pituitary hormones. It has also been shown that maturation of “naked” oocytes in pituitary suspension can occur if a large amount of follicle shell is added to the medium (Masui, 1967).
From the data obtained, both researchers concluded that: gonadotropins induce maturation by acting on the ovarian follicle cells, which secrete progesterone or progesterone-like substances, which act directly on the oocyte to cause maturation (manifested by the dissolution of the nuclear membrane or germinal vesicle). The ability of the ovarian follicle cells to respond to the gonadotropins of the brain lobe and of the oocytes to progesterone arises in the gonads not simultaneously and the ability to respond by complete maturation of the oocyte arises before the ability of the ovarian epidermis to respond to gonadotropins by synthesizing progesterone-like substances.
In nature, mature eggs will be released and the female fish can perform egg laying.
3.2. Mechanism of ovulation and ovarian degeneration
3.2.1 Ovulation mechanism
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Preparation for ovulation includes changes in the eggshell, the gradual dissolution of collagen fibers, changes in the structure of the ovarian epidermis and the accumulation of a certain fluid.... Oocytes with normal edema are more likely to ovulate than oocytes with low edema. When in contact with the oocyte, the gonadotropin activates the enzyme hyalurodinase, which dissolves hyaluronic acid on the oocyte surface, causing the oocyte to erode; on the other hand, in addition to its maturation effect, it indirectly stimulates steroid production to increase the secretion of fluid in the oocyte. Excessive secretion increases pressure, at which time the follicle shell is too thin, causing the follicle to rupture and the egg to ovulate (Le Xuan Tho and Le Xuan Cuong, 1979).
3.2.2 Mechanism of ovarian degeneration
When fish are exposed to unfavorable conditions, eggs can degenerate. Degeneracy can occur at various levels of oocyte development, from oocytes in the early stages of yolk formation to oocytes that have completed their growth (maximal growth).
Disruption of the maturation process under unfavorable conditions for reproduction usually occurs when the ovary moves from stage III to stage IV and from stage IV to stage V. Disruption of the maturation process will lead to the destruction of the germ cells.
for that spawning. The death and decomposition of oocytes can occur in several ways. Usually in degenerated oocytes, the nucleus first decomposes, then the radioactive membrane is broken down, and gradually the oocytes themselves are absorbed by the follicular cells. The yolk and the fat droplets absorbed in the follicular cells undergo a complete change and form bright orange substances. Therefore, degenerated oocytes appear to be bright in color, their shape is unstable due to the loss of part of the cell mass and the follicular walls begin to contract. Sometimes during the degeneration of oocytes, there is a phenomenon of water absorption and even a mixing of the yolk into a mixed mass (in this case, the dead oocytes are initially transparent).
In fish ovaries, when there is total degeneration, there are always oocytes remaining in the largest stage; sometimes there are oocytes in the vacuolar phase. When there is partial degeneration, the ovary still has oocytes that have not been damaged by the cortex and oocytes in the final stage of yolk formation.
Degeneration always begins asynchronously for all oocytes. The presence in the ovary of degenerating oocytes does not exclude the possibility of maturation of intact oocytes without shell injury, due to their relative independence. Thanks to this, parents who have just begun to degenerate can still participate in reproduction. Degeneration of a mature oocytes does not prevent the development of the next new oocytes.
4. Fertilization and hatching mechanism
4.1. Fertilization
In Lampetra (jawless fish) and many bony fishes, fish eggs secrete gamones that activate sperm and serve as chemical guides to move sperm toward the egg. Meanwhile, there are gamones that have the effect of immobilizing or coagulating sperm after fertilization of the egg. In addition, sperm also secrete androgamones that have the effect of reducing sperm activity to save energy and dissolving the eggshell mucus to penetrate the egg. In bitterlings, Acheilognathus and Rhodeus, sperm concentration and activity have been recorded in the micropore region of the eggshell.
The eggshell, also called the vitelline envelope or zona pellucida or zona radiata or chorion, is relatively tough with a
funnel-shaped micropyle at the animal pole. In the eggshell there is a protoplasmic membrane surrounding the cytoplasm (ovoplasm). In fish that lay sticky eggs, on the radiolucent shell there is often another shell formed to serve the purpose of sticking the eggs to the attachment (substrate - substrate). In some fish it is usually a jelly layer, and in other species it is filaments.
Fertilization requires the presence of small concentrations of Ca2+ and Mg2+ ions. In bony fish, fertilization is usually monofertilization: the micropores are very small and allow only one sperm to pass through. The chorion splits when the egg is activated by the sperm and a plug forms in the micropore, which then blocks the sperm. In polyspermy, which occurs in some cartilaginous fish, several spermatozoa penetrate the egg but only one fuses with the egg nucleus, while the others are probably absorbed and used as nutrients.
Separation of the chorion from the cytoplasmic membrane results in the appearance of the peri-yolk space. The chorion is permeable to water and small molecules, but larger colloidal molecules are retained in the peri-yolk space.
After fertilization, the chorion hardens and serves to protect the embryo in the early stages of development. The hardening of the chorion involves glycoproteins, Ca2+ ions and phospholipids under the action of hardening enzymes. Eggs and sperm can prolong their fertilization after leaving the parent fish. According to Yamamoto (1961), fish eggs lose their fertilization capacity after a very short time, but their fertilization capacity can be prolonged if they are kept in isotonic Ringer's solution for freshwater fish. Nikolsky (1963) estimated that sperm motility is very short in fish spawned in fast-flowing water, 10–15 seconds in Oncorhynchus. In slower-flowing water, sea bass sperm motility is 230–290 seconds and herring sperm motility is
can exercise for hours to days.
4.2. Expansion
After the embryo has developed to a certain stage, the larvae will break free from the egg shell and enter the aquatic environment. Hatching of fish larvae is influenced by many environmental factors, of which temperature and oxygen supply have a significant influence. Hatching is the result of physiological effects. Hatching is the result of softening of the chorion by enzymatic or other chemical substances from the ectoderm glands on the surface or from the endoderm glands in the pharynx. The development of the embryo is mainly
Catabolism produces waste products that increase osmotic concentration, leading to increased water absorption and increased pressure on the chorion. In addition, when the embryo begins to hatch, there is a lot of movement. Larval activity is often enhanced by increased temperature or light intensity or by decreased oxygen pressure, contributing to chorion rupture.
5. External factors affecting fish reproduction
5.1. Nutrition
Nutrition is a source of energy for the body to function and is also a source of raw materials for the development of the gonads, so nutrition greatly affects the maturation process of fish.
During the breeding season, the growth of fish almost stops. The energy absorbed from food is mainly for the development of the gonads and is stored for the period of not eating prey. For example, silver carp and grass carp in the north of our country, around the beginning of February, have a gonad maturity coefficient of about 3-5%, by April-May, the gonad maturity coefficient increases to 17-22%. Thus, in just a short time, a mass of nutrients equivalent to 14-17% of the fish's body mass is converted into gonad products. In the initial development of the gonads, it depends greatly on nutrients from the outside and then can rely on the energy accumulated inside the body. If the fish are well-bred and accumulate a lot of nutrients, they will have a higher maturity rate than fish of the same age but poorly bred.
The development of the gonads also depends on the quality of food. According to Chung Lan (1965), if grass carp broodstock are raised, in addition to plant food, if they are supplemented with food rich in protein, fat and vitamin E such as silkworm pupae, soybeans, rice germ, and oil cake, their relative fertility will increase by 2 times. In addition, the quality of food also affects the quality of reproductive products and fry later. For example, if roach is raised and deficient in vitamin B12 or cobalt, the female fish can produce eggs but cannot be fertilized and hatched.
5.2 Temperature
Fish are poikilothermic animals, so temperature is the environmental factor that most strongly affects metabolism, thereby affecting the entire reproductive process of fish. Each species of fish requires a certain total temperature to mature. For example, silver carp needs about 18,000-20,000 degrees per day, so the growth rate of fish is proportional to water temperature. The same species of fish living in low-temperature water often have different ages of maturity and times of maturity.
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longer than fish of the same species that live in warmer water. Each species of fish only spawns within a certain temperature range. For example, carp (temperate) at 17-18oC, crucian carp (temperate) at 20-22oC, silver carp at 25–27oC is best.
If the temperature is too low, the fish will not spawn, but if the temperature is too high, it will often affect the quality of the fry. Temperature also greatly affects the ovulation process. During the spawning season, if the temperature is too low, even though the gonads have reached the end of stage IV and the hypophysis has accumulated enough gonadotropin, the eggs will not spawn, and must wait until the temperature rises to a suitable temperature before ovulation begins. In artificial reproduction, low temperatures often prolong the effect time to induce ovulation. Inappropriate temperatures also affect fertilization and embryo development. If the temperature is too high, it often reduces the fertilization rate, hatching rate, and increases the rate of deformities.
5.3 Flow
Some fish mature well in conditions of flowing water; for example: silver carp, silver carp, silver barb, silver carp. Fish species that prefer flowing water such as silver carp, silver carp, silver carp in our country can spawn naturally in rivers. The natural spawning season coincides with the rainy and flood season. At that time, due to heavy rain and floods, the strong current and high water level stimulate fish to spawn naturally in the river. Doan Van Dau and Nguyen Van Hai studied the natural spawning of silver carp on the Red River and wrote: "The spawning ground is a place with complex conditions, the water flows rapidly and in a circle. When there is flood, the water flows in a circle with high velocity, and the high water level is a factor that stimulates the spawning process of silver carp". In addition to the flow factor, the authors also emphasized the rise in water level: "During spawning, each time coincides with a rise in water level; during the spawning period, if the water level drops or stands still, the fish stop spawning. There is no time when the water level drops and the fish spawn. Perhaps the rise in water is a factor that stimulates fish to spawn".
6.4 Light
The intensity of sunlight varies throughout the year, so this change can be considered as an activating factor for maturation and spawning. It is known experimentally that some fish respond to photoperiod (the length of the day), while others do not. According to Kuronuma (1968), by reducing the photoperiod of Pluoglossus altivelis, it was stimulated to spawn 2 months earlier than usual to take advantage of the time when there was plenty of food in the pond (at that time in autumn); conversely, increasing the photoperiod can make it spawn in February and April of the following year instead of spawning in the previous autumn. For Brachyraphis episcope, which spawns in the spring, light stimulates
Sexual maturity. Turner (1938) stimulated maturity by adding light, which made this fish spawn in winter. However, photoperiod does not affect the maturity of fish alone, but this process depends a lot on temperature and other factors. For fish spawning in autumn and winter, they can be stimulated by reducing the photoperiod. For fish spawning in spring, increasing the photoperiod is the stimulating factor. In addition, there are some factors such as: substrate, dissolved oxygen, the presence of fish of the opposite sex.
B. Questions and practice exercises: Question:
Question 1. Describe sexual maturity in fish. State the signs of mature fish. Question 2. Describe the mechanism of ovulation and egg laying. Apply this mechanism in artificial breeding.
Question 3. Describe the stages of development of fish ovaries.
Question 4. Describe the development stages of the ovary of a shrimp.
Exercise: Observe and identify the stages of ovarian development in fish.
C. Note: Signs of sexual maturity in aquatic animals and stages of ovarian development.
REFERENCES
* Main documents:
[1]. La Thi Noi, (2020). Lecture on aquatic animal physiology . College of Economics
- Bac Lieu Technology.
* Additional documents:
[1]. Nguyen Van Mui, (2010). Textbook of aquatic animal physiology . Hanoi University of Agriculture.
[2]. Textbook of fish and crustacean physiology . (2012). Ho Chi Minh City University of Forestry and Agriculture.
Bright.
CHAPTER 5. ENDOCRINE GLANDS
Introduce:
The characteristic of hormones is that in very small amounts they cause a very strong impact and bring about clear physiological effects. Hormones are generally difficult to quantify by chemical methods, so people often use biological methods to qualitatively and quantitatively identify them. The role of hormones is to participate in regulating physiological processes.
Target:
Describe the location and hormones of some endocrine glands in fish and crustaceans. Identify the location of some endocrine glands in fish and crustaceans.
State the role and application of endocrine glands in aquaculture.
Main content:
1. Endocrine glands in fish
1.1. General Concept
In vertebrates, glands are divided into two types: - Exocrine glands: are glands with ducts, secretory products are released to certain locations.
These exocrine products may have some biological activities such as gastric juice, pancreatic juice and intestinal juice which help digest food but can also be just waste like sweat. - Endocrine glands: are glands without ducts, the secretory products are released directly into the blood and through the circulatory system to the organs that produce the excitatory or inhibitory effects.
The products of endocrine glands are called hormones and the organs affected by hormones are called target organs.
There are hormones that only affect a certain organ, but there are also hormones that affect many different organs in the body; for example, the pituitary gland has the hormone TSH (thyroid stimulating hormone) that only affects the thyroid gland, the pituitary gland has the hormone GH (growth hormone) that affects different organs. There are hormones that support each other, but there are also hormones that inhibit each other; for example, the hormone insulin of the pancreas has the effect of reducing blood sugar and the hormone glucagon of the pancreas has the effect of increasing blood sugar. Among the hormones, which species have hormones that only affect
on that species, this property is called species specificity. The non-specificity of hormones is that hormones of one species can act on many other species. The activities of endocrine glands are all controlled by the central nervous system, so hormones are often considered chemical collaborators, along with nerve activities, regulating all physiological processes of the body.
1.2. Thyroid
1.2.1. Location:
In teleost fish there is ample evidence that the distal part of the hypothalamus contains a thyroid-stimulating hormone and that removal of this gland results in a reduction in thyroid size and function, and that hypothalamus glands transplanted to a site more distal to the hypothalamus may secrete more TSH than the normal site.
1.2.2. Thyroid function and thyroid hormone synthesis
The function of the thyroid gland is to produce thyroid hormones. Typically, the thyroid hormones of fish are relatively small molecules and their names are the same as in all vertebrates: triiodothyronine (T3) and tetraiodothyronine (T4) or thyroxine.
1.2.3. Effects of thyroid hormones
In fish, thyroid hormones primarily affect metabolic activities, secondarily affect structure, and thirdly affect the central nervous system and behavior.
- Metabolism
- Growth
- Nervous and behavioral.
1.3. Endocrine Pancreas
The pancreatic islets of bony fish play an important role in protein synthesis and, in particular, in insulin biosynthesis. All insulins contain 51 amino acids (MW: 12,000) consisting of two polypeptide chains (chain A has 21 aa and chain B has 30 aa) linked by two disulfide bridges (SS). Glucagon is a linear polypeptide chain of 29 amino acids. Metabolic pathways of pancreatic islets As in the liver, glucose diffuses freely into the pancreatic islets of bony fish and mammals.