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Types Of Reproduction In Fishes


Smit Ramesh Lende1,Ramchandra khileri2

Department of Aquaculture

Department of Fisheries Resource Manegment

College of fisheries JAU, Veraval

Gujrat-362265



Reproduction


In biology, reproduction is the process by which new individual organisms are produced. Reproduction is a fundamental feature of all known life; each individual organism exists as the result of reproduction. Although the term reproduction encompasses a great variety of means by which organisms produce new offspring, reproductive processes can be classified into two main types: Sexual reproduction and asexual reproduction.


  • Reproduction in Fishes


The methods of reproduction in fishes are varied, but most fishes lay a large number of small eggs, fertilized and scattered outside of the body. The eggs of pelagic fishes usually remain suspended in the open water. Many shore and freshwater fishes lay eggs on the bottom or among plants. Some have adhesive eggs. The mortality of the young and especially of the eggs is very high, and often only a few individuals grow to maturity out of hundreds, thousands, and in some cases millions of eggs laid.

Males produce sperm, usually as a milky white substance called milt, in two (sometimes one) testes within the body cavity. In bony fishes a sperm duct leads from each testis to a urogenital opening behind the vent or anus. In sharks and rays and in cyclostomes the duct leads to a cloaca. Sometimes the pelvic fins are modified to help transmit the milt to the eggs at the female's vent or on the substrate where the female has placed them. Sometimes accessory organs are used to fertilize females internally-for example, the claspers of many sharks and rays.

In the females the eggs are formed in two ovaries (sometimes only one) and pass from the ovaries to the urogenital opening and to the outside. In some fishes the eggs are fertilized internally but shed before development takes place. Members of about a dozen families each of bony fishes (teleosts) and sharks bear live young. Many skates and rays also bear live young. In some bony fishes the eggs simply develop within the female, the young emerging when the eggs hatch (ovoviviparous).

Others develop within the ovary and are nourished by ovarian tissues after hatching (viviparous). There are also other methods utilized by fishes to nourish young within the female. In all live-bearers the young are born at a relatively large size and are few in number. In one family of primarily marine fishes, the surfperches from the Pacific coast of North America, Japan, and Korea, the males of at least one species appear to be born sexually mature, although they are not fully grown.

Some fishes are hermaphroditic, an individual producing both sperm and eggs, usually at different stages of its life. Self-fertilization, however, is probably rare. Successful reproduction and in many cases defense of the eggs and young is assured by rather stereotyped but often elaborate courtship and parental behaviour, either by the male or the female or both. Some fishes prepare nests by hollowing out depressions in the sand bottom (cichlids, for example), build nests with plant materials and sticky threads excreted by the kidneys (sticklebacks), or blow a cluster of mucus-covered bubbles at the water surface (gouramis). The eggs are laid in these structures. Some varieties of cichlids and catfishes incubate eggs in their mouths.

Some fishes, such as salmon, undergo long migrations from the ocean and up large rivers to spawn in gravel beds where they themselves hatched (anadromous fishes). Others undertake shorter migrations from lakes into streams or in other ways enter for spawning habitats that they do not ordinarily occupy.


  • Types of reproduction

Generally two types of reproduction are seen, these are-


  1. Sexual reproduction

  2. Asexual reproduction


  1. Sexual reproduction


A from of reproduction that involves the fusion of two reproductive cells (ova and sperm) in the process of fertilization normally especially in animals, it requires two parents, one male and the other female. The female of the species produces an egg which is fertilized by sperm from the male. Depending on the species, fertilization takes place either within the female's body or externally, after the female lays her eggs and the male releases his sperm close by. When the egg is fertilized the genes of both parents are combined to produce zygote which develops into a new individual. Nearly all animals reproduce sexually.


  • Bisexual

Which is the prevalent kind; sperm and eggs develop in separate male and female individuals.


  • Hermaphrodites

Both sexes are in one individual and, as among certain errands and a dozen or more other families. Self-fertilization or true functional hermaphrodites sexists. Hermaphroditic sex glands are known for several species, including some trout relatives (salmonoids), perches (Perca), walleyes (Stizostedion). Darters (Etheostoma). And some of the black basses (Micropterus).


  1. Asexual reproduction


Reproduction in which new individuals are produced from a single parent without the formation of gametes. Asexual reproduction enables animals to reproduce without a partner. It occurs chiefly in lower animals, microorganisms, and plants. Many invertebrates reproduce asexually, including coral and starfish. Coral grow small buds that break off to become separate organisms, whilst starfish are able to generate an entirely new being from a fragment of their original body.


  • Other Types

Without above classification, another three types of reproduction are also possible.


  • Parthenogenesis

Is the development of young without fertilization, and a condition that has been called parthenogenesis (more properly gynogenesis) occurs in a tropical fish, the live-bearing Amazon molly (poecilia Formosa), and is also known in poeciliopsis. Mating with a male is required, but the sperm serves only one of its two functions, that of inciting the eggs to develop; it does not take any part in heredity. The resultant young are always females, with no trace of parental characters.



  • Sexual reproduction


Sexual reproduction is a process of biological reproduction by which organisms give rise to descendants that have a combination of genetic material contributed by two different gametes, usually i.e. two different individuals, male and female. A gamete is a mature reproductive or sex cell. Sexual reproduction results in increasing genetic diversity, since the union of these gametes produces an organism that is not genetically identical to the parent(s).

Sexual reproduction is characterized by two processes: meiosis, involving the halving of the number of chromosomes to produce gametes; and fertilization, involving the fusion of two gametes and the restoration of the original number of chromosomes. During meiosis, the chromosomes of each pair usually cross over to achieve genetic recombination. Once fertilization takes place, the organism can grow by mitosis.

While typically sexual reproduction is thought of in terms of two different organisms contributing gametes, it also includes self-fertilization, whereby one organism may have "male" and "female" parts, and produce different gametes that fuse.

Sexual reproduction is the primary method of reproduction for the vast majority of visible organisms, including almost all animals and plants. The origin of sex and the prominence of sexual reproduction are major puzzles in modern biology.


  • Overview: Gametes, meiosis, fertilization, and mitosis


In the first stage of sexual reproduction, 'meiosis', a special type of that takes place in gonads prior to the a production of gametes, the number of chromosomes is reduced from a diploid number (2n) to a haploid number (n). During 'fertilization', haploid gametes come together to form a diploid zygote and the original number of chromosomes (2n) is restored in the offspring.

Sexual reproduction involves the fusion or fertilization of gametes from two different sources or organisms.

Typically, a gamete or reproductive cell is haploid, while the somatic or body cell of the organism is diploid. A diploid cell has a paired set of chromosomes. Haploid means that the cell has a single set of unpaired chromosomes, or one half the number of chromosomes of a somatic cell. In diploid organisms, sexual reproduction involves alternating haploid (n) and diploid (2n) phases, with fusion of haploid cells to produce a diploid organism. (See life cycle.) Some organisms, however, exhibit polyploidy, whereby there are more than two homologous sets of chromosomes.

Meiosis and mitosis are two diverse type of cell divisions. Mitosis occurs in somatic (body) cells. The resultant number of cells in mitosis is twice the number of original cells. The number of chromosomes in the daughter cells is the same as that of the parent cell. Meiosis occurs in reproductive or sex cells and results in gametes. It results in daughter cells with half the number of chromosomes present in their parent cell. Essentially, a diploid cell duplicates it's chromosomes, then undergoes two divisions (tetroid to diploid to haploid), in the process forming four haploid cells. This process occurs in two phases, meiosis I and meiosis II.

Fertilization involves the fusion of haploid gametes to give a diploid organism, which can then grow by mitosis. Thus, in sexual reproduction, each of two parent organisms contributes half of the offspring's genetic makeup by creating haploid gametes that fuse to form a diploid organism. For most organisms, a gamete that is produced may have one of two different forms. In these anisogamous species, the two sexes are referred to as male, producing sperm or microspores as gametes, and female, producing ova or megaspores as gametes. In isogamous species, the gametes are similar or identical in form, but may have separable properties and may be given other names. For example, in the green alga, Chlamydomonas reinhardtii, there are so-called "plus" and "minus" gametes. A few types of organisms, such as ciliates, have more than two kinds of gametes.

Sexually reproducing organisms have two sets of genes (called alleles) for every trait. Offspring inherit one allele for each trait from each parent, thereby ensuring that offspring have a combination of the parents' genes. Having two copies of every gene, only one of which is expressed, allows deleterious alleles to be masked.

As with the other vertebrates, sexual reproduction is the overwhelming dominant form of reproduction. However, there are several genera of fish that practice true or incomplete parthenogenesis, where the embryo develops without fertilization by a male.

Although vertebrates in general have distinct male and female types, there are fish species that are both males and females gonads(hermaphrodites), either at the same time or sequentially. For example, the amenone fish spend the first part of their lives as males and later become females, and the parrot fish is first female and then male. Some members of the Serranidae (sea basses) are simultaneous hermaphrodites, such as the Serranus and their immediate relatives, Hypoplectrus (the synchronoous hermaphroditic hamlets).

Fertilization may be external or internal. In the yellow perch, eggs are produced by ovaries in the female and sperm is produced by the testes, and they are released through an opening into the environment, and fertilization takes place in the water (Towle 1989). In some live bearers, such as guppies and swordtails, females receive sperm during mating and fertilization is internal.

Other behaviors related to sexual reproduction include some species, such as the stickleback, built nests from plants, sticks, and shells, and many species that migrate to spawn.


  • Asexual reproduction


Asexual reproduction is a form of reproduction in which an organism gives rise to a genetically-similar or identical copy of itself without a contribution of genetic material from another individual. It does not involve meiosis, ploidy reduction, or fertilization, and only one parent is involved genetically. A more stringent definition is agamogenesis, which refers to reproduction without the fusion of gametes.

Asexual reproduction is the primary form of reproduction for single-celled organisms such the archaea, bacteria, and protists. However, while all prokaryotes reproduce asexually (without the formation and fusion of gametes), there also exist mechanisms for lateral gene transfer, such as conjugation, transformation, and transduction, whereby genetic material is exchanged between organisms. Biological processes involving lateral gene transfer sometimes are likened to sexual reproduction (Narra and Ochman 2006). The reproductive variances in bacteria and protists also may be symbolized by + and - signs (rather than being called male and female), and referred to as "mating strains" or "reproductive types" or similar appellations.

Many plants and fungi reproduce asexually as well, and asexual reproduction has been cited in some animals, including bdelloid rotifers, which only are known to reproduce asexually, and various animals that exhibit parthenogenesis under certain conditions. In parthenogenesis, such as found in some invertebrates and vertebrates, an embryo is produced without fertilization by a male. Generally, parthenogenesis is considered a form of asexual reproduction because it does not involve fusion of gametes of opposite sexes, nor any exchange of genetic material from two different sources (Mayr 2001) however, some authorities (McGraw-Hill 2004) classify parthenogenesis as sexual reproduction on the basis that it involves gametes or does not produce an offspring genetically identical to the parent (such as a female domestic turkey producing male offspring).

A wide spectrum of mechanisms may be exhibited. For example, many plants alternate between sexual and asexual reproduction (see Alternation of generations), and the freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Many protists and fungi alternate between sexual and asexual reproduction.

A lack of sexual reproduction is relatively rare among multicellular organisms, which exhibit the characteristics of being male or female. Biological explanations for this phenomenon are not completely settled. Current hypotheses suggest that, while asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.


  • Hermaphroditism


A hermaphrodite is defined as any individual organism that possesses both male and female reproductive organs during their life span. The main advantage of hermaphroditism is the assurance of a reproductive partner. Although hermaphroditism is quite common in invertebrates and plants, it is an exceedingly rare occurrence in vertebrates. Hermaphroditism in mammals and birds are almost always a pathological condition (often leading to infertility). Only in Perciforms (fish) does hermaphroditism occur naturally and in high frequency.

Hermaphrodites are divided into two main categories: synchronous hermaphrodites, and sequential hermaphrodites. In the synchronous hermaphrodites, organisms possess both active male and active female reproductive organs at the same time. In sequential hermaphrodites, both male and female reproductive organs may be present, but only one is active and viable at any given time.

Synchronous, or simultaneous hermaphrodites in reef fish are relatively atypical. A few Serranids (sea basses, e.g. Serranus sp.) and Hamlets are known synchronous hermaphrodites. During mating, one individual will lay eggs while another fertilizes the eggs, after which both will reverse roles and perform fertilization again. Synchronous hermaphrodites do not fertilize themselves; Self-fertilization does not promote genetic diversity, and can amplify genetic defects from parent to offspring. The interesting fact is most synchronous hermaphrodites form monogamous pairs.

Sequential hermaphrodites are so named because they are capable of transforming from one sex to another. Theses transformations entail a full conversion of gonads from one sex to another. The gonads of sequential hermaphrodites possess the genetic information to produce both male and female reproductive organs, but only the dominant gene is expressed at any giventime. Different cues - varying from species to species - may induce sex changes.

Sequential hermaphrodites are further categorized into two main categories: protogynous and protandrous. Protandrous hermaphrodites are those that develop into males first, then possibly to females. Protogynous hermaphrodites are the exact opposite, with juveniles first developing female reproductive organs that may possibly change into male reproductive organs in select circumstances. It should be noted that hermaphrodites do not necessarily have to change sexes, but by definition, are capable of this feat.

Protandrous hermaphrodites are the rarer of the two types. Pomacentrids (damselfish) are the most famous of these hermaphrodites. For example, clownfish of the genus Amphiprion live in communities that consist of one dominant female specimen and several smaller male (or asexual juvenile) specimens. If the female should be removed, a male will convert to a female, insuring a reproductive partner for the community.

Protogynous hermaphrodites are most often haremic fish. These fish form monoandric harems comprising of 1 male overseeing numerous females for life. The two primary responsibilities of the male are to defend its territory against other conspecific males, and to court and fertilize females of its territory. If the male should die (either of natural causes or conflict-related mortality), the dominant female of the harem will undergo a sex change from female to male. This sex change may take as little as 5 days. The new male will then resume the full responsibilities of the previous male until he should die. Protogynous hermaphrodites that form harems include the wrasses of the genus Cirrhilabrus and Paracheinlinus, Dwarf Angelfish of the genus Centropyge, and Anthias (e.g. Pseudoanthias sp.).

Some protogynous fish do not form harems, but may form pairs. The dottybacks (Pseudochromis sp.) are presumably protogynous hermaphrodites that fall under this category.

There are also sequential hermaphrodites that waver between sexes with no discernable order. The sexes of these fish are often determined by the ratio of sexes in an immediate community. These types of hermaphrodites include numerous gobies.


  • Modes of reproduction and development


The ability to produce new living individual is a basic characteristics of all plants and animals. All reliable evidence indicates that new life comes only from pre-existing life; this is the process of biogenesis or reproduction. The different modes of reproduction can be classified according to the environment in which the embryos develop, and the sources of the nutrients supporting embryonic growth. These are mentioned into the following term:


  • Oviparity


Oviparity or egg laying refers to the situation where the development or the fertilized of egg occurs outside the body of the female. The young hatch when the egg envelope, shell or capsule is broken. Oviparous fish may be further categorized as being either ovuliparous or zygoparous.

ovuliparity refers to the release of ova from the reproductive tract of the female followed by fertilization or activation in the external environment. Thus, all organisms that have external fertilization and this includes most teleost are said to be ovuliparous.

zygoparity refer to the oviparous condition in which the zygotes (i.e. fertilized ova products of fusion between the eggs and sperm) are retained within the body of the female for a short period of time before being released into the environment. Obviously, zygoparous species display internal fertilization with their being a transfer of male sperm to the reproductive tract of the female. zygoparous reproduction characterizes all skates some sharks and a small number of teleost.

Irrespective of weather the fertilization of the eggs occurs internally or the egg yolk provides externally the nutrient for the developing embryos of oviparous species.


Oviparity in fishes

Amongst fish which reproduce by ovipartity, there are several groups.


  • Egg Scatterers

Fish which breed in this way either spawn in pairs or in groups. Males and females release milt (the sperm and spermatic fluid of the male) and eggs into the water at the same time. These mix together, fertilising the eggs. The fertilised eggs are broadcast (or spread) into the plankton column and float away in the current or sink to the bottom. No parental care is given, so large amounts of eggs are produced. It is easy to produce many eggs and because they are in the water, they don't dry out - necessary oxygen and nutrients aren't scarce. When theoffspring settle out of the plankton, they might be in totally new environments: this gives the young a chance to survive across a wide area. The main disadvantage of this method is that the fish must go through a larval stage before they transform into adults. In this larval stage, they are very vulnerable while they try to find food and avoid predators. Also, they may not find a suitable environment when they settle out of the plankton column. The survival rate for individual eggs is very low, so the parent has to produce millions of eggs.


  • Egg depositors

These fish either lay eggs on a flat surface, like a stone or plant leaf or may even place them individually among fine leaved plants. The parents usually form pairs and guard the eggs and fry (young fish) from all danger. The Cichlids such as Koi are the best known species for this. Some Catfish and Rainbow fish are also egg depositors.


  • Nest Builders

Many fish species build nests. These might be a simple pit dug into gravel (trout do this) or an elaborate bubble nest. When they are ready to spawn, the fish may construct a nest by blowing bubbles, and they often use vegetation to anchor the nest. The male will keep the nest intact and keep a close eye on the eggs. The Gouramis, Anabantids and some catfish are the most common of this type.


  • Mouthbreeders

These are particularly odd, since eggs are fertilised externally, but raised internally. The females usually lay their eggs on a flat surface where they are then fertilised by the male. After fertilization the female picks up the eggs and incubates them in her mouth. Broods tend to be small, since by the time the fry are released by their mothers they are well formed and suffer minimal losses. The best known mouthbreeders are the African lake Cichlids.


  • Egg buriers

The annual Killifish reproduce in this way. As the pools they live in dry out, the fish spawn, pressing their eggs into the mud. The pools eventually dry out completely, killing the adults, but the eggs remain safe in the dried mud. When it rains and the pool refills the eggs hatch and the cycle is repeated. Killifish eggs can survive for many years in dried out mud.


  • Ovoviviparity


In the ovoviviparous species the eggs usually develop within the uterus (modified oviduct) of the female. Fertilization of the egg occurs internally and the eggs are retained until hatching or beyond. The developing embryos do not receive any supply of foodstuffs from the female but must rely on the yolk of the egg for nutrition. This form of nutrition is known as lecthotrophy. The developing embryos must, however, rely on the female for supply of oxygen. During pregnancy the wall of the oviduct becomes enlarged and richly supplied with blood vessels.


  • Viviparity


Viviparous species have eggs, which develop either within the uterus (i.e. several species of shark), or within the ovary (i.e. teleosts). The developing embryos receive some nutrient supply from the female in edition to that provided as yolk in the egg. This form of nutrition is known as matrotrophy. The additional nutrient can be supplied in various forms. in some species of female secrets a nutrient-rich fluid which is taken up by the developing young has a form of 'soup'. In other species some form of 'placenta' may develop allowing a more direct transfer of nutrient from the blood of the female to the developing embryo.

A distinction is sometimes made between viviparous animals in which the embryos derives nourishment from the mother's tissues as in most mammals and animals that are called ovoviviparous in which the embryo is nourished entirely by food stored in the egg, the hatching before being laid.


  • Viviparity in fishes

Fishes provide a key to understanding vertebrate viviparity in as much as the first viviparous vertebrates were fishes and they display the most diverse maternal fetal relationships of all the live bearing vertebrates. Viviparity occurs in three major fish groups: the chondrichthyans, the teleosts, and the actinistians. Although widespread, it represents the dominant mode of reproduction only among the sharks and rays. Approximately 420 of the estimated 600-700 species of chondrichthyan fishes are viviparous in contrast, only 510 of the estimated 20,000 species of teleost fishes are viviparous. The coelacanth, the only extant species of actinistian, is viviparous.


  • Semelparity and iteroparity


Semelparity and Iteroparity refer to the reproductive strategy of an organism. A species is considered semelparous if it is characterized by a single reproductive episode before death, and iteroparous if it is characterized by multiple reproductive cycles over the course of its lifetime. Some plant scientists use the parallel terms monocarpy and polycarpy.

In truly semelparous species, death after reproduction is part of an overall strategy that includes putting all available resources into maximizing reproduction, at the expense of future life. In any iteroparous population there will be some individuals who die between their first and second reproductive episodes, but unless this is part of a syndrome of programmed death after reproduction, this would not be called semelparity.


  • Semelparity

The word semelparity comes from the Latin semel, once, and pario, to beget. It is also known as "big bang" reproduction, since the single reproductive event of semelparous organisms is usually large, as well as fatal. A classic example of a semelparous organism is Pacific salmon (Oncorhynchus spp.), which lives for many years in the ocean before swimming to the freshwater stream of its birth, for laying eggs, and dying. Other semelparous animals include many insects, including some species of butterflies, cicadas, and mayflies, some molluscs such as squid and octopus, and many arachnids. Semelparity is much rarer in vertebrates, but in addition to salmon, examples include smelt, capelin, and a few lizards, amphibians, and didelphid and dasyurid marsupial mammals.


  • Iteroparity

The term iteroparity comes from the Latin itero, to repeat, and pario, to beget. An example of an iteroparous organism is a human-though many people may choose only to have one child, humans are biologically capable of having offspring many times over the course of their lives. Iteroparous vertebrates include all birds, most reptiles, virtually all mammals, and most fish. Among invertebrates, most mollusca and many insects (for example, mosquitoes and cockroaches) are iteroparous. Most perennial plants are iteroparous.


  • FECUNDITY


Fecundity, the number of eggs ripened by female fish during a spawning season, or event, varies from a few dozen in some continuously reproducing livebearing fishes to millions in some species that spawn pelagic eggs on an annual basis. In general, fecundity varies among species inversely with the amount of "care" given to the individual progeny: viviparous fishes have lower fecundity than ovoviviparous fishes, which in turn have lower fecundity than oviparous fishes, and nest builders have lower fecundity than pelagic spawners. Within species fecundity is positively related to size; generally it is close to a function of the cube of fish length. Although fish eggs range from about 0.5 to 20 mm in diameter, the size of the adult limits the fecundity in smaller species. In most species with high fecundity, several batches of eggs are usually produced at intervals of a few days to weeks during the spawning season.

Some general reasons for determining fecundity is that it is useful for making total population estimates, it is useful in studies of population dynamics or productivity, and it is useful for characterizing specific populations, subpopulations, and/or stocks of fishes.


  • Absolute fecundity : is the number of ripe eggs produced by a female in one spawning season or year (this is the usual meaning when the general term "fecundity" is used, although on occasion it might also mean the number of eggs produced in a lifetime).

  • Relative fecundity : is the number of eggs produced in a season per unit somatic weight of the fish (i.e., eggs/gram), and is useful if it is shown that the fecundity of a fish is proportional to its weight, which is not uncommon.


  • Population fecundity : is the number of eggs spawned by the population in one season, is the sum of the fecundities of all females, and is usually expressed as the product of the expected fecundity of an average female, the number of breeding females in the population (an example of why classifying maturity stages may be useful).


  • SPAWNING


Spawning refers to the release of unfertilized planktonic eggs by female fish, which is the reproductive pattern for most marine fishes. The eggs are fertilized shortly after release by males. Some fishes also deposit unfertilized Fish Reproduction eggs in nests where they are fertilized and develop. Fishes with internal fertilization release free-swimming larvae, or juveniles. The ripening of eggs and spawning are controlled by hormones, nutrition of the female, and external (ecological) factors (Hempel 1979). Usually maturation and spawning are controlled by a combination of endogenous and exogenous controls and are not governed by any specific factor.


  • Terminology used in discussing spawning


  • Mating: pairing (one-on-one) for the purpose of fertilizing eggs; copulatory organ present.

  • Spawning: release of unfertilized eggs into the environment or release of larvae into the environment; mating and spawning need not occur simultaneously (e.g., surfperches). Spawning can occur without true mating (e.g., herring, which are broadcast spawners).

  • Fertilization: fusion of eggs and sperm (creating diploids from haploids); mating and fertilization need not occur simultaneously (e.g., surfperches and rockfishes).

  • Incubation time: time from egg fertilization to hatching. Gestation applies only to live-bearing fishes; it is the time young stay within the female.

  • Hatching: when the larva frees itself from the egg.

  • Breed: to produce offspring by hatching (or gestation).

  • Brood: guard and groom eggs until they hatch.


  • DESCRIPTION OF MATURITY STAGES


  • Male


Diagrammatic representation of different maturity stages of male

Diagrammatic representation of different maturity stages of male


  • Stage I (Immature) : The testes are extremely small, often recognized only as translucent filamentous strands in the early stages. Later, when easily visible, they are small, opaque, pinky-white, leaf-like structures with a fairly long vas deferens, which get easily snapped when the testes are removed. There is very little asymmetrical development and, if any, the left testis is slightly longer. The testes with the vas deferens occupy roughly 50 % of the length of the body cavity or occasionally slightly more, measuring a maximum overall length of about 35 mm, of which the testicular portion may form 10 to 15 mm only. The gonad weight ranges up to 0.2 gm, but it is usually less than that. Their relative weight to body weight is normally below 0.8%.


  • Stage II-A (Developing Virgin) : The testes are thicker and more elongated. They are opaque, pink or white in colour, with vas deferens reduced but thread-like. Asymmetrical development has set in and the left testis is almost always longer. The testes with the ducts extend to 50-60% of the body cavity and measure 30 to 40 mm and the testicular portion measures about 25 to 30 mm. Their weight ranges from 0.2 to 0.5gm, but it is usually around 0.4gm. The relative weight varies from 1 .0 to 1.7%.


  • Stage II-B (Spent-resting) : The testes are pinkish or brownish-white in colour but different from the previous stage in having shrunken and wrinkled appearance when viewed against light, showing that the organs are not compactly filled with germ cells. The left testis is usually longer. The vas deferens is a much wider duct than the thread-like passage of the previous stage and is covered by the lower halves of the testes which are narrower and extend almost to the posterior end of the body cavity. With the degree of opacity varying, there are a few patches of semi-opaque regions. The organs fill up 50 to 55% of the body cavity and measure almost the same length as the previous stage, i.e., 30 to 40 mm, but theif absolute and relative weight are much less. While the former ranges up to 0.29 gm with an average of about 0.15 gm, the latter is about 0.5%.


  • Stage III (Maturing) : The testes are well developed and thickened, white in colour. Vas deferens, being filled with spermatogonia, are reduced and measure less than 15 mm. The left testis is distinctly longer and this condition persists in the subsequent stages also. The gonads varying in length from 36 to 50 mm occupy 70 to 75 % of the body cavity. Their weight ranges up to 2.0 gm but it is usually around 1.0 gm forming 2.5 to 4-0% as relative weight.

.

  • Stage IV (Maturing) : Quite massive and creamy-white are the testes with vas deferens hidden under them. The organs, while extending to the entire breadth of the body cavity, occupy 85 to 90% of the body cavity length, measuring 40 to 70 mm. Their absolute weight ranges from 2.5 to 5.0 gm, but in majority of cases it is around 3.5 gm. Their relative weight varies from 5.0 to 7.0%.


  • Stage V (Mature) : The testes are opaque white in colour, soft, more extensive than Stage IV and occupy the entire length of the body cavity but very often even more with the result their anterior ends tend to fold down along the ventral body wall. On a little pressure internally at the posterior end, spermatic fluid oozes out. Their usual length is 65 to 70 mm but can attain even 80 mm sometimes. Their absolute weight ranges from 6.0 to 8.0 gm, with an average around 7.0 gm. The relative weight works up to 10.0 to 14.0%.


  • Stage VI (Running) : The testes are very extensive, white in colour and fill the entire space of body cavity displacing the intestines to a fraction of space. Not only the anterior tips are folded down along the ventral body wall but even the outer margins extending along the sides of the body wall curve towards the middle so much so very often an insertion along the mid-ventral line cuts through the outer edges of the testes. Under a slight pressure externally on the flanks of the fish or even while handling, milt extrudes out. The organs always measure more than 70 mm and weigh from 9.0 to 13.0 gm with an average around 10.0 gm and the relative weight ranges between 15 and 20%.


  • Stage VII A (Partially Spent ) : The testes are meat-coloured, a bit leathery in texture, shrunken with wrinkles and semi-opaque spaces visible when viewed against fight. Measuring 40 to 60 mm, they occupy 70 to 80% of the body cavity and weigh usually 2.0 gm but may range from 1.0 to 2.5 gm, which forms 2.5 to 4.5% as relative weight.


  • Stage VII B (Spent) : The testes are deep flesh-coloured, shrunken, flat, strap-like, shrivelled with translucent patchy regions. They occupy 50 to 60% of the body cavity and measure 30 to 45 mm. Their weight is around 0.5 gm, forming about 1.5% as relative weight.





  • Female

Diagrammatic representation of different maturity stages of female

Diagrammatic representation of different maturity stages of female


  • Stage I (Immature) : The ovaries are soft cylindrical and almost translucent, pink or flesh-coloured. Sometimes due to post-mortem changes they appear purple in colour. The surface of the ovary is smooth with no distinct blood vessels. There is very little asymmetry in the size of the ovaries. The oviduct is fairly long and completely transparent with the result, the ovarian bodies look like detached stubs, short and plump. The entire length of the ovaries with their ducts occupies about or slightly more than 50% of the body cavity and measures up to about 35 mm of which the length of the ovary alone ranges from 10 to 25 mm. Their absolute weight may be about or below 0.25gm forming a maximum of 0.8% as relative weight. The ovaries are compactly filled with oocytes, not visible to naked eye. The oocytes are yolkless and transparent and measure up to 0.13 mm with the majority of them in the size range of 0.07 to 0.09 mm.


  • Stage II A (Developing Virgin) : Cylindrical, soft, translucent ovaries pink or flesh-coloured. Asymmetry is not quite distinct yet. Oviducts, thin and thread-like, are a little reduced in length and not more than 10 mm. The overall length of the gonads ranges from 30 to 35 mm forming 55 to 60% of body cavity. The ovaries alone measure about 20 to 25 mm in length and usually weigh around 0.4 gm, but may range from 0.3 to 0.6 gm. The relative weight is 1.0 to 1.5%. Majority of the ova are transparent with signs of yolk formation in some, which are mostly semi opaque but sometimes fully opaque with or without translucent periphery. However, even these do not appear as distinct grains to be easily recognised with naked eye. Maximum diameter of ova recorded is 0.30 mm with a large number of ova ranging in size from 0.15 to 0.18 mm.


  • Stage II B (Spent-resting) : The ovaries are dark-red or brownish red or deep flesh-coloured, having a collapsed and flattened appearance. External surface is wrinkled. The tunica is thicker and the Oviducts much wider and shorter than in the previous stage. The length of the organs is 30 to 45 mm occupying 55 to 60% of the body cavity. Their weight is commonly around 0.2 gm, forming less than 0.5% as relative weight. Occasionally, late spawners resting in January-February period may record a maximum gonad weight of about 0.4 to 0.5 gm with their relative weight of about 0.8%. Majority of ova are transparent, not visible to naked eye and measure 0.07 to 0.11 mm. A few scattered opaque ova may be present without transparent periphery and measure up to 0.15 mm. This stage is characteristically distinguished by the presence of clots of blood cells appearing as brownish masses in between the oocytes.


  • Stage III (Maturing) : The ovaries are turgid, opaque and yellow in colour with granular appearance. Development of blood vessels is perceptible. The oviducts are very much reduced. Usually there is asymmetrical development in the size of the ovaries, either gonad longer than the other. This condition persists in the subsequent stages also. The ovaries occupy about 65 to 70% of the body cavity and measure from 35 to 50 mm. Their weight ranges from 0.8 to 1.5 gm, but usually it is below 1.0gm. The relative weight may accordingly vary from 2 to 4%. The maximum diameter of ova recorded is 0.54 mm. The size frequency distribution of ova normally shows one distinct mode of maturing ova around 0.35 to 0.40 mm, which are opaque with translucent periphery and visible to naked eye. In these ovaries, some semi-opaque ova with yolk deposition around the centre measure 0.15 to 0.17 mm and form a good percentage. Sometimes, ovaries advanced a little further but not entered into Stage IV show two modes of maturing ova, an advance one 0.42 to 0.47 mm and a minor one around 0.22 to 0.27 mm. Both sets of ova are opaque and are provided with translucent periphery.


  • Stage IV (Maturing) : The ovaries are compact, vascular with conspicuous blood vessels on the tunica and bright yellow in colour. Oviducts are not quite distinct. The organs almost extend to the entire body cavity forming 80 to 90% of the latter's length, with its own length varying from 45 to 65 mm. They weigh about 2.0 to 4.5 gm with an average of 3.0 gm, forming 4.5 to 7.0% as relative weight. The largest ova may measure about 0.67 mm and the size frequency polygons show two distinct modes, one Any where between 0.51 and 0.57 mm and the other around 0.27 to 0.34 mm. The former are completely opaque, while the latter are provided with translucent periphery.


  • Stage V (Mature) : The ovaries are orange-yellow in colour, fully vascular with prominent blood vessels ramifying on the surface. Tunica is very thin and bursts at slight pressure. Ovaries are very often more than the length of the body cavity with the anterior tips taking a loop down. Their normal length is 65 to 70 mm, but according to the size of fish may even extend up to 80 mm. Their average weight is 6.5 gm with a range of 5.5 to 7.5 gm. Relative weight ranges from 9 to 13%. Maximum diameter of ova observed is 0.82 mm. The distribution of ova shows two groups, an advanced mature one anywhere between 0.62 and 0.67 mm. and a maturing group around 0.35 to 0.47 mm. The ova of former group Present varying appearances; some are completely opaque, some provided with narrow or wide transparent periphery, some vacuolated, some partly Transparent and a few fully. Completely transparent ova have one big Oil globule or two-three smaller droplets of oil globule which measure from 0.054 to 0.109 mm and the other partly transparent and vacuolated ova have a number droplets of oil globule. The ova of less advanced mode are fully opaque.


  • Stage VI (Running) : The ovaries appear as a sort of cream-coloured cellophane bags filled with boiled sago. At a slight prick, a gelatinous mass of transparent ova flows out, the tunica being so thin. Ova can be extruded on slight pressure externally on the flanks of the fish or even while handling. The ovaries measure more than 70 mm and fill the entire-space of abdomen cavity displacing the intestine to a narrow space in between the two ovaries- They may weigh from 8.0 to 12.0 gm, but ordinarily around 9.0 gm, forming 13 to 17% as relative weight. The largest ova are transparent and jelly-like reaching a maximum diameter of 1.2 mm, but the majority of these range from 0.80 to 0.91 mm in diameter with one or two oil globules very rarely cleaved into three which measure 0.091 to 0.127 mm. There is a significant number of medium-sized ova forming another distinct mode between 0.46 and 0.51 mm, which are completely opaque or provided with transparent periphery.

.

  • Stage VII A (Partially Spent) : The ovaries are dark red in colour either throughout the length or only at the posterior half They are a bit flaccid and collapsed with slight wrinkles on the surface. The ovarian lamellae are clearly seen as book leaves especially at the posterior region indicating that the lamellae are not compactly filled with maturing ova and that some have been shed. The ruptured blood capillaries produce blood clots which appear as brownish or reddish masses. Blood capillaries penetrate deeply into the interior. Although the ovaries are shrunk in volume and reduced in breadth, they extend to 70 to 80% of the body cavity measuring 40 to 60 mm. Their weight ranges from 1.5 to 3.0 gm but is usually around 2.0gm, amounting to 2.5 to 5.0% as relative weight. The maximum diameter of ova is about 0.60 mm and the frequency distribution shows only one distinct mode anywhere within 0.35 to 0.51 mm. These ova are completely opaque but a few of these perhaps in the process of resorption are translucent with greyish yolk in the centre within which may be found a few droplets of oil-globule.


  • Stage VII B (Spent) : The ovaries are elongated, honey-coloured, bloodshot, flabby, limp and gelatinous with wrinkles on surface due to collapsed condition. The tunica is leathery and the wide oviduct is now Recognizable. The ovaries measure about 30 to 45 mm and occupy 55 to 60% of the body cavity. They almost always weigh around 0-5 gm with a maximum limit of 1.0gm, forming 0.6 to 1.5% as relative weight. Recently, spent fish have remnants of mature ova, measuring a maximum diameter of 0.47 mm, as resorbing and disintegrating opaque structures. The blood cells from ruptured capillaries appear as reddish clots. Sometimes, there may be a few scattered droplets of oil globule. The resorbing eggs are sometimes translucent in appearance, with the yolk in the form of small spherules, light grey or brownish in colour with man droplets of oil globule clustering around the centre. These ova form a small mode in the size frequency distribution around 0.27 to 0.31 mm. At a later stage, a few blood-coloured or brownish masses are seen which represent the unspawned ova broken down and covered up by the blood cells. In this state, the ovaries appear deep flesh-coloured. The rest are all immature transparent oocytes less than 0.25 mm in diameter.


  • Factors Triggering Maturation and Spawning

There are three primary factors that influence the events leading up to spawning: nutritional state of the female, physiological factors (hormones), and ecological factors.


  • Nutrition of the Female

The feeding condition of the mother can have an important effect on the final maturation of the eggs. Two examples from Hempel (1979) show that in some of the Atlantic herring populations spawning may occur only every other year if environmental conditions, particularly those affecting food supply, are poor. Also, it has been found in the laboratory that in Atlantic sole (Solea solea) no spawning occurred when the flatfish were fed a diet (mussels only) deficient in certain amino acids; however, when the flatfish were force-fed the missing amino acids they spawned, indicating the ovary had been unable to obtain the needed amino acids from maternal tissue when the nutrition of the female had been inadequate (Hempel 1979).


  • Physiological Factors

Hormones govern migration and timing of reproduction, morphological changes, mobilization of energy reserves, and elicit intricate courtship behavior. The pituitary is the major endocrine gland that produces gonadotropin, which controls gametogenesis, the production of gametes, namely sperm (spermatogenesis) and eggs (oogenesis), by the gonads. The pituitary also controls the production of steroids (steroidogenesis) by the gonads; once the gonads are stimulated by the pituitary they begin producing steroids, which in turn control yolk formation (vitellogenesis) and spawning. The control of spawning by the pituitary is often used in fish farming such as in the production of caviar from sturgeon (Acipenser spp.) where spawning is induced by injecting pituitary extract at a late stage of gonadal development, usually in combination with changes in temperature and light periodicity.

  • Ecological Factors


Often ecological factors are associated with timing so that food availability is optimal for the larvae. Some ecological factors important to spawning are temperature, photoperiod, tides, latitude, water depth, substrate type, salinity, and exposure.

  • Temperature : An important factor in determining geographical distributions of fishes. Although little is known about the mechanism by which temperature controls maturation and spawning in fishes, for many marine and freshwater fishes the temperature range in which spawning occurs is rather narrow, so that in higher latitudes the minimum and maximum temperature requirement for spawning is often the limiting factor for geographical distribution and for the successful introduction of a species into a new habitat. For example, Pacific halibut (Hippoglossus stenolepis) are found spawning primarily in areas with a 3-80C temperature on the bottom and therefore do not spawn in Puget Sound, although the adults are caught in the northern areas of Puget Sound. In fact, even in highly migratory tuna, spawning is restricted to water of specific temperature ranges.


  • Photoperiod and periodicity : The day length (photoperiod), in some cases at least, is thought to influence the thyroid gland and through this the fishes' migratory activity, which is related to gonadal development (maturation). In the northern anchovy, by combining the effects of temperature and day length, continued production of eggs under laboratory conditions was brought about by keeping the fish under constant temperature conditions of 150C and a light periodicity of less than 5 hours of light per day (Lasker personal communication). In high latitudes, spawning is usually associated with a definite photoperiod (and temperature), which dictates seasonal pulses of primary production in temperate regions to assure survival of larvae. In low latitudes, where there is little variation in day length, temperature, and food production, other factors may be important such as timing with the monsoons, competition for spawning sites, living space, or food selection.

Reproductive periodicity among fishes varies from having a short annual reproductive period to being almost continuous. There is a tendency for the length of the reproductive period to shorten with increasing latitude. Thus tropical fishes spawn nearly continuously, whereas subarctic fishes spawn predictably during the same few weeks each year. Presumably times of spawning have evolved so larval development will coincide with an abundant food supply. Within spawning seasons, fish may spawn on a daily or monthly tidal cycle or on a diel cycle, or in association with some other environmental cue, such as a change in daylength, temperature, or runoff. A notable instance of spawning periodicity associated with the tidal cycle is the California grunion (Leuresthes tenuis), which spawns intertidally at the peak of the spring high tides (Walker 1952). Within species, spawning times may vary with latitude: Generally, in species that spawn as daylength increases, spawning occurs earlier in the year in lower latitudes than at higher latitudes. In species that spawn as daylength decreases, spawning takes place earlier in the year at higher latitudes than at lower latitudes.


  • Tides (moon cycles) : The dependence of spawning on moon cycles in California grunion spawning on California beaches is an extreme example of external factors controlling reproduction in fishes. Grunion are adapted to spawning on the beach every two weeks in the spring during a new or full moon. Spawning is just after the highest high tide; therefore, eggs deposited in the sand are not disturbed by the surf for 10 days to a month later. Eggs will hatch when placed in agitated water (which simulates surf conditions). In Puget Sound, surf smelt (Hypomesus pretiosus) spawn year-round, except in March. Surf smelt deposit eggs at high tide in sand and gravel (but not necessarily at the highest tide). On the open ocean shores, spawning occurs at midtidal heights (for different subpopulations) (Dan Pentilla personal communication).


  • Latitude and locality : Pacific herring show a definite relationship between latitude and spawning time. Spawning is early in San Francisco (December, January); later in Washington State (February, March, April, May); and still later in Alaska (April, May, June). These fishes are perhaps of different, distinct subpopulations. In temperate waters a biomodal distribution of eggs is usually seen, which indicates discontinuous spawners. The smaller-sized mode represents resting eggs for a future spawning, and the larger mode represents maturing eggs (oocytes), which will presumably be spawned within the year. Temperate water fishes are also usually deterministic, which means all eggs to be spawned are determined at the start of the year. A polymodal distribution of eggs is typical of tropical areas and some temperate water fishes, which signifies continuous or serial spawners, and indicates several spawnings. A well-known temperate example would be Pacific sardine (Sardinops sagax), which spend 7 months spawning and 2 months developing/maturing (Clark 1934). Batch spawning has been described for northern anchovies (Laroche and Richardson 1983). Tropical spawners are usually nondeterministic, which means the eggs to be spawned are not determined at the start of the year but are produced throughout the year; however, nondeterministic can also represent the spawning potential for successive years, an example being the Atlantic cod (Gadus morhua), which will have several years' spawn in the ovary.

In general, older fish usually spawn first and younger fish later, which means that a prolonged spawning period for a population may not be true for individual fish. Once a set of eggs is mature and hydrated, the female may release them all at once or in several batches. An example of releasing several batches is plaice (Pleuronectes platessa), where a single female two weeks after releasing one batch of eggs releases more eggs, and then three weeks later she releases the remaining eggs. Another example is the Pacific herring, which spawn once a year and females lay about 100 eggs per spawning act, which they repeat several hundred times over a few days (Hourston and Haegele 1980). In the lab, walleye pollock spawned an average of nine times in an average period of 27 days (Sakurai 1983).

It also needs mentioning that a long duration of the spawning season of a population cannot necessarily be taken as an indication of prolonged spawning of the individual fish. The prolonged period may be due to differences in spawning time between age groups since older fish tend to spawn earlier in the season. Furthermore, the coexistence of different spawning subpopulations must be taken into account, since winter and summer spawners may be distinct stocks, although shifts from one seasonal spawning pattern to the other may occur. An example of how unpredictable this can be is that certain Atlantic herring of low fecundity have been found to always spawn in the winter, regardless of whether they originated from winter or summer spawning (Hempel 1979).


  • Water depth : Pacific herring spawn along beaches, marine grasses, and algae. Atlantic herring do not spawn along shore but in deeper water up to 200 m (the clearest difference between the Pacific and Atlantic herring, which are usually designated distinct species on the basis of genetic analysis). Of course fishes often spawn at one depth but live at different depths during other times of the year. For example, petrale sole (Eopsetta jordani), in which spawning occurs in a specific offshore area 300-400 m deep, were found by fishermen and eventually had to be protected with regulations to prevent overfishing (A.C. Delacy personal communication).


  • Spawning substrate type : Pacific herring spawn on vegetation whereas Atlantic herring spawn on solid substrate (e.g., gravel). Lingcod spawn on rocks, pilings, and cracks in solid substrate; this species protects the egg mass. Some species such as buffalo sculpin (Enophrys bison) and plainfin midshipman (Porichthys notatus) spawn intertidally and will stay with the egg mass even when they are exposed at a low tide.


  • Salinity : Also a factor affecting spawning. There are varying salinities in many areas of estuaries. Some species will shift spawning sites because of salinity changes. Various degrees of mixing, precipitation, and freshwater runoff may alter spawning habits.


  • Exposure and temperature : A clear example of shifting spawning sites in response to temperature and exposure is the black prickleback (Xiphister atropurpureus), where spawning is shifted from winter in protected areas to spring in exposed areas (Marliave 1975). The complex effects of lower or higher wave action and lower or higher temperatures on courtship, gonadal development, and spawning behavior that result in the spawning site shift.


  • Reference :


  • Antony raja B. T., On the maturity stages of Indian oil-sardine, sardinella longiceps val., with notes on incidence of atretic follicles in advanced ovaries, Indian journal of fisheries, pp. 27 - 47


  • Hames, B. D., (1998), Instant notes in animal biology, Viva books private limited.

pp, 241 - 278.


  • Kay, I., (1998), Introduction to animal physiology, BIOS scientific publishers Ltd.

pp, 175 - 191.


  • New World Encyclopedia contributors, "Asexual reproduction," New World Encyclopedia,

http://www.newworldencyclopedia.org/entry/Asexual_reproduction?oldid=947438


  • New World Encyclopedia contributors, "Sexual reproduction," New World Encyclopedia,

http://www.newworldencyclopedia.org/entry/Sexual_reproduction?oldid=863848


  • Sandhu, G. S., (2001), An introduction to fishes, campus books international.

pp, 337 - 358.



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