The organs of movement of all fish are fins. They are paired - thoracic and abdominal and unpaired - dorsal, anal and caudal. The body of the fish is protected by scales, which are bone plates that overlap each other like tiles. To reduce friction on water, the scales are covered with mucus.
The two-chamber heart of fish, consisting of the atrium and the ventricle, pumps blood through its vessels. The central nervous system is formed by the brain and spinal cord passing through a canal formed by processes of the vertebrae.
Fish breathe gills.
The lateral line as a sensory organ is present in the Majority of the jerk. In the shark, it consists of the thinnest channels that lie almost on the skin of both sides of the body. The shafts of the nerve ganglia stretch along the entire body of the shark, from which structures resembling hairs enter the cavity of the canals filled with fluid.
450 million years ago a huge number of various fish appeared on Earth. Among them were fish with real jaws and jawless, in scales and in bone armor. Fish were the first creatures that have jaws. Fish, presumably, are the ancestors of all quadrupeds.
Cartilage and bone
Fish are divided into two large classes: cartilaginous and bone. The main criterion for such division is the substance that makes up the internal skeleton of fish: cartilage or bone. Unlike bony, cartilaginous fish do not have a swim bladder. Therefore, in order not to sink to the bottom, the cartilage fish must be in constant motion.
First, the female lays its eggs, and then the male must water it with a special liquid, milt, so that life begins in the eggs. All this fish do in certain places - spawning grounds. Some spawning fish travel from the sea to the rivers or, conversely, from the rivers to the seas.
Fish breeding features
“How do fish breed?” Is a question that cannot be answered unambiguously. All types of fish breed in different ways and at different ages. In most cases, reproduction occurs sexually. However, some species are characterized by parthenogenesis and genogenesis - methods of reproduction without fertilization by the male.
What is parthenogenesis and gynogenesis?
Parthenogenesis is one of the breeding methods in which fertilization by the male is not required. Eggs develop until the moment of crushing, after which, adjacent to the spawning mounds with fertilized eggs, they begin their development in this way. Only from fully developed eggs do fry appear. With this method fish manage to save all egg laying, avoiding rotting of eggs. Parthenogenesis breeds the following species of individuals:
- carp and some other species.
It is worth noting that the majority of eggs, developing by parthenogenesis, still die, only a small part lives to the hatching stage. Only in the Issykkul chebachka such a method of reproduction leads to a large number of viable offspring.
Genogenesis is the method of giving birth to fry without being fertilized by males of this individual. With this method, only female fry are born. Fertilization of eggs occurs due to sperm males of other species of fish that penetrate the eggs, being nearby. After fertilization has taken place, the egg begins its development.
As a rule, many species of fish are same-sex. However, among such individuals there are species called hermaphrodites. Hermaphrodites are individuals capable of developing both spawn and sperm. Although such individuals cannot produce self-fertilization their eggs, since their sex products (eggs or sperm) mature not at one time, but in turn. For example, a red pagell can change its sex throughout life: young pagella individuals, as a rule, are females, as the fish grows older, the ovaries change to testes.
This ability - hermaphroditism, is also inherent in herring, salmon, carp, perch species.
Fertilization of eggs also occurs in a different way:
Most fish species breed outside the method of fertilization, in which the eggs are fertilized while in the water. Internally, cartilaginous plants breed, as a rule, although this type of increase in the population is also characteristic of bony (sea bass, eelpout) and carpsong (swordtails, guppies, gambusia) species of individuals.
Depending on the development of eggs, all types of fish are conventionally divided into the following categories:
Oviparous development is the most common and is the deposition of eggs (eggs) directly into the water. With the egg-borne method, fertilized eggs are attached to the posterior part of the oviduct of the female there continues to develop until the birth of the fry. In a word, at the moment of sweeping, the eggs break and fry are born.
The egg-breeding method of reproduction is applicable to the majority of cartilaginous fish species: katranu, white shark, fox shark, pylonos and others. Some species, such as the stinging tail, on the walls of the posterior part of the oviduct acting as the uterus, have special outgrowths, thanks to which they can feed their young with a nutrient fluid.
The vivifying method consists in the formation of a structure in the back section of the oviduct in the female resembling the placenta of mammals. It is necessary in order so that the mother can feed the embryo nutrients. Live birth is inherent in some species of deep-sea sharks.
The main advantage of the egg-breeding and live-giving methods is to achieve the greatest survival of the young, due to the intrauterine development.
By the breeding pattern, all fish are divided into the following groups:
Monocyclic species are characterized by the only reproduction during life. With a single spawning all the eggs of the individual may die. River eels, Pacific salmon, river lamprey, Baikal golomyanka and some other fish are called monocyclic, due to the single multiplicity of reproduction.
But, most species breed several times throughout their existence, so they belong to polycyclic individuals.
The age of puberty in all species of individuals is also different. At the same time, the interval from the earliest to the late puberty in fish is significant. Thus, small species of individuals, such as gambusia and others, can reach puberty within 1 to 2 months after birth. Sturgeon, in turn, reach sexual maturity only in 15 - 30 years.
Fish with a short life cycle develop faster, respectively, reach puberty earlier. In individuals with a longer life cycle, puberty occurs much later. In Atlantic cod, for example, puberty occurs at 8 - 9 years. Sea bass matures only closer to 15 years.
But the age of sexual maturity of individuals is influenced not only by the life cycle and species. Large role for the age of maturation play fish habitat and their food. The better and more abundant the individual eats, the faster it reaches the required size, which means that its sexual maturity comes faster. Males of most species of fish faster than females reach puberty.
Great importance to the speed of sexual development of fish have the climatic features of their existence. So, for example, bream living in the Aral seas, ripen closer to 3 years, in the Northern Caspian - closer to 6, and individuals inhabiting the Middle Volga reach sexual maturity at the age of seven.