Morphology and impacts of the burrows of the Chinese mitten crab, Eeriocheir sinensis H. Milne Edwards (decapoda, grapsoidea), in South San Francisco Bay, California, USA

Juvenile Chinese mitten crabs, Eriocheir sinensis, create burrows, in both their native and introduced ranges, in soft-sediment banks of intertidal portions of streams. When mitten crab abundance is high, burrows can become very dense. In Europe, this behavior has been linked to erosion and bank collapse. To assess burrow morphology, patterns of use, and impacts to stream bank integrity, we conducted burrow casting and burrow transects in the intertidal portions of tributaries to South San Francisco Bay, California, between winter 2000 and winter 2002. Burrows were found between the high and low tide lines of streams, and contained at minimum a downward-sloping tunnel and saturated terminal chamber. Burrow morphology ranged from single tunnels with one terminal chamber containing one crab, to highly complex burrows with multiple surface openings and tunnels containing multiple crabs. Casted burrows ranged in volume from 199 cm(3) to 1994 cm(3). Sediment adjacent to casted burrows was dominated by sand and silt; mitten crab burrows casted on a stream gravel bar were found to stop above a coarser gravelly mix underlying the sand/silt top layer. Densities and occupancy rates of burrows on sampled streams were variable among streams and seasons, with the highest occupancy rates generally occurring in spring and summer. Crabs removed an estimated 1 to 6% of sediment per 0.5 m(3) of stream bank through burrowing activities over the period of study. Localized areas of bank slumping were also noted, particularly in spring following rain events. The complex morphology of mitten crab burrows suggests that they are created, maintained, and reused over long periods of time, by multiple year classes of crabs. Sediment loss from burrowing activities may be substantial in this part of South San Francisco Bay, and is influenced by multiple factors including population abundance, connectivity of the burrow systems, and sediment composition.