One of the most popular pursuits is bridwatching. More than 150 species of birds have been sighted in the marsh and mudflats of the San Francisco Estuary. During the fall and winter, huge mixes-species flocks congregate on the edges of the bay at low tide to probe and peck for invertebrate meals. That the estuary can support such large numbers and kinds of birds attests to the great biological productivity and diversity of the mollusks, crustaceans, worms, and other invertebrates that call this bay home. But many of these marsh and mudflat denizens shouldn't be calling the San Francisco Bay home because it isn't.
Home is in reality, the Atlantic Coast, Asia, Europe, and Australia.
As many as 200 species of marine invertebrates have been introduced to the San Francisco Bay by humans, either accidentally or purposefully, in the last 130 years. The concern of the mitten crab is multi-faceted. The crabs dig burrows that may be destructive to levees and earthen dams. Mitten crabs are also an intermediate host for a parasitic liver fluke that affects humans. Both the mitten crab and the Asian bivalve are prolific breeders and, in vast numbers, can have an impact on the availability of food for native animals.