Reproduction is an interesting phenomenon in the life cycle of organisms, which ensures not only their existence, but also their evolution. The life of fish surprises with its diversity and adaptability to a wide variety of conditions. Reproduction can occur at different ages and in different ways. Some fish lay eggs with or without subsequent fertilization, others give birth to small fish, and some even lay eggs in mollusks. Let's learn more about fish reproduction.
What is parthenogenesis
Parthenogenesis in fish is a method of sexual reproduction in which the female eggs do not need fertilization by the male.
This method is considered sexual, since the development of the organism comes from the germ cell, although without the fusion of male and female gametes. In fish, it is quite rare and just as rarely leads to the “hatching” of viable young.
Unfertilized eggs, which develop in this way, perfectly coexist with the fertilized ones, do not rot and do not destroy the clutch. It can develop to the larval stage, but when the yolk sacs are reabsorbed, the larvae usually die. However, most often the eggs develop only to the crushing stage.
Parthenogenesis can be found in fish such as herring, sturgeon, carp, and salmon.
Mating fish: briefly about the methods of fish reproduction
Aquarist with many years of experience
Reproduction is one of the most interesting phenomena that exist on planet Earth. Asexual reproduction appeared about three billion years ago, and sexual reproduction more than one billion. Methods of procreation changed over time and took on different forms. Both on land and in the ocean, there are many species of organisms that have their own way of reproducing offspring. For example, fish mate in several ways and also reproduce through parthenogenesis and histogenesis.
These fish reproduce in several ways
What is hermaphroditism
If a fish simultaneously or sequentially has male and female sexual characteristics, as well as reproductive organs, then this is hermaphroditism. In other words, such fish can develop both eggs and sperm. However, they are not capable of self-fertilizing eggs, since the maturation of the reproductive products does not occur simultaneously, but alternately. Some fish can change sex several times during their lives. The ability for hermaphroditism is found in such fish as guppies, sea wrasse, red pagella, herring, cyprinids, salmon, and perch.
Sexual method
This type of reproduction is most common among fish. It requires a female and a male. The eggs can be fertilized in different ways:
- Internally. Insemination occurs inside the fish's body. Found in swordtails, guppies, gambusias and other viviparous species.
- Externally. Fertilization of eggs occurs in water. Found in most fish species.
The development of eggs is also unequal. And depending on it, fish are:
- Viviparous. The female's posterior oviduct contains a structure similar to the mammalian placenta. Thanks to her, the mother can provide the embryos with the necessary substances. The eggs develop from 30 to 50 days. During this period, a dark-colored “pregnancy spot” can be observed near the anal fin. If the breeder notices that the female’s abdomen has become rectangular, then fry can be expected in three days. They are born swimming independently and capable of feeding. Guppies, swordtails, mollies, formosa, etc. are viviparous.
- Ovoviviparous. The fertilized eggs are attached to the oviduct, more precisely its posterior section, and develop there until the fry appear. This happens in most cartilaginous fish - sharks and rays.
- Oviparous. They lay their eggs directly into the water. This method is the most common.
Scientists also identify fish that reproduce monocyclically, that is, once in a lifetime (river eel, river lamprey, Pacific salmon and others) and polycyclically, that is, several times throughout life (this is most fish).
There is also a classification depending on the substrate in which spawning occurs:
- lithophiles with masonry on rocky ground;
- psammophiles with masonry on sand;
- phytophiles with clutches on plants;
- ostracophiles with clutches in the mantle cavity of the mollusk;
- pelagophiles with eggs floating in the water column;
- pelophiles with clutches in the mud.
Some species of fish are indifferent to the substrate, others build special nests from plants, air bubbles, etc.
Imitation of spring conditions and selection of pairs
First, reduce the temperature of the liquid in the tank to 11 degrees. Before you start breeding goldfish, provide a favorable 22 degrees, but not immediately, but adding 1-1.5 degrees per day.
Then choose the future parents. A boy is distinguished from a girl by the following characteristics:
- genital opening - rounded and protruding in shape for the reason that through it she lays eggs, and does not release sperm;
- the stomach is soft to the touch and mobile;
- the fins in the chest area are rounded and they are also shorter;
- the female of this type of fish is larger than the partner.
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“Men” have other distinctive features:
- on the body there are white stars or light tubercles - these small protuberances located on the fin, head or gills indicate readiness for reproduction;
- the boy's genital opening is concave, unlike that of females;
- the stomach is much firmer and more elastic;
- pectoral fins are long and pointed.
Another indicator of gender is the behavior of fish. A girl will never show interest in the opposite sex. Boys always chase after their friends, lazily at first, but then more and more actively.
When breeding goldfish, it is always recommended to separate representatives of the opposite sex from each other for a greater desire to mate. This period may be several weeks.
How to determine whether fish are ready to reproduce
Everything is individual here. Some species, such as gambusia, are ready to give birth 1-2 months after their birth. Others will take 15-30 years to reach sexual maturity. The timing of ripening is affected by:
- life cycle: the shorter it is, the earlier the maturation, and vice versa;
- species affiliation;
- living conditions and climate.
If a fish has a “nuptial outfit”, it means it is ready to give birth to offspring. This may be a bright color, age spots, skin growth, etc.
You can identify female representatives of the carp and characin families that are ready for spawning by looking at their abdomen - when it is filled with eggs, it becomes round and full, even if the fish is hungry.
Here are a couple more interesting facts:
- Males reach sexual maturity earlier than females.
- In young fish, the first few spawnings are unproductive.
How many eggs do fish lay?
The eggs, larvae and fry of ordinary, non-viviparous fish pass their life path in less favorable conditions than the eggs of viviparous fish. A huge number of eggs die before embryos appear in them.
Imagine millions of eggs scooped up by fish in the open sea and left to the elements. Birds, sea animals, and most importantly, fish immediately rush to the defenseless eggs as if they were tasty food. No one has calculated how many eggs remain from the hundreds of thousands that are spawned by fish, but even without calculations it is clear: very few.
In addition, there is no doubt that part of the spawned eggs turns out to be unfertilized and, therefore, cannot develop at all. This is dead caviar.
The larvae that hatch from the eggs are also quite defenseless. They are destroyed in countless numbers by other animals. Fish fry are also exterminated in large numbers.
Salmon, as already mentioned, bury their eggs in the ground, which contributes to their preservation. But even before this operation, a lot of eggs die. When spawning on fast currents, part of it is carried away by the river, the eggs become crumpled and torn, ending up in silty pits. Near the places where salmon lay and bury their eggs, predatory fish lie in wait for the eggs carried from their nests and immediately destroy them. Before the Far Eastern salmon have time to spawn, they are attacked by greedy loaches.
The life of eggs and larvae is also unsafe in the nest itself: they are eaten by insects and their larvae. I have observed whitefish feeding on smelt eggs many times. It is not for nothing that the fishermen of Lake Onega call such whitefish “smelts.”
Frogs are the worst enemy of eggs and fry. It is known that in the hollows of the Volga delta, frogs mainly feed on eggs and juvenile fish. In July, up to 70 percent of the weight of the stomach contents of frogs consists of fry.
Not only predatory fish feed on caviar, but also the most peaceful fish - roach, bleak, etc.
There are other reasons leading to the death of eggs and fry. Let's name some of them. Autumn-spawning fish often lay their eggs in shallow places. In winter, such places sometimes freeze and the eggs die. This phenomenon is common in salmon rivers of the Far East. True, caviar has great endurance: if freezing is short-lived, the caviar is preserved. (The eggs of vendace and some other fish are also able to withstand freezing.) However, the spawning grounds of Amur chum salmon sometimes freeze to a depth of 1 meter, in which case both the eggs and the larvae that emerge from them die.
No less disastrous for eggs is the drying out of spawning grounds, which is often observed in rivers, the level of which can change sharply due to the opening of dams or the rapid decline of spring floods. In rivers with low banks this happens every year.
Fish are also plagued by various diseases, which are especially dangerous for eggs and fry. On the eggs and on the fish themselves, it is not so rare to see, in greater or lesser quantities, whitish mold in the form of threads and flakes of Saprolegnia, which causes rotting of the eggs and the appearance of wounds on the body of the fish. The disease often ends in death. We have already seen this in the example of Far Eastern salmon.
There are plants that kill tiny fish - larvae that have just emerged from eggs. As soon as the larva touches the leaves of aldrovardia, it becomes captive: the leaves of the plant fold and capture it. Fish larvae serve as food for bladderwrack. The plant has small chambers (vesicles) placed on the stems. The chambers are open, but as soon as the fish enters such a trap chamber, the lid closes and the larva dies.
Among worms, crustaceans and mollusks there are also many that cause harm to eggs and larvae.
So, the survival rate of eggs, fry and young fish (up to the stage of sexual maturity) is influenced by many different factors, and since these factors are very variable, there are no specific, more or less constant, survival rates. Indirectly, mainly based on observations in ponds and aquariums, it has been found that of the hundreds of thousands of eggs laid and fertilized, a negligible number survive to adulthood.
Here's some data. Of the 600,000 eggs laid by a female carp, 97 percent do not reach the age of three, and 93 percent die in the first 7-10 days. In bream, with an average fecundity of 103,000 eggs, from 16 to 45 individuals survive to sexual maturity, and only two females can come to spawning, since the others are used by fisheries.
It is known that with age, the ability to spawn in fish weakens and then completely disappears. In bream of the Volga-Caspian region, the ability to reproduce begins to fade from the age of six, and after 12 years all bream become barren. Among Karelian breams there are specimens that, even at the age of more than 20 years, have not lost their ability to reproduce.
Fish barrenness can occur not only depending on age, but also on other reasons. A sterlet raised in a fish hatchery and reaching an age of more than 30 years has never shown the ability to reproduce.
Fish farmers associate this with sterlet obesity. Age-related barrenness and obesity (“fat”) barrenness have been found in many fish (sturgeon, whitefish, cyprinids). In a fish farm, the barrenness of fish producers is a negative fact, but the “fat” barrenness can also have a certain positive meaning when growing a high-quality food product.
This is due to the survival strategy of the species and its living conditions.
Caring parents
No matter how strange it may sound, some fish show reverent care towards their offspring. It is most pronounced in representatives of such families as labyrinthidae, loricariaceae and cichlids.
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South American cichlids form pairs that are able to stay together throughout the life of the fish. During spawning, couples select a substrate suitable for the nest, clean it and spawn eggs, which are closely monitored, periodically fanning them with fins and removing dead eggs. During the breeding season, the character of the fish changes dramatically - they become aggressive and often do not allow other inhabitants of the aquarium to live a quiet life, jealously guarding their clutch and newborn offspring.
Discus fish stand out from the family not only by their appearance and character, but also by their reproductive characteristics. Like other cichlids, they guard the clutch, but when the young emerge from the eggs, they stay close to their parents. During this period, adult fish secrete a special nutritious secretion, which young fish feed on in the first days of life.
African cichlids have an equally interesting way of caring for eggs. Many of them live in harems, where there are several females per male. The spawning itself takes place according to the usual pattern for fish - the female lays eggs on the substrate, and the male fertilizes them. Only then, instead of guarding the nest, the caring mother collects the eggs in a special crop and incubates them for several weeks, after which she releases several dozen completely independent fry.
Another interesting representatives of cichlids are lamprologus fish, whose life cycle takes place next to the empty shells of gastropods. They lay eggs in them and keep their young.
Representatives of labyrinths are characterized by good, responsible fathers. They build nests from foam and plant particles, after which they invite the female to evaluate the home. If the lady is satisfied with the “cradle”, then spawning begins - the male hugs her with his body and squeezes out the eggs, which he then transfers to the nest. After spawning, his attitude towards his girlfriend changes radically: he drives her away and continues to care for the offspring, periodically repairing the nest and picking up fallen eggs and larvae.
Good fathers are also found in loricariid catfish, such as ancistrus. Males of this species occupy oblong caves, clean them, invite females for spawning, and then independently “hatch” the eggs.
Representatives of the ichthyofauna familiar to us can also be surprising. For example, bitterlings inhabiting the Dnieper and Amur basins lay eggs in the mantle of bivalves. As soon as the juveniles hatch, they swim out of their living shelter, carrying the toothless larvae on the surface of their bodies. Pike perch exhibit spawning behavior similar to cichlids - males jealously guard their eggs.
Representatives of marine fish also demonstrate care for their offspring. Male seahorses have a special pouch in which they bear their offspring after spawning.
Video about the development of fish - from eggs to adults:
In most fish, egg fertilization occurs in water. Spermine, entering the water, acquire mobility and, penetrating through the micropyle into the egg, fertilize it. Sperm motility in water lasts for different periods of time in different species. In fish that spawn on the current, where contact between sperm and eggs can be carried out in a very short period of time, sperm moves in the water for a shorter time than in fish that spawn in stagnant water. Thus, in pink salmon and chum salmon, which spawn in fast currents (current speed 0.5-1.0 m sec), sperm motility in water is maintained only for 10-15 sec. In Russian sturgeon and stellate sturgeon, spawning at a slower current (about 0.2 m/sec), sperm motility remains for about 230-290 sec. In the Volga herring - Caspialosa volgensis (Berg) - a minute after placing the sperm in water, only 10% of the sperm remained motile, and after 30 minutes only a few sperm moved. The addition of ovarian fluid to water activates the movement of sperm, which is an adaptation that ensures increased movement when approaching the egg.
In fish species that breed in stagnant water, the possibility of sperm contact with eggs remains much longer than in flowing water, and, accordingly, the viability of sperm is much higher. For example, the sperm of the oceanic herring - C/irea harengus pallasi Val. - retains the ability to fertilize in sea water for more than a day.
In most fish, insemination is external, but in some species, both spawning and those in which eggs develop inside the mother’s body, internal fertilization takes place, and therefore they often develop special copulatory organs. In their simplest form, the organs of copulation are presented in the form of an anal papilla - in lampreys (cyclostomes), as well as in a number of fish - sculpins, gobies and some others.
In male sharks and rays, both viviparous and spawning, internal fertilization occurs. In connection with this, a special copulatory apparatus develops - the pterygopodium (Fig. 80), formed from modified outer inner rays of the ventral fins. These rays increase in size, forming a massive burrow on each pelvic fin, on the inside of which there is a groove. At. During mating, the male puts both outgrowths together and inserts them into the female’s cloaca. Interestingly, the most ancient fossil sharks apparently did not have a copulatory apparatus (pterygopodia), and fertilization was probably external. The modern polar shark, Somtiiosus microcephalus (Bloch.), also lacks pterygopodia, but this is undoubtedly a secondary phenomenon.
Food for goldfish fry
Breeding goldfish involves constant care for the offspring that appear. Diet plays a huge role in influencing the growth and development of young animals. Many novice breeders do not know what is suitable as food for newly hatched fry. Babies eat the same food as adults, but it must be small so that babies swallow it. To do this, it is ground into powder.
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At first, the babies get their nutrition from the yolk sacs, but once the fry begin to swim, they will be able to feed on small foods. Ciliates and rotifers are ideal for the first week of life of the fry. After 14 days, you can give Cyclops, crushed tubifex. When the fish are a month old, they will be able to eat bloodworms.
They are fed 4-5 times in small portions, but the weight of food you give per day should be twice the weight of the fry. The main mistake beginners make is overfeeding their pets. As a result, the food sinks and then rots, which worsens the growth conditions of the young animals.
Coloring appears at 3 or 5 months.
Among bony fish there are also a large number of viviparous forms.
In the species-rich order Cyprinodontiformes, a significant proportion of species give birth to live young. In these fish, the modified rays of the anal fin (usually the third and fourth) are transformed into a very complex copulatory organ - the gonopodium. It is interesting that in viviparous carnivores this organ acquires mobility and can turn back and forth.
Rice. 80. Pterygopodium (copulation organ) of the stingray (from Kyle and Eherenbaum, 1927)
A complex gonopodium develops in the fish Horaichthys setnai Kulkarni from Cyprinodontiformes (Fig. 81). Its function is to suspend spermatophores from the female's genital opening. In fish from the order Phallostethiformes, the copulatory apparatus develops on the throat and is a derivative of the first pair of ribs and parts of the pelvic and shoulder girdles. Fish of some other groups also have copulatory organs.
Normal, non-pathological hermaphroditism in fish is very rare.
It is reliably known only among representatives of groupers - Serranidae and sea crucians -
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Rice. 81. Copulatory organs of different fish (not to scale):
L, 2 - male and gonopodium of Gambusia (from Lindbergh, 1934); 3, 4 — male and gonopodium of Horaichthys setnal (from Kulkarni, 1940); 5, 6 — male and Neostethus amaricola (Vill.et Man.)
:Sparidae. In some species of the genus Serranus, self-fertilization is even possible; in other genera Serranidae and Sparidae, the male and female gonads mature alternately, and self-fertilization does not take place.
In some individuals of Chrysophrys aurata (L.), which are hermaphrodites, the milt ripens first and then the eggs, while in others it is the other way around. Therefore, during the spawning period, there are always both “males” and “females” in the herd, although each individual acts differently.
U86 is sometimes either male or female, but self-fertilization does not occur.
The spawning process itself is very different for different fish. In a number of fish, the laying of eggs is preceded by the construction of a special nest. Many catfish of the families Bagridae and Loricariidae lay their eggs in burrows specially dug for this purpose, which are then guarded by males. In this case, something like nesting colonies is formed. Fish such as the common catfish, Silurus glanis L., make nests of plants in various shapes. fish - Amia calva L., some wrasses - Labridae and sticklebacks - Gasterosteidae. A complex floating nest of plants is made by the gymnarchus - Gymnarchus niloticus Cuv. Some labyrinths - Macropodus, Betta, etc. - make nests from air bubbles. The construction of the nest is usually carried out by one male, but in some species, for example, labyrinthine gouramis, the female also takes part in the construction of the nest.
Among fish there are both monogamous and polygamous.
Thus, among salmon, pink salmon usually spawn with one female several males (up to ten), which is apparently associated with spawning in a fast current, when a significant part of the sperm is carried down the river and dies. In the masu salmon, each female usually spawns with one large male. (In masu salmon, dwarf freshwater males often take part in spawning along with large ones); In stickleback, on the contrary, one male spawns with several females, taking turns driving them into a nest that he has built. The wrasse - Creniiabrus ocellatus Forsk. - has two
Rice. 83. Change in the strength of the shell of salmon eggs (according to Zotin):
1 - Sal then salar m. lacustris L.; 2 – Coregonus lawaretus lavaretus awirensls Pravdin
groups of males - large and small. A large male makes a nest of Cladophora, where eggs are laid, inseminated by both large and small males, after which the small males are expelled, and the large one remains at the nest to guard the laid eggs.
Fights over females occur, for example, between male salmon, as well as among some catfish and labyrinth fish. Among salmon and some labyrinths, these fights sometimes end in the death of one of the rivals.
The struggle between males for a female, i.e., so-called sexual selection, is an adaptation that, by weeding out weak individuals, contributes to the preservation of the species.
When sperm penetrates the shell of the egg, as noted above, the droplets in the plasma quickly flow under the shell, which peels off from the yolk, and further penetration of sperm under the shell becomes impossible.
The release of droplets under the egg shell increases its strength (Fig. 83). In the case of unfertilized eggs of sturgeon, salmon, whitefish and other fish that are adapted to reproduce in the current in the water, droplets shoot out under the shell within a few minutes, and the eggs lose the ability to further fertilize. In the herring - Clupea harengus L., the eggs of which develop in sea water that is not very mobile, in most eggs the release of droplets under the shell sometimes occurs later than a day later, due to which such eggs retain the ability to fertilize longer. The liquid contained in these droplets has a negative effect on sperm, causing them to stick together.
In some fish in nature, parthenogenetic development sometimes occurs, which usually differs from normal and, as a rule, does not lead to the formation of a normal larva.
In salmon, unfertilized eggs that are placed in a nest-mound along with fertilized ones do not die, but continue to develop in a unique way.
This parthenogenetic development lasts. for a long time, until the embryos hatch from the fertilized eggs. If unfertilized eggs did not have the ability to develop, but, once in the nest, died and decomposed, this would lead to the death of the entire clutch. The presence of such a peculiar adaptation to parthenogenetic development ensures the preservation of fertilized eggs.
In herring - Clupea harengus membras L. - and in Pacific herring - Clupea harengus pallasi Val. - parthenogenetic development sometimes reaches the stage of a free-swimming larva. Parthenogenesis also occurs in some other species, but there are no known cases of normal development during parthenogenesis in fish. During gynogenesis, that is, when the sperm penetrates the egg, but the fusion of the nuclei of the egg and sperm does not occur, in some species of fish development proceeds normally, but only females are produced in the offspring. This phenomenon was noted in the silver crucian carp - Carassius auratus gibelio (Bloch.) - and in the viviparous fish Molienisia formosa (Girard). Goldfish in East Asia have both males and females in their populations, which ensures normal insemination of eggs. In Central Asia, Western Siberia and Europe, males are extremely rare in populations of silver crucian carp, and in some populations they are even completely absent. In such populations, insemination leading to gynogenesis occurs at the expense of males of other fish species. The adaptive significance of this phenomenon apparently lies in ensuring the reproduction of the population even with the preservation of one female, i.e., under extremely unfavorable conditions.
Caring for offspring is expressed in many fish belonging to different orders.
As mentioned above, many fish make nests from plants, pebbles, and even air bubbles. Many labyrinths lay their eggs in a foam nest built on the surface of the water; In this nest the eggs are guarded by the male. Interestingly, the air bubbles from which the nest is built are enveloped in a film that is stronger than water, secreted by the fish from the mouth. Thanks to this film, the bubbles do not burst.
A very complex nest is made by sticklebacks (Gasterosteus and Pungitius). In these fish, the male builds a spherical hollow nest from scraps of plants, where the females lay their eggs, after which the male remains at the nest and guards it.
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spawning of various fish
The spawning of pike differs from other fish. After the ice breaks up and the water level in the river rises, when the shallow areas have already warmed up sufficiently, a female pike comes here, accompanied by 3-7 males. The female releases eggs, and the males release milk, which fertilizes the eggs.
Bream begins spawning in the morning, before sunrise, and continues until almost mid-day. The fish first swim around the reservoir, looking for the appropriate spawning site. Then the fish begin to swim in a circle with a diameter of 4-8 m, with the male bream staying with the females. During circular movements, the fish hit the water with their tail, scattering eggs onto the plant substrate, and fertilize them. After active movement there comes a period of rest. After some time, the fish spawn again.
Before spawning, the female catfish digs a nest in the ground with her front fins. After laying eggs, the male and female catfish guard the nest until the larvae hatch from the eggs.
The examples given indicate that the spawning of fish is unique for each species. Even for the same species in different reservoirs, spawning may be different. And for many fish, spawning has not yet been studied.
