (580), dehydroepiandrosterone (3200), testosterone (10), oxymetholone (10), 11-ketotestosterone (10), stanazol (833), thyroidin (60) , 1-dehydrosterone acetate (15), dry thyroid (6), krezacin (10-20) and triiodothyronine (20 mg/kg). You can get 100% males by adding methyltestosterone and ethinyltestosterone to the fish feed (from 50 to 100 mg/kg), and 100% females can be obtained by treating the fry with ethinyl estradiol (50 mg/kg) and estradiol (20 mg/kg). Using supersolvents (dimethyl sulfoxide), you can administer any hormones, adaptogen drugs, vitamins, etc. to fish. without injections, direct transport of substances through the integument.
Most fish do not care about eggs, laying them in excess so that at least some of the offspring can survive. But in a limited aquarium, all eggs can become food for other inhabitants. Some species give birth to live fry, which also become a delicacy. But there are also many who actively care for both caviar and fry. These include, for example, cichlids. However, their strict adherence to their territory often results in tragedy for other fish. Courtship behavior of some species can also result in injury. That's why, if you seriously decide to start fish breeding, don't let this process take its course. And for a breeding pair, as a rule, a separate aquarium is needed.
Depending on the type of reproduction, fish are divided into egg-laying, ovoviviparous, and viviparous.
Spawn-marking- the main group of fish that spawn eggs into the water column, where fertilization occurs.
Ovoviviparous– fertilization is internal, the embryo develops in the female’s body in special extensions of the oviducts, but is nourished by the nutrients of the yolk sac, and the mother’s body serves only as protection from external factors.
Viviparous– in these fish, the union of the egg and sperm occurs in the female’s genital tract, the formation of the placenta occurs, which ensures the connection of the mother’s body with the embryo and supplies nutrients.
Viviparity is a rare phenomenon, typical for aquarium fish (guppies, swordtails), and sharks. There is no larval stage; an embryo develops in the oviducts of the female, and an already formed fry is born, which is capable of independent existence.
Features of fish reproduction
Fish are dioecious animals. Females produce eggs - eggs that develop in the ovaries and have a thin, translucent membrane for quick and easy fertilization. Moving along the oviducts, they exit through the external opening located near the anus.
Males form sperm in paired testes - milk, which is a system of tubules that flow into the excretory duct. There is an expanded part in the vas deferens - this is seminal vesicle. The laying of eggs and the release of seminal fluid occurs almost simultaneously.
Exception - rock perch, has gonads of two sexes, but they do not mature at the same time, which prevents self-fertilization.
Fish only reproduce sexually, by the fusion of male and female reproductive cells.
The process of laying eggs by females and fertilizing them with sperm from males is called spawning. During the spawning period, fish look for favorable conditions for the development of offspring, so they often leave their usual habitats. Some move from the seas to the mouths of the rivers flowing into them, while others, on the contrary, rush to the seas.
If, due to unfavorable conditions, the fish were unable to spawn, they are characterized by resorption of eggs and milt (gradual resorption of the reproductive material).
Fertilization in most cases is external; the larva develops outside the female’s body (live birth is rare).
Fish lay eggs in huge quantities (from 100 thousand to millions of eggs). Such fertility ensures the preservation of the species, because not all eggs will be fertilized, and some will die altogether.
When the eggs are spawned, sperm can enter the egg through a special hole - micropyle. After the fusion of germ cells, the egg membrane becomes more permeable (adsorbs water) and stronger.
After fertilization is complete, eggs form zygote, in which multiple divisions occur with the formation of a multicellular embryo. In the abdominal region, the remains of the yolk sac are preserved, which provides nutrition to the larva in the first days.
Larval stage begins with the rupture of the egg shells, when the formed individual comes out and begins to feed on its own (single-celled organisms, crustaceans, algae). The body shape is elongated, large eyes, and no fins.
In the first days, the larva hangs motionless, attached to some substrate; after depleting its supply of nutrients, it begins to actively move in search of food. During this period, scales begin to form. Small fish have functioning temporary organs that are needed for survival in a new environment:
- Fin fold;
- additional external gills;
- blood vessels.
This stage is also called critical; if the larvae cannot find food, their mass death will occur.
For fry stage Characteristic is the reduction of temporary organs and the formation of a structure similar to that of adult individuals. From this stage, the fish looks like all representatives of the species, only smaller in size. The body is completely covered with scales, fins of all types are formed.
Adult fish has fully formed systems and organs, is covered with mucus and scales, has glands and sensory organs. Having reached sexual maturity, they soon begin to reproduce.
What kind of development do fish have: direct or indirect?
Indirect development occurs in larvae, which, when emerging from the egg, do not resemble the adult. Such organisms develop gradually, acquiring the characteristics of their parents through a series of successive stages, differing in their mode of nutrition and lifestyle.
After the eggs ripen, a larva emerges from it, with undeveloped fins and scales, and in appearance it is not similar to the adult. Therefore, such fish belong to organisms with an indirect type of development (mainly bony fish).
When babies are born that are similar to adult organisms, only smaller in size and with incompletely formed organs, such development is called direct. Thus, fish that are characterized by viviparity (for example, sharks) develop in a direct way.
Caring for offspring
The spawning of eggs in large quantities is due to the fact that fish do not tend to care about their offspring. The eggs left behind die from enemies, drying out, and unfavorable conditions; only a relatively small part can survive to the stage of a mature individual.
Some fish that care for their young choose spawning sites in crevices, build nests for protection, or carry eggs in their mouths. Thus, a female salmon uses her caudal fin to clear a place for laying eggs, making a depression on the sandy bottom, then covers the eggs with sand (protecting them from predators and freezing).
Parents provide constant access of oxygen to their offspring and use their fins to aerate the water. To prevent the eggs from drying out, the male waters them with water from his mouth. Manifestations of care in fish are at the instinctive level, when the larvae are able to obtain food on their own, can swim well, and their parents leave them.
- Read more: Fish reproduction: procreation
Features of fish reproduction.
The main method of reproduction of fish is sexual. The vast majority of fish species reproduce sexually. At the same time, parthenogenesis and gynogenesis occur in fish, although rarely.
Parthenogenesis is a method of reproduction when the development of eggs occurs without fertilization by males. At the same time, the development of eggs usually reaches only the crushing stage, as for example in herring, sturgeon, salmon and carp. It has been noticed that unfertilized salmon eggs, once in the spawning mound along with fertilized eggs, often develop parthenogenetically. Thanks to this, they do not immediately rot, which is why the entire clutch of eggs does not die, and fry hatch from the fertilized eggs. In burbot and herring, the development of eggs can, in exceptional cases, reach the larval stage, but the latter survive only until the yolk sac is reabsorbed. Thus, in most cases, parthenogenetic development does not lead to the production of viable juvenile fish. The exception is the Issyk-Kul chebak, which produces normal offspring with parthenogenetic development of eggs.
Gynogenesis is the birth of only females from eggs unfertilized by males of a given species. At the same time, the development of eggs is stimulated by sperm from males of related fish species, which are able to penetrate the eggs, but their actual fertilization does not occur. As a result of the gynogenesis path of reproduction, only females are observed in the offspring. Thus, in the reservoirs of Central Asia, Western Siberia and Europe, populations of silver crucian carp are often found in which there are practically no males. Here, the development of crucian carp eggs is stimulated by spermatozoa of carp, roach and some other related species. In the reservoirs of Mexico, there are populations of mollies belonging to the order Cyprinodontiformes, also consisting almost entirely of females.
Most fish species are usually unisexual, although true hermaphrodites are also found among them. The hermaphrodite species among bony fishes includes the rock perch, in which both eggs and sperm develop in the reproductive gonads. However, the maturation of reproductive products usually occurs alternately, due to which self-fertilization of eggs does not occur. In the red pagella, sex reversal occurs during life, i.e. gender reversal. If in young individuals of red pagella the gonads function as ovaries, then in older individuals they function as testes. In addition, hermaphroditism occasionally occurs in some representatives of herring, salmon, carp, and perch fish.
As for the methods of fertilization of eggs in fish, there is external and internal fertilization of eggs. External fertilization occurs in most fish species and occurs in the external aquatic environment. Internal fertilization is more common in cartilaginous fish, although it is also typical for some bony fish, in particular sea bass and eelpout; many carp-toothed fish (gambusia, guppies, swordtails, etc.).
Based on the location of egg development in fish, the following three groups can be conventionally distinguished: oviparous, ovoviviparous and viviparous. Most fish species are oviparous - they lay eggs (eggs) directly into the external aquatic environment. In fish from the ovoviviparous group, fertilized eggs are retained in the posterior sections of the oviducts and develop there until the young are hatched. Those. Almost at the moment the eggs are born, they rupture and immediately the fry are born. Most cartilaginous fish, such as the katran, white shark, fox shark, sawfish and others, are ovoviviparous. In species such as the stingray, the walls of the posterior sections of the oviducts, which act as a “uterus,” even have special outgrowths through which nutrient fluid enters the oral cavity of the embryos through special squirters.
In viviparous fish, a structure is formed in the posterior sections of the oviducts (“uterus”), somewhat similar to the placenta of mammals, thanks to which the embryo receives nutrients along with the mother’s blood. Viviparity is typical for many species of sharks, especially deep-sea ones, and in particular for blue, mustelids, cats and other sharks.
The adaptive significance of viviparity and ovoviviparity in fish lies in the fact that during intrauterine development, greater survival of the juveniles is ensured.
Depending on the nature (multiplicity) of fish reproduction, they are divided into monocyclic and polycyclic. Monocyclic fish reproduce only once in their lives and after a single spawn they all die. Monocyclic fish include river eel, Pacific salmon, river lamprey, Baikal golomyanka and some others. Most fish species are polycyclic, i.e. They reproduce many times during their lives, and the number of reproduction cycles depends mainly on their life expectancy.
The age of puberty in different fish species can vary widely - from 1 - 2 months in small species, such as mosquito fish, and up to 15 - 30 years in sturgeon. Typically, fish with a short life cycle mature faster: at the age of about 1 year, sprat, smelt and some gobies mature. Fish with a long life cycle become sexually mature much later. Thus, Atlantic cod matures at 7–10 years, sea bass at 12–15 years, etc.
At the same time, the age of sexual maturation of fish depends not only on the species, but also on the living conditions of the fish, and primarily on the feeding conditions. Therefore, the onset of sexual maturity in fish usually occurs when they reach a certain length. It follows that the better a fish is fed, the faster it grows, and therefore the faster it matures. Males in most fish species usually mature earlier than females.
Climatic conditions also have an important influence on the rate of ripening. For example, in the Aral Sea, bream reaches sexual maturity at the age of 3–4 years, in the Northern Caspian Sea at 3–6 years, while in the Middle Volga at 6–7 years, and in Lake Ladoga only at 8–9 years.
To determine the size of fish withdrawal (catch) from a particular reservoir and assess raw material resources, the age of puberty is important. Therefore, for short-lived fish that mature in the 2nd–3rd year of life, the permissible commercial removal from the population can be 40–60% (sprat, sprats, sardines, etc.), while for long-lived fish the annual removal should not exceed 5–20%.
