Spawning river crabs fall down the river migration into the sea, is the result of its long-term evolution of natural selection. River crabs are seawater born, two together water transition desalination, fresh water in the growth of species. Sexual maturity in the fall of each year will be in groups downstream into the sea, in a certain salinity under the stimulation of mating, male crabs one after another to stay on the bottom of the sea forever, impregnated female crabs hibernate in the river into the sea, for the second year of the spring bloom breeding progeny to prepare.