Carcinus maenaswas reported in the western Atlantic in 1817 (Cohen et al. 1995). In this area, the crab is commonly found from the high tide level down to 3 fathoms, though some have been reported as deep as 10 fathoms (Elner 1981). C. maenas occurs on all shore types, but is most common on sheltered shores, as in Europe, where they out compete all other crab species (Elner 1981). The existence of many sheltered waterways on the east coast probably helped with the early spread of the crab (Le Roux et al. 1990). Its introduction has had devastating impact on the soft shell clam Mya arenaria, young oysters, Cancer crabs, and artificially created mollusc beds (Lafferty and Kuris 1996). The original introduced range ran from Cape Cod to New Jersey, but the crab eventually spread as far north as Nova Scotia (Grosholz and Ruiz 1996). The crab began to move out of its original range in 1905 (Vermeij 1982). An increase in air and surface water temperatures north of Cape Cod was an important factor in the northern expansion of the range (Ropes 1967). Temperature changes still affect the range; in cold years, the crab is not common above central Maine. Feeding is suppressed at less than 7°C. The life cycle is slightly different in Maine than in slightly warmer European waters (Figure 6) (Berrill 1982). Growth is more rapid in the warmer waters of Europe. In Maine, the life history is delayed, and smaller individuals may be sexually mature, to allow more breeding in a life time. Crabs south of Cape Cod are more comparable in life cycle to those in the Netherlands and Britain (Berrill 1982).

The crab's widespread movement above Cape Cod wasn't possible until the 1950s, when there was a period of warmer ocean temperatures. The crab had been detected in Maine's Casco Bay in the early 1900s, in central Maine by the 1930s, and northeastern Maine and New Brunswick by 1951. In 1961, the crab was detected in Nova Scotia. The greatest increase in population on the Maine coast occurred in the period from 1948-51. A decline began after 1954. The rise and decline of the population correlates with warming and cooling trends in ocean temperatures. 1953 marked the peak of a warming trend with a mean temperature of 11.1°C. There was then a subsequent downtrend, with 1967 having a mean temperature of 7.3°C. Previous observations in Essex, Massachusetts during a warming trend in the 1926 to 1933 period also showed population growth with a decline after the severe winter of 1933-34 (Welch 1969).

On this coast, the main foods for C. maenas are bivalves, with Mytilus edulis being the most frequently found species in gut analysis. Gastropod mollusks are also important food resource. The crabs also eat detritus, true to their omnivorous habits. Spartina grasses are the most frequent plant found in gut analysis (Ropes 1967). The crab doesn't prey on the native sea urchin Strongylocentrotus droebachiensis, which correlates with European behavior. Native crabs such as Cancer irrotatus do prey on the urchin.

Many native animals have yet to evolve adaptations to the predation stresses of C. maenas. Ilynassa obsoleta snails lay their eggs in capsules that are attached to solid surfaces in the intertidal zone, such as wood, algae, and shells. The egg capsules protect the larvae from salinity stress, but aren't strictly necessary for development. I. obsoleta capsules are preyed on by moon snails, but are not a major prey item. Spines on the egg capsules deter most mollusk predation, but not decapod predation. In lab studies, C. maenas often detached the egg capsules and ate them. The main advantage to the capsules is keeping the larvae out of the plankton stage. This allows the larvae to avoid many of the native predators, but introduced predators such as C. maenas are making this expenditure worthless (Brenchley 1982).

I. obsoleta has a chemoreceptory burrowing response to native predator species, but not to C. maenas (Brenchley 1982). The snail also has a burrowing response to the chemoreception of juices from an injured member of its own species (Ashkenas and Atema 1978). This conspecific reception works in the snail's favor for C. maenas avoidance, as the green crab is a much messier feeder than native predator species such as the moon snail Euspira heros, and it is therefore more likely that C. maenas feeding will cause this particular alarm response (Atema and Stenzler 1977).

A contrasting example is that of the periwinkle Littorina littorea, which is also an introduced species from Europe. C. maenas is a known predator of L. littorea in Europe. L. littorea appears to have been in North America for quite a bit longer than C. maenas (Vermeij 1982). The snail will crawl to pool sites such as crevices, or under rocks or algal fronds, where they are less visible, when juices from C. maenas are detected. A significant portion of the snails hid in ten minutes or less, due to an average fourfold increase in crawling velocity. This ten minute escape period is adaptive; crabs take an average of nine minutes and fifty-four seconds to fully consume one snail, allowing for the others to escape (Hadlock 1980). The snails also display thicker shells in areas with heavy crab predation, as is observed in Europe. Repaired shells became more frequent after C. maenas was introduced (Vermeij 1982).

C. maenas has also affected the crustacean populations. One study examined four decapod species that all coexist in the shallow rocky subtidal zones of the Gulf of Maine (Moody and Steneck 1993). The American lobster Homarus americanus, C. maenas, the rock crab Cancer irroratus, and the jonah crab Cancer borealis all show morphological characteristics of predatory decapods that feed on prey such as the blue mussel, M. edulis, which is common in the area. These morphological and behavioral adaptations are for foraging and feeding, particularly on shelled mollusks. Because these species coexist in the same area, they would be expected to compete for common resources (Elner 1981). These four species use different behaviors in their foraging expeditions. C. maenas and C. irrotatus had the widest variety of behaviors. They used crushing force across broad areas of the shell, and also perimeter force applied to the edges of the shell. Perimeter tactics were used when crushing wouldn't work, and these crabs were likely to reject larger mussels. These species feed at high tide, directly on the mussel bed. The other two species, H. americanus and C. borealis, only used crushing techniques. These two species fed more at night and were likely to take a clump of mussels back to their burrows. The smaller species could open shells that were wider than their chela gap; the larger species couldn't. This allowed the smaller crabs to use a class of prey unavailable to the larger species (Moody and Steneck 1993). There are also slight differences in diet between the native species and C. maenas. The lobster and the Cancer crabs will eat echinoderms, while C. maenas won't. Even with the differences in diet, the intense competition for bivalves, gastropods, polychaetes and crustaceans are enough to make C. maenas at least an indirect competitor for these species. H. americanus and C. borealis are not as affected because of their different feeding habits, but C. irrotatus has been negatively impacted (Elner 1981).

The biggest impact of the green crab introduction north of Cape Cod was the devastation of the soft shell clam, M. arenaria, beds, leading to a near collapse in the industry in the 1950s. (Figure 7) In 1938, 14.7 million pounds of clams were harvested, in 1959, only 2.3 million pounds were taken (Ropes 1967). The clams are sometimes included in the diet of a wide variety of predators including other crabs, snails, fish and birds, but these animals could not have been responsible for such a drastic decline in the clam population. C. maenas was the only possible explanation for the reduction in numbers. Large crabs preferentially fed on the clam beds, even though the whole intertidal zone was available to them (Ropes 1967). The population of M. arenaria rebounded when water temperatures returned to 'normal' in the 1960s, but as we can see, there has been another decline in the 1980s (MER 1997).