CATDOLL : CATDOLL: How many kinds of fish are there in the world? Please name them.

There are about 20,000 species of fish in the world. Among them, there are about 73 species of Cyclostomes, about 800 species of Chondrichthyes, and about 20,000 species of Osteichthyes. There are about 3,000 species of fish in my country. Among them, there are about 2,100 species of marine fish. There are about 1,010 species of freshwater fish. Common freshwater fish species! my country has abundant freshwater fish resources, and with artificial breeding, the market supply is sufficient. Among them, carp, silver carp, grass carp, and black carp are the most common, in addition to eels, turtles, etc. Carp is a kind of good fish among freshwater fish. Its scales are white with metallic luster, red tail, tender meat, and delicious taste. The Yellow River carp is particularly popular. Grass carp is also called grass green. It has a tea yellow body color and is the most produced freshwater fish. This fish is characterized by fast growth, heavy weight, big head and fat meat, but the meat is coarse and the quality is inferior to carp. Silver carp is divided into two types: silver carp and silver carp (commonly known as fathead fish). Silver carp has a whitish body, small scales, a large head, fat meat, and delicious taste. The head is the fattest, and it is especially suitable for making casserole fish heads. Crucian carp, also known as crucian carp, has a flat and wide body, a prominent back bulge, and small scales. It is characterized by tender meat and great freshness, but has many small thorns. Crucian carp is most suitable for making soup. Small crucian carp is suitable for making crispy fish. Black carp, also known as black fin, has a long, cylindrical body, a black spine, a milky white belly, and white and full meat. It is a kind of freshwater fish with tender meat; it contains more fat, especially the meat of a section of the pectoral fin and the meat of the head and tail. The lung of black carp is the most tender part of the fish body and contains a lot of fat. The famous dish "burned bald lung" is made with black carp lung. It is cooked and eaten hot, and it is extremely plump and delicious. Eel, also known as long fish and yellow eel, has a long and thin body, a thick head and a thin tail, a dark brown back, a yellow belly, small eyes and no scales. The meat of this fish is extremely tender and delicious, and is regarded as a top-quality fish. The soft-shelled turtle, also known as the turtle fish, has tender meat and rich nutrition. It is a high-end tonic. Ornamental fish refers to those fish with bright colors or peculiar shapes that have ornamental value. There are usually three major strains, namely temperate freshwater ornamental fish, tropical freshwater ornamental fish and tropical marine ornamental fish. Temperate freshwater ornamental fish mainly include red crucian carp, Chinese goldfish, and Japanese koi. There are three major series of tropical freshwater ornamental fish, one is the lantern species, the second is angelfish, and the third is arowana. Marine ornamental fish mainly come from the coral reef waters of the Indian Ocean and the Pacific Ocean. There are many varieties and strange colors, with a primitive, simple and mysterious natural beauty. Etc.

Fish are the oldest vertebrates. They live in almost all aquatic environments on Earth, from freshwater lakes and rivers to saltwater seas and oceans.

There are about 26,000 species of fish in the world. Two-thirds of them live in the ocean, and the rest live in fresh water. There are 2,500 species in China, of which more than 100 can be used for medicinal purposes. Common medicinal animals include seahorses, sea dragons, eels, carp, crucian carp, sturgeons (swim bladder is fish swim bladder glue), large yellow croaker (otolith is fish brain stone), sharks, etc. In addition, they are often used as raw materials in the pharmaceutical industry. For example, the liver of cod, shark or ray is the main raw material for extracting cod liver oil (vitamin A and vitamin D). Hydrolyzed protein, cytochrome C, lecithin, cephalin, etc. can be extracted from various fish meats. The liver and ovaries of tetrodotoxin contain a large amount of tetrodotoxin, which can be extracted to treat neuropathy, spasms, tumors and other diseases. The bile of large fish can be used to extract "bile pigment calcium salt", which is the raw material for artificial production of bezoar. The differences between the various classes of fish are as great as those between the various classes of terrestrial vertebrates. Fish are generally considered to be aquatic animals with smooth, spindle-shaped, streamlined bodies, fins, and gills for breathing, but many species do not meet this definition. Some fish have extremely long bodies, while others are extremely short; some are laterally compressed, while others are flat; some have large or complex fins, while others are degenerate or even digested; the shapes and positions of the mouth, eyes, nostrils, and gill openings vary greatly; some fish breathe air and will drown if immersed in water. Fish are an important food for humans. Overfishing, pollution, and environmental changes can all damage fish resources. Fish prey on mosquito larvae, which helps control mosquito-borne diseases such as malaria. Fish are important experimental animals in behavior, physiology, ecology, and medicine. Many fish are kept for ornamental purposes, and many species are game fish. Fish range in length from less than 10 cm (0.4) to more than 20 meters, and weigh from about 1.5 grams to about 4,000 kilograms. The body color is often consistent with the environment and has a concealing effect. Some fish have bright colors and stripes, which are of identification significance. Some fish can expand and contract pigment cells to change their body color, and some fish can glow.

Fish are a type of aquatic, cold-blooded vertebrate that breathes with gills and has jaws and fins. Existing fish can be divided into two main groups: cartilaginous fish (such as sharks) and bony fish (fish with thread-fins and wavy fins). Both groups of fish first appeared in the Early Devonian. The more advanced group of thread-finned fish, called bony fish, began to evolve in the Jurassic period and have become the most numerous fish today. There are also several extinct fish.

Fish have been with humans for more than 5,000 years and have formed an indissoluble bond with humans. They have become an extremely important food and ornamental pet in human daily life. However, few people know what animals are "fish" and how to define fish. With the development of science, people's definition of fish has also changed a lot.

Nearly 500 million years ago, a major leap forward occurred in the history of life on Earth. The earliest fish-like animals appeared, marking the beginning of the history of vertebrates, which led to the development of the animal kingdom and ushered in a new historical stage. True fish first appeared more than 300 million years ago. Throughout the long history, a large number of fish have long since died out with the passage of time. The fish that survive on Earth today are only a very small part of the species that appeared and evolved later.

Humans have been able to identify species and give them names since a long time ago. The term "fish" usually refers to all animals in water, so many animals living in water are given the name of fish, including whales, seals, giant salamanders, cuttlefish, squids, octopuses, starfish, jellyfish, sponges, lancelets, etc. Which aquatic animals are the real "fish"? The classification of "fish" has different definitions in different eras.

Two thousand years ago, the ancient Greek philosopher Plato defined fish as follows: "This kind (fish) is created by completely ignorant and unconscious things. The Lord of Transfiguration thought it was no longer worthwhile to give pure breath to this kind, because they are the offspring of various sins and have unclean hearts. The Lord of Transfiguration threw them into the water and made them pass through the deep mud to breathe the magical and pure air. This is how fish and oysters and all other aquatic animals are separated far away as a punishment for their great sin of ignorance." Plato's view is full of creationism. Due to the development of modern science, this view has long been completely denied.

There are about 20,000 species of fish on Earth. How to classify and distinguish them is both a rigorous task involving biological taxonomy and a fascinating topic.

We know that the levels used in modern taxonomy (including the classification of fish) are mainly phylum, class, order, family, genus, and species. If necessary, some additional levels can be added, such as subphylum, superclass, subclass, superorder, suborder, superfamily, subfamily, subgenus, etc. A certain organism exists as a species, and it is not artificially classified. There are all kinds of organisms in nature. In most cases, there are clear boundaries between species, and species exist in the form of populations. There is reproductive isolation between different species.

Generally speaking, there are three specific ways for biological evolution: one is the differentiation of one group into two groups with little difference; the second is the specialization in a certain direction of an individual, which causes a large change in some aspects of morphological structure; the third is from low to high, from simple to complex, the so-called "complex evolution". It should be pointed out that the evolution of organisms is intertwined with each other, and also includes specialization and degeneration. Therefore, in classification, the first way is usually represented by subspecies, species, and genus, while some genera, families, and orders are consistent with the second way, and some orders, classes, and phyla are consistent with the third way. When classifying organisms, we must arrange the natural system according to the natural situation and in accordance with the actual situation.

There are two ways to classify fish. One is to use one or several characteristics of the external morphology and habits of fish as the classification standard, without involving kinship, and without considering the basic structure and evolutionary relationship of fish. This is based on people's subjective opinions. The other is to classify based on the knowledge of fish morphology, ecology, physiology, occurrence, fossil evolutionary relationship, etc. This is natural classification. With the development of science and technology, some new methods have emerged in taxonomy. Such as cell classification, chemical classification, molecular classification, etc.

In 1844, Muller first classified fish as a class of vertebrates, which was divided into 6 subclasses and 14 orders. After that, Regen, Goodrich and Jones used their own methods to classify fish. In 1955, Berg divided living and ancient fish into 12 classes and 119 orders in his book "Modern and Fossil Fish-Shaped Animals and Fish Taxonomy". Each class, order and family had a characteristic description. In 1966, Greenwood, Rosen and others divided teleost fish into 3 major categories, 8 superorders, 30 orders and 82 suborders based on embryonic development, whether juvenile fish were metamorphosed, and internal morphological anatomy. In 1971, Lars divided fish into the class of Chondrichthyes and the class of Osteichthyes. In 1994, Nelson made a more systematic classification of fish. In his book "World Fishes", he classified fish more completely based on the principles of osteology, phylogeny, embryology, morphology, comparative anatomy, paleontology and comparative biochemistry.

At present, the world's marine fish are divided into the subphylum Cephalochordata and the subphylum Vertebrata. In the subphylum Cephalochordata, the fish species have a notochord and neural tube that runs through the whole body and are retained throughout life. They have no head and no spine. They have no cartilage or hard bones, and their heart is a beating abdominal blood vessel. They have no red blood cells: they have a hepatic caecum, and their muscles are segmented: their epidermis is composed of a single layer of cells. They have many gill openings that open into the peribranchial cavity. The pronephric ducts are segmented, and the primitive common ducts open separately, with inner columns. They have no true brain, but have two pairs of cerebral lobes and nerves, and the upper and lower branches of the spinal nerves are not connected. The gonads are segmented, and there is no fossil record yet. Fish with these characteristics can be named under the subphylum Cephalochordata sequence.

Currently, only lancelets belong to this subphylum. The fish of the subphylum Vertebrata are divided into: Jawless, Hagfish, Cephalomorpha; Jaws, Chondrichthyes, Holocephalians, Elasmobranchs, Sarcopterygii, Coelacanths, Porites and Lungfishes, Actinopterygii, Chondrostegoscelis, Neopterygii, etc. The biggest feature of the fish belonging to the jawless class is that they have no jaws. There are 2 families, 12 genera, and 84 species in the world; the marine fish of the jawed class first appeared in the acanthids in the early Silurian. It also includes Chondrichthyes (divided into 2 subclasses, 13 orders, 45 species, 170 genera, about 846 species), Sarcopterygii, Actinopterygii (2 subclasses, 4 subgroups, 9 orders, 42 orders, 431 families, 4075 genera, 23681 species).