CATDOLL : CATDOLL: How to tell if a fish is male or female

Unlike some animals, fish cannot be identified as male or female at a glance. However, for some species, if you observe carefully, you can still use certain external morphological characteristics to identify their gender.

1. Body length

In most species of fish, females are generally larger than males of the same age. This difference is due to the fact that males mature earlier and live a little shorter, which ensures a higher fertility of the population.

This difference is not very obvious in some species, such as carp and crucian carp, where male fish are only a few centimeters smaller than female fish. But in some species, the difference is very large and very obvious. Anchovy (commonly known as anchovy) is a small fish with a unique flavor that Chinese people like to eat very much. Female fish are generally about 16cm long, while male fish are only 10cm long. The size of the mesh of the gill net used by fishermen is determined by the size of the mesh that can catch female fish. Among large-mouthed fish, the female of the strange-spined fish can reach a body length of 27cm, while the male fish is only 4cm. Swordfish is known as the swimming champion. In the northern hemisphere, swordfish caught below 60kg are almost all male fish, while female fish are all over 100kg. The difference between the sexes of conger eels is more obvious. Female fish can reach 45kg, while male fish are less than 1.5kg. The weight difference is 30 times. The fish with the largest difference in body length between the sexes may be the horn eel. The female is about 1.5 meters long, while the male parasitic on it is only 1/10 or even 1% of the female's length, so that for a long time, people regarded them as two completely different fish.

On the contrary, there are some species, such as yellow catfish, cod, cod, tilapia, etc., where males are larger than females. This phenomenon only occurs in species where males have the task of protecting offspring.

2. Head shape

In some species, the shape of the head and the shape and position of the head organs can be used to distinguish the sex of the species.

The head shape of the female loach, which feeds on flying fish in the ocean, is not special, but the dorsal part of the male head is very high, and the front part is straight up and down when viewed from the side, like a square head. The male catfish, which feeds on organic matter and benthic organisms in the mud, is different not only in the shape of the snout and mouth, but also in the number and shape of the bristles on the head. The male has more, thicker and more fleshy bristles. The difference between the sexes of some flatfish is reflected in the distance between the eyes. Among them, the distance between the eyes of male flounders and fish is much wider than that of females.

3. Fin shape

Silverfish are small fish with slender and transparent bodies. The silverfish from Taihu Lake in my country are famous for their delicious meat. Male silverfish have a row of scales above their anal fins, but females do not. The fin rays in front of the anal fins of male fish, such as eels and carps, are significantly extended. Many fish can be distinguished by the difference in their pectoral fins. The pectoral fins of male eels and carps are sickle-shaped with long and pointed ends, while those of female fish are round and blunt at the ends. The opposite is true for loaches, where the pectoral fins of females are rounded at the ends, while those of males are pointed. The males of many species of the family Cobitidae have extended fin rays on their pectoral and pelvic fins. Swordtails are tropical fish and are good ornamental fish in aquariums. The lower lobe of the male's tail fin is elongated, like an outstretched sword, which is completely different from the normal-shaped tail fin of the female. The difference between the sexes of the sailfish, which is in the same family as the swordtail, is mainly reflected in the dorsal fin. The male fish's dorsal fin not only looks like a sail, but also has beautiful eye-like patterns on it.

4. Body color

Generally speaking, male fish are much prettier than female fish. Female rainbow killifish are grey-green in color, while males have a variety of beautiful spots. In some species of wrasse, the male's body and fins are yellow or orange with red in between, and there are five or six blue stripes behind the eyes; females do not have the beautiful stripes, and their body color is red. Siamese fighting fish are particularly beautiful in both appearance and color. The male's fins are particularly long, like a colorful skirt, floating like a fairy. The body color is generally brown-red, with colorful spots all over the body, and golden light flashes from head to tail, and the whole body of the fish is like a gorgeous pattern. The female's body color is dull, and it looks much inferior to the male.

5. Sexual characteristics

Many fish have obvious sexual characteristics, and it is easy to distinguish between males and females. There is a common small fish called mullet in my country's freshwater waters. The female fish has a long ovipositor hanging under its belly. In elasmobranch fish such as sharks and rays, the pelvic fins of male fish have a pair of organs for transporting sperm - fin feet. Among killifish, many species of male fish have a complex copulation organ evolved from the anal fin. In some species, during the reproductive season, "stars" will appear on the whole body or a certain part of the male fish. They are white and hard cones. This is the result of the extra hypertrophy and keratinization of epidermal cells. The distribution of star-chasing varies from species to species. Sweetfish and yarrow are all over the body; goldfish have them on the gill cover, pectoral fin, anal fin and dorsal fin; my country's four major carps (black carp, grass carp, silver carp, bighead carp) have them on the pectoral fins. When fishery technology personnel are artificially breeding domestic fish, they can distinguish between males and females by touching the pectoral fins. This is because the pectoral fins of female fish are very smooth, while those of male fish are particularly rough and even feel prickly.

Gonadal characteristics

For some fish, if the male and female cannot be distinguished from the external morphology and sexual characteristics, then the only way to distinguish them is to observe the gonads with the help of anatomical means. The gonads of most fish are paired, and only a few fish, such as the yellow eel, have single gonads. The ovaries of fish are green, yellow, orange, red and other colors, and are usually thin and elongated. When the reproductive season is approaching, the ovaries will swell very large, filling most of the body cavity, and the eggs inside can be seen. During the reproductive season, gently press the fish's abdomen. If eggs flow out of the reproductive opening, the fish is female. The testes of male fish are milky white and much smaller in size than the ovaries. Behind each testis, there is a short vas deferens that leads to the reproductive opening. During the reproductive season, gently press the fish's abdomen. If milky white semen flows out of the reproductive opening, the fish is definitely a male.

Hermaphroditism and sex reversal

As far as we know, hermaphroditism has not been found in cartilaginous fish. Among bony fish, herring, cod, yellow bream, flounder and other fish have this phenomenon. In the Perciformes, there are several fish that are permanent hermaphrodites. The only fish that can fertilize themselves is sea bass. It is said that there is a small hermaphroditic grouper in the Percidae family in the Gulf of Mexico that also fertilizes itself. Among vertebrates, this rare phenomenon is only found in fish.

The distribution of gonads in hermaphroditic fish varies from species to species. Some fish, like herring, have testes on one side and ovaries on the other, while others, like yellowtail sea bream, have both testes and ovaries on each side.

With the exception of self-fertilizing sea bass, hermaphroditic fish mate in a way that, like earthworms, requires two individuals for fertilization to occur.

The sex of the yellow eel often raises questions. The bodies of small yellow eels are full of eggs, while the bodies of large yellow eels are full of milt. The gonads of almost all yellow eels are ovaries from the embryonic stage to maturity, that is, the yellow eels are female when they are young, and the ovaries change after laying eggs, and slowly become testes. In other words, the yellow eel changes from female to male, and remains male until death, and its sex no longer changes. In biology, this special physiological phenomenon of the yellow eel changing from female to male is called sex reversal. The sex reversal of the yellow eel is not a mutation of a certain individual, but a developmental law of the entire race. The young eels hatched from eggs, after laying eggs in the first year, become male eels in the second year, and mate and lay eggs with the next generation of female eels. In this way, there is a batch of male eels that change sex every year, and a batch of newly grown female eels every year, so the marriages of yellow eels are all "old man and young wife". Some hermaphroditic fish also often have sex reversal, which is mainly caused by the different development time and changes of male and female gonads. The hermaphroditic yellow sea bream has both male and female gonads when it is young, but after ovulation, the ovaries enter the resting phase and the testes begin to develop. Therefore, among the individuals that have just begun to mature, female fish account for the vast majority, and as the individuals grow, the number of male fish gradually increases.

The black silk porgy is an example of the testes maturing first. When the black porgy is young, it only has testes. When it grows to 5cm, the ovarian cavity begins to appear and produces eggs. After the fourth year, some individuals have testes degenerated, and some have ovaries degenerated, thus becoming dioecious.