Carcinus maenas were first collected in San Francisco Bay in 1989 (Cohen et al. 1995). By 1993, the crab had spread north to Bodega Bay, a movement of 120 km (Grosholz and Ruiz 1995) By 1994, they had traveled 125 km south to Monterey Bay (Grosholz and Ruiz 1996). Specimens were found in Oregon in 1997 (Raloff 1998), and in Willapa Bay, Washington in 1998 (Tangley 1998). Theories of introduction include: ballast water, on fouled ships, in algae with imported live bait or lobsters, or by intentional release (Cohen et al. 1995). Further colonization appears to be by larval dispersal (Grosholz and Ruiz 1995). Northern colonization appears to be proceeding more rapidly than southern, due to a difference in surface water dispersal from Coriolis effects (Grosholz and Ruiz 1995). C. maenas has become abundant in shallow, warm lagoons (Cohen et al. 1995), in protected bays, and on soft bottoms (Lafferty and Kuris 1996). It is not found in rocky habitats on this coast, and it is as common in estuarine habitats as marine (Grosholz and Ruiz 1996). Data indicate that the crab's effects will be felt most in estuaries and protected marine embayments (Grosholz and Ruiz 1995). By 1991, crab densities in traps were high enough to merit the attention of biologists (Cohen et al 1995). Washington state has a $75-million shellfish industry, which is being monitored closely for negative effects (Tangley 1998). The potential range on the west coast, within environmental limits, is southern Alaska to Baja California (Cohen et al 1995).

San Francisco Bay is an interesting area because it has been invaded so many times. There are 212 recognized invasions, and the Bay and Delta are known as the most invaded aquatic ecosystem in North America. Since 1850, the Bay and Delta region have been invaded by an average of one new species every 36 weeks (Cohen and Carlton 1995). Additionally, there are many species that are cryptogenic. It is estimated that approximately 123 species of plants, animals and protists in the Bay are cryptogenic, including large numbers in such ecologically important groups as phytoplankton (Carlton 1996b). The Estuary may have twice the number of cryptogenic taxa (Cohen and Carlton 1995).

Lab studies show the crab will feed on all of the predominant macroinvertebrates in the Bay system (Lafferty and Kuris 1996). Since introduced species make up 90% of the benthic biomass in San Francisco Bay, except for in the deepest areas, C. maenas will feed on a variety of introduced species. The Asian clam Potamocorbula amurensis, which was introduced to the Bay in 1986, was readily accepted as prey in the lab. In 1995, the clam was one of the most abundant subtidal organisms, and predation by C. maenas should help to increase diversity by controlling the clam's population. C. maenas grows larger than the native crabs (Hemigrapsus oregonensis, Hemigrapsus nudus and Pachygrapsus crassipes), and so could compete with them for food. It also may compete with Cancer species subtidally. Several species of shrimp, fish, birds and mammals may become predators of the crab (Cohen et al. 1995).

One area of concern is the population Transennella sp., bivalves which are often numerically dominant in bays on the west coast (Grosholz and Ruiz 1995). They are an important food for Dungeness crabs (Cancer magister), migratory birds and surfperches. A significant reduction of bivalve populations in Bodega Bay has occurred since the introduction of the crab. There has also been a decrease in the numbers of H. oregonensis, which almost entirely overlaps in distribution with the green crab (Grosholz et al. 1996).

The direct and indirect effects of recent fluctuations of the Dungeness crab population are an early look at the possible effects by C. maenas. When the Dungeness population was low, densities of Transennella and Leptochelia dubia, a tanaid crustacean, sharply declined. The resident invertebrate community underwent several changes, and the numbers of shorebirds diminished (Grosholz and Ruiz 1995). Dungeness crabs are important selective forces in the community, and C. maenas has the potential to be the same.