Chinese Mitten Crab
 
The Chinese mitten crab is indigenous to China, but since the beginning of the twentieth century it has spread in Europe. The species first came to Sweden in the 1930's, in 1965 it was reported for the first time from North-America. It is believed that it was transported in the ballast water of ships. It is not common in Sweden, but e.g. in Germany it occurs in large number and causes much damage to fishing nets, which the crabs cut apart with their strong claws to get at the fish. The crabs' burrows can undermine shores and dams.
Physical Characters
The main identifying features of the Chinese Mitten Crab are the dense patches of hairs on the white-tipped claws of larger juveniles and adults. The claws are of equal size. The shell (carapace) has four spines on either side, is somewhat round in shape, and reaches a width of approximately four inches. The legs of the adult crab are generally twice the length of the shell. The shell color is grayish green to dark brown with two pairs of pale spots.
Food
The Chinese Mitten Crab is omnivorous, consuming a wide variety of plant and animal materials.
Reproduction
The Chinese Mitten Crab reaches sexual maturity in its fourth or fifth year. During the summer months, this crab will begin migrating from fresh water streams towards the sea. Male crabs are the first to arrive in the estuaries and once the females arrive, mating begins. After mating, the females will continue to migrate seaward, over-wintering in deeper water before returning to brackish water in the spring to hatch their eggs. The eggs are spawned on a substance excreted by the female which will harden and hold the eggs to the abdominal limbs. Throughout the winter, the females remain in deep waters while the eggs slowly develop. A single female crab can produce from 250,000 to over 1 million eggs depending on the size of the female. The female will return to shallow brackish waters in the spring to spawn its eggs.
Lifecycle Stages
The Chinese Mitten Crab begins its life cycle as an estuarine pelagic larva. At this stage, it will settle to the bottom of an estuary and begin migrating to freshwater streams. It may spend one to several years in the stream before returning to the coast to breed. This crab may cover hundreds of kilometers during its migrations, rapidly spreading throughout a new ecosystem. Larval development is similar to that in most marine grapsid species, where there is normally a prezoea stage, five zoeal stages, and a megalopa stage. The larval development of the Chinese Mitten Crab shows variability in the number of its larval instars, sometimes passing through an additional zoea stage. This may be related to unfavorable environmental conditions. During most of the larval stages, except the first zoeal stage and the megalopa stage, the Chinese Mitten Crab has very little tolerance for low salinities. During the first zoeal stage and megalopa stage, the larvae continually leave and re-enter estuarine habitats, whereas the larvae of intermediate stages develop in coastal marine waters. Thus, adaptation to freshwater environments is restricted in the mitten crab to its benthic juvenile and adult life-cycle stages.
Habitat
The Chinese Mitten Crab spends most of its life in fresh water but migrates to the sea to breed. These crabs prefer hard bottom surfaces and areas covered with submerged plants.
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