The river crab belongs to the phylum Arthropoda, class Crustacea, order Decapoda, suborder Reptilia, group Short-tailed, family Fasciatidae, subfamily Bow-legged, and genus Mitten Crab. There are four species of mitten crabs in China, namely, Chinese mitten crab, Japanese mitten crab, narrow-fronted mitten crab and straight-fronted mitten crab (Figure 2-1). Narrow-fronted mitten crab and straight-fronted mitten crab are not only small in size, but also small in yield and have no economic value; Chinese mitten crab and Japanese mitten crab, which are usually called river crab, hairy crab, crab, clear water crab and hairy crab, are large in size, have good meat quality, and have high yield. Their main differences are:

Figure 2-1 Morphological comparison of four species of mitten crabs

(1) Chinese mitten crab

The cephalothorax is obviously elevated, the frontal margin has four cuspate teeth, the notch between the teeth is deep, and the center one is the deepest, and the frontal angle has six warty protuberances at the back, and the anterior lateral margin has four teeth, the fourth lateral tooth is small and obvious, and the toe joints of the terminal pair of steppers are in the form of pointed claws.

(2) Japanese mitten crab

The cephalothoracic armor is not obviously elevated, the frontal margin has 4 pointed teeth, but the middle two teeth are rounded and blunt, the outer two teeth are more pointed, the notch between the teeth is shallow and thin, and the one in the middle is the shallowest, the frontal horn is posterior to the frontal horn with 4 warty projections, the anterior lateral margin has 3 teeth, the 4th lateral tooth is degraded, and the toe node of the terminal pair of steppers is wide and flat.

The Chinese mitten crab is distributed in the Bohai Sea, Yellow Sea and East China Sea coastal provinces, and it has two populations, of which the northern population is represented by the Liaohe River (Yellow River) system, and the southern population is represented by the Yangtze River (Oujiang River) system. The Japanese mitten crab is mainly distributed in Guangdong, Guangxi, Fujian, the coast of Taiwan, the coast of Japan, and the Far East of Russia, and it also has two populations, of which the northern population is represented by the Suifen River system (locally known as the "Russian big crab"), and the southern population is represented by the Nanliujiang River system (commonly known as the "Hepu crab"). Hepu crab").

Research has shown that the Liao crab, after migrating to the Yangtze River system, is not adapted to the growth environment in the middle and lower reaches of the Yangtze River, and the commercial crab matures one month earlier (in late August), with fewer moults, smaller individuals, and poorer quality. More seriously, the Liaohe River system of Chinese mitten crab and Japanese mitten crab mixing, unfortunately, our country in the last decade or so of the "big crab", will be different water systems or different populations of crab seedling crab species transplanted to a different place of culture, resulting in the mixing of germplasm resources of mitten crab in various water systems, and the loss of its intangible assets is incalculable. Therefore, production must be strictly prohibited in different water systems of the river crab transplanted to other water systems culture.