The Chinese mitten crab is primarily a freshwater species as it spends most of its life in rivers and streams, migrating into the brackish waters of estuaries to reproduce and die. In the spring, mitten crab larvae escape from their eggs and disperse throughout the water column. These prezoeae develop immediately into a 1.7 mm zoeae, three additional stages follow subsequently before the larvae reach the last larval stage of a megalopa, which is 3-4 mm long. As a megalopa the larvae change their shape to that of a crab; instead of drifting freely about, they take up a life at the bottom (Panning 1938). During this development the animals move gradually from the salt to the freshwater near the river mouths. The crabs are too small at first to be able to make their way upstream against the strong currents.
The distance of their migration is determined by the salinity gradient, and possibly by population density (Panning 1938). The second reason for the migration seems to be that converted rivers do not hold sufficient nutrition for the crabs. Panning describes that in the 1920's when mitten crabs were first discovered in Germany the migrations ended in the rich tidal freshwater feeding grounds, "it was th[e] enormous increase (in numbers) that forced them to move on farther upstream in their search for food" (Panning 1938).
During the fall and winter months from August to November, maturing adults migrate downstream to reproduce (Panning 1938). Return migration occurs after a several year period (2-3 years in San Francisco, 3-5 years in Germany) spent upstream, whereupon they begin their migration downstream undergoing their terminal molt as their sexual organs develop (Hieb 1997, Nickles 1997, Panning 1938) After migrating down the channels of stream beds, the adults gather in large swarms to breed in the estuarine waters (Hieb 1997, Panning 1938). Within 24 hours of fertilization, the female mitten crab lays her eggs, holding them in her pleopods [pic] (Hieb 1997). The eggs are "cemented" to the female's underside (the process requiring an ambient salinity of 25ppt) where they remain until the larvae hatch into the estuary in early summer (Panning 1938).
In the following summer after mating, males and females set out for the river banks in the estuaries where they gradually perish. The observed coverage by barnacles of older adults has been suggested as a sign of their fading strength after migration and breeding period: they are too slow and weak to fight off encrusting organisms (Panning 1938). Under normal circumstances (what are normal circumstances) the single breeding period (semelparity) is compensated by an enormous egg production (250,000 to one million eggs per female (Cohen and Carlton 1997)),a characteristic which renders the Chinese mitten crab a potentially successful invasive species.
The timing of hatching and migration is presumed to be influenced by water temperature (Hieb 1997, Panning 1938), i.e., hatching and migration may be delayed in colder springs
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