Mariana Trench

Only 1.2% of the world's oceans have a depth greater than 6,000 meters. Among them, the Mariana Trench, located east of the Mariana Islands in the western-central Pacific Ocean, is a very famous trench. The tower extends 2,850 kilometers from north to south, and is only 70 kilometers wide. It is an almost vertical cliff, deeply cutting into the bottom of the sea. This trench is estimated to have formed over 60 million years ago.

In 1957, a marine research ship "Fichaz" of the Institute of Oceanography of the Soviet Academy of Sciences conducted a detailed exploration of the Mariana Trench, and used an ultrasonic detector to detect it in its southwest on August 18. The Ministry discovered a particularly deep abyss, located at 11°20.9?0?7 north latitude and 142°11.5?0?7 east longitude. The maximum depth reaches 11,022 meters, which is the deepest place in the known world ocean so far.

According to investigation. There are currently 21,723 species of fish, including about 12,000 species of marine fish. The total number of marine fish species in China is 3,187. The classification of these fishes is generally divided into three classes.

First, Cyclostomata is the most primitive fish and has no upper and lower jaws, so it is also called jawless. The body surface is naked and scaleless, the body is slender and eel-shaped, and the bones are completely cartilage. No fins, no shoulder straps and belt, notochord exists throughout life, no vertebrae. It has separate unpaired nostrils and gills formed by endoderm in a muscular sac that opens outside the body.

The second is Chondrichhyes. The endoskeleton is fully composed of bones, but is often reinforced by calcification and does not contain any real bone tissue: the body surface is covered with shield scales, spines or is exposed without scales. The brain has no seams. There are gill slits on each side of the head, opening outside the body. The intestine is short and has spiral valves. No fat. Males have specialized mating organs called flippers on the inside of their pelvic fins. Perform internal fertilization. Oviparous, ovoviviparous or viviparous, with a crooked tail, cartilaginous fish are widely distributed in the ocean, but mainly in low latitude waters.

The third is Osteichthyes. It is the most advanced class of fish and the most prosperous class now. The endoskeleton appears ossified, the head floats and often has membranous bones, and the bones have sutures. The body surface is covered with hard scales or bony scales, or is exposed without scales. There is one pair of external gill openings, the gill space is degraded, the gill filaments are supported by double rows of gill rays, usually with fat, the fin rays are multi-segmented, and there is no spiral valve in the intestine. Some fish have dorsal and ventral ribs and solid otoliths. Usually in vitro fertilization, no cloaca. There are currently about 18,000 species of bony fish in the world.

The body shapes of marine fish can generally be divided into: ①Torpedo type. Fish of this type live in the middle waters and are best at swimming, such as mackerel, mullet, tuna, etc. ②Arrow shape. Similar to the torpedo type, but the body is more elongated, the odd fins are moved backward, inhabit the surface water, and are good at swimming, such as pike, needlefish, etc., ③ flat side type. Fish of this size have increased dorsal and ventral axis height, and extremely flat left and right sides. They can be divided into spot fish type, sunfish type and flatfish type, which inhabit near the bottom and bottom respectively. ④Snake type. This type of fish has a slender body and an almost circular cross-section. It usually inhabits seabed plants, such as eels, sea dragons, etc. ⑤Belt type. The body height is elongated into a side-flat type, which is not good at swimming, such as hairtail, oarfish, etc. ⑥Spherical shape. The body of fish of this size is almost spherical, and the tail fin is generally underdeveloped, such as box fish, some round-fin fish, etc., ⑦ vertically flat type. This type of fish has a reduced dorsal and ventral axis and a flat body, such as various rays, anglers, etc.

The fins of fish are the main movement and balance organs of fish. Paired fins are called even fins, including pectoral fins and pelvic fins; unpaired fins are called odd fins, including dorsal fins, anal fins and tail fins. The caudal fin is born at the end of the tail of the fish and has the functions of turning and pushing. It is generally divided into round tail (the upper and lower lobes are equal in size, such as mackerel, tuna, etc.), and crooked tail (the upper lobe is more developed than the lower lobe, which helps in upward movement). , such as sharks), orthomorphic tail (the lower lobe is more developed than the upper lobe, which helps in downward movement, such as flying fish).

There are three types of reproduction methods of fish, namely oviparous, ovoviviparous and viviparous. The vast majority of fish are oviparous.

Oviparous fish generally discharge mature eggs directly into the water for external fertilization and complete the entire development process. However, there are also a few fish (such as some sharks) that are fertilized internally, and the fertilized eggs still develop outside the body. Ovoviviparous fish generally have eggs fertilized in the body, and the fertilized eggs develop in the body, but the nutrition of the embryo relies on its own yolk supply and has nothing to do with the mother, such as white-spotted starfish, white-spotted dogfish, Japanese shark, and Xuya Plowshare ray, sea crucian carp and black (fish king) etc. For viviparous fish, the eggs are usually fertilized and developed within the mother's body. The embryo formed by the fertilized eggs is connected to the mother's body through blood circulation. Its nutrition not only comes from the yolk of the body, but also needs to be supplied by the mother, such as gray star sharks. The number of eggs laid by fish is much higher than that of terrestrial vertebrates. The number of eggs they lay varies greatly depending on the species. For example, it ranges from laying a few large eggs (such as many kinds of sharks) to laying 300 million floating eggs (such as a car that rolls over). fish). Generally, fish that do not protect their eggs after spawning will lay larger eggs, such as sea bream, which lays about 1 million eggs, and eels, which lay 7 to 15 million eggs. Fish that protect their eggs after spawning will lay fewer eggs, such as Seahorses lay dozens to hundreds of eggs.

The growth rate and individual size of fish at each stage are very different. Research shows that the smallest individual fish is the microgoby, with a body length of only 7.5-11.5 mm; the largest can reach 20 meters, such as the whale shark. The most rapid period of fish growth is usually before sexual maturity. After that, most of the food consumed by fish is used for maturation of sexual products and fat storage, and only a small part is used for growth in length. Therefore, the growth slows down and reaches the senescence stage. , length growth almost completely stops. The growth rates of various fish species are also very different. Some fish can grow to the same size as their parents in one year after hatching, while others may take many years to grow to the same size as their parents. The lifespan of fish also varies depending on the species. Some species of the Gobiidae and Lanternidae families live less than a year, while some Sturgeon fish can live to be more than 100 years old. The maximum age of the large yellow croaker produced in the coast of Zhejiang, my country has been found to be 29 years old, and the maximum lifespan of the Atlantic croaker is 23 years old. Some fish species, such as salmon and European eels, die after their first sexual maturity and spawning.

Due to different species, fish also show great differences in feeding. If divided according to the nature of the food they eat, fish can be divided into three types: herbivorous fish, carnivorous fish and omnivorous fish. Herbivorous fish feed mainly on phytoplankton, such as milkfish, barracuda, bluefish, etc.; carnivorous fish generally feed on animals, and most fish in the ocean are carnivorous fish, such as hairtail and rockfish. Spot fish, large yellow croaker, whale shark, basking shark, etc. Omnivorous fish refer to eating two or more types of food with different properties, including animals and plants, and also eat underwater humus, such as spotted bream, leaf trevally, etc. The different feeding habits of fish directly affect their exercise intensity. Generally, carnivorous fish have high exercise intensity and violent nature, while herbivorous fish have weak exercise intensity and mild temperament. The difference in feeding habits and the strength of exercise intensity also directly affect the meat quality of the fish.

Marine fish are friends of mankind and their favorite food. Humans must provide opportunities for fish to recuperate, protect the ecological environment of the ocean, and create a good natural environment for fish to survive.