Unlike some animals, fish can tell the male from the female at a glance. However, some species can be identified by some external morphological characteristics as long as they are carefully observed. 1. For most fish of the same age, females are usually bigger than males. This difference is due to the earlier maturity and shorter life span of male fish, which can ensure the higher fecundity of the population. This difference is not obvious in some species, such as carp and crucian carp. The male fish is only a few centimeters smaller than the female fish. Some species are very different, very obvious. Anchovy (commonly known as anchovy) is a small fish with unique flavor that people in China like to eat very much. Generally, the female fish is about 16cm long, while the male fish is only 10cm long. The mesh size of gill nets used by fishermen for fishing is determined by their ability to catch female fish. Among the big mouth fish, the female fish can reach 27cm, and the male fish is only 4cm. Swordfish is called swimming champion. Almost all swordfish under 60kg caught in the northern hemisphere are males, while females are above 100kg. The sex difference of Jikang eel is more obvious. The female fish can reach 45kg, and the male fish is less than 1.5kg, with a weight difference of 30 times. Perhaps the biggest difference in the body length between the sexes of fish is the horn. The female body length is about 1.5m, while the male parasitized on it is only110 or even 1%. So that for a long time, people regarded them as two completely different fish. On the contrary, there are some species, such as Pelteobagrus fulvidraco, pike, cod, tilapia and so on, but the male fish is bigger than the female fish. This phenomenon only occurs in species where males have the task of protecting their offspring. 2. There are several head shapes that can be used to distinguish men from women. There is nothing special about the shape of the female fish head, but the back of the male fish head protrudes very high, and the front looks straight up and down from the side, like a square head. Silurus meridionalis, which feeds on organic matter and benthos in mud, not only has different nose and mouth shapes, but also has different numbers and shapes of bristles on its head. Male fish are plentiful, thick and fleshy. The sex difference of some flounders is reflected in the distance between their eyes. Among them, the distance between the eyes of catfish and the eyes of fish is much wider than that of females. 3. The whitebait with a long beard is a slim and transparent small fish, and the whitebait in Taihu Lake in China is more famous for its delicious meat. The male whitebait has a row of scales above the anal fin, but the female does not. The fin in front of the gluteal fin of Pistacia chinensis and Elk is obviously prolonged. Many fish can distinguish sex by the difference of pectoral fins. The male pectoral fin of Japanese eel and carp is sickle-shaped, with long and sharp ends, while the female is round and blunt. Misgurnus anguillicaudatus, on the other hand, is round at the end of the female pectoral fin, while the male is pointed. ? ? ? ? Many species of male fish in the family have slender fins on both pectoral fins and ventral fins. Swordtail fish is a tropical fish and a good ornamental fish. The lower leaf of the male caudal fin extends like a sword, which is completely different from the common shape of the female caudal fin. The sex difference of swordfish, which belongs to the same family as swordfish, is mainly manifested in the dorsal fin. The dorsal fin of a male fish is not only like a sail, but also has beautiful patterns like eyes. 4. Body color Generally speaking, the body color of male fish is much more beautiful than that of female fish. The female is grayish green, while the male has all kinds of beautiful spots. Some species of arowana, the male's body and fins are yellow or orange, mixed with red, and there are five or six blue bands behind his eyes; The female has no beautiful stripes, and her body color is red. Siamese fighting fish are beautiful in all shapes and sizes. The male fish's fins are very big, just like colorful skirts, flying high. The body color is generally reddish brown, and the whole body is covered with colored stripes. The golden light flashed from beginning to end, and the whole fish body was like a gorgeous pattern. Compared with the male fish, the color of the female fish is much inferior. 5. Sexual characteristics Many fish have obvious sexual characteristics, making it easier to distinguish between males and females. There is a common pomfret in fresh water in China, which is a small fish. The female fish has a 1 long oviposition tube under her abdomen. Plate fish such as sharks and rays. Male fish has 1 pair of organs for transporting sperm on its ventral fin. Among the salamanders, many kinds of male fish have 1 complex mating apparatus evolved from the gluteal fin. In some species, during the breeding season, the whole body or a certain part of the male fish will appear "idolization", which is a white and hard cone, which is the result of special hypertrophy and keratinization of epidermal cells. Different species have different distribution parts of idols. Fragrant fish and yarrow fish are all over the body; Goldfish are divided into gill cover, pectoral fin, anal fin and dorsal fin; The four major fishes (herring, grass carp, silver carp and bighead carp) in China are distributed on pectoral fins. Fishery scientists and technicians can distinguish males from females by touching pectoral fins when artificially breeding domestic fish to promote greening. This is because the female fish's pectoral fins are very smooth, while the male fish is particularly rough and even feels prickly. Gonadal characteristics