The juvenile mitten crabs do the most physical damage and disruption to the ecosystem, usually in tidal freshwater regions. In this area the crabs burrow holes in banks and levees. As primarily freshwater organisms, spending 2-5 years in freshwater they have the potential to contribute significantly to erosion rates and bank instability. Their burrows are situated between the mean high and low tides, so that the downward sloping burrows retain some water in them at all times (Nickles 1997, Hieb 1997). The openings to the burrows are oval in shape with a width of 1-3 inches and may extend downwards into the river or stream bank up to a meter in length (Nickles 1997, Cohen and Carlton 1995, Panning 1938). Mitten crabs prefer hard clay soils in which to dig their burrows (Hieb 1997), they may also reuse burrows left by other crabs or crayfish (Nepszy and Leach 1973, Nickles 1997).
Juvenile mitten crabs also are a nuisance to "ecosystem services" and commercially important resources associated with freshwater regions. The mitten crab poses a threat to agriculture by eating the shoots of plants, an impact has been described in rice paddies in the crabs native Asian range (Vogel, 1994; Hieb, 1997). Additionally, diked fields may be endangered by the collapse of levees triggered by the crab's burrows. Sheer numbers of crabs have the potential to clog the intake pipes of power plants and irrigation systems (Halat and Resh 1996; Hieb, 1998, Veldhuizen 1998). A possible impact on commercial fisheries may consist of direct consumption of catch by the crab or damage caused to the catch and the nets or traps (when the crabs cut the threads with their claws) (Hieb 1997). In addition, sportfishermen complain that mitten crabs in San Francisco have been "stealing" their bait (Hieb 1997). It has also been proposed that juvenile crabs may compete for resources with other aquatic invertebrates including crayfish which support a commercial fishery in California and the Pacific Northwest (Hieb 1997, Bliss 1982).
One of the most potentially devastating effects of the mitten crab is its facilitation of a parasite, the Oriental lung fluke (Paragonimus westermannii), that it is traditionally associated with in China (Cohen and Carlton 1995). This parasite can be transferred directly to humans and other mammals through consumption of raw or undercooked crabs, or through the consumption of another parasite host. The records from the early invasion of Germany did not report any instances of the parasite infecting human hosts, possibly due to the lack of interest in the crab as a food source or lack of knowledge about the fluke and its association with the mitten crab. However, German farmers were reported to have ground up tons of mitten crabs caught in weirs and fed them to their livestock (Panning 1938), a behavior that could have led to the loss of livestock infected by the parasite and could have opened a vector for indirect transfer to humans. Parasite transfer could potentially occur at any stage where infected crabs could be harvested and consumed.
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