In Europe, Carcinus maenas is most often found on semi-protected rocky coasts (Lafferty and Kuris 1996). Other habitats utilized include hard and soft bottoms, protected embayments, and moderately exposed shores (Grosholz and Ruiz 1996). The crabs range from Norway to Mauritania (Bulnheim and Bahns 1996). They are a conspicuous member of the outer coast fauna in all but the most wave exposed areas. The adult crabs move into deeper water when temperatures fall below 8C, and in warmer months migrate into the intertidal zone with the flood tide (Sanchez-Salazar et al. 1987).

The winter migration is not a complete migration; specimens can be found in the littoral zone throughout the year, and very few specimens actually stay in the sublittoral zone permanently (Figure 2). On the shore, the crabs exhibit a migration up and down the shoreline according to the tide. Peaks of activity are during darkness and high tide (Ropes 1967). Males commonly forage higher on the shoreline than females (Naylor 1962).

C. maenas is euryhaline and can be found in salinities from 4 to 34 ppt. Breeding is successful in salinities down to 13 ppt, but larvae require at least 17 to 19 ppt to metamorphosize and settle (Cohen et al. 1995). The crabs are able to withstand even lower salinities, and so can be found slightly downstream from the mouths of tidal rivers (Gonzalez-Gurriaran 1986). If this is the case, females with eggs tend to migrate to deeper and more saline waters until the larvae hatch (Naylor 1962). Eggs are less tolerant of lower salinities at lower temperature (Cohen et al. 1995).

Larval settlement is concentrated in August and early September. The water temperature must be above 18C, and salinity must be above 19 ppt (Eriksson and Edlund 1977). Development occurs fastest in water temperatures between 12 and 25C (Beukema 1991). Juveniles are mostly found in the intertidal zone, regardless of the state of the tide (Aagard et al. 1995) In one study in Sweden, 90% of crabs of the 0-group, which are those less than a year old, were found in beds of the blue mussel Mytilus edulis (Eriksson and Edlund 1977). Crabs of this age prefer an environment with places to hide, and a substrate rich in organic matter. The 0-group crabs feed primarily on substrate; 78% of feces examined in one study were composed of ash (Eriksson and Edlund 1977).

Molting crabs are common at all times of the year, with a peak in January and February, where the proportion rose from 17% soft-shelled in the population to 50% soft-shelled (Naylor 1962). Crabs in the later intermolt stages prefer to stay in subtidal waters, while freshly molted adults forage actively in the intertidal zone with the flood tide (Aagard et al 1995). Pairing occurs most frequently in August and September (Naylor 1962). Females can only mate when they are soft, following a molt (Berrill and Arsenault 1982). Females with eggs are most common in April (Naylor 1962). Mature females produce 185,000 to 200,000 eggs per year (Cohen et al. 1995). Zoea larvae are common in late spring and early summer, and megalopa larvae are most common in June and July (Naylor 1962).

There is argument over the classification of certain Carcinus species in Europe. Some scientists recognize two separate species, C. maenas and Carcinus mediterraneus (Cohen et al. 1995). C. maenas are found in the Atlantic Ocean, while C. mediterraneus are found in the Mediterranean Sea. The two species vary slightly in color, carapace shape, and shape of the sexual pleopods. Recent genetic testing supports instead a subspecies classification for these two populations, as Carcinus maenas maenas and Carcinus maenas aestuarii (Bulnheim and Bahns 1996). However the classification debate is eventually solved, there are two distinct populations of the crabs in Europe.

C. maenas have been shown to consume a large variety of prey items, including organisms from 104 families and 158 genera in 5 plant and protist phyla, and 14 animal phyla (Figure 3) (Cohen et al. 1995). The crab is an opportunistic feeder. One study compared crabs in three habitats. Those in Port Herbert, Nova Scotia ate mostly molluscs, with crustaceans and algae as secondary food. Crabs from Massachusetts and New Hampshire had a more diverse diet with a slight dependence on bivalves. Crabs in the Menai Straits, Wales, ate mostly crustaceans and algae. These differences reflect differences in habitat rather than any adaptive behaviors (Elner 1981).

The crab is thought to be a tactile and chemosensory hunter rather than a visual hunter (Cohen et al. 1995). It is a voracious predator on benthic invertebrates (Lafferty and Kuris 1996). Adults can detect shallowly buried prey. They generally feed in the top few centimeters of the sediment, but they have been observed digging pits up to 15 cm to extract large clams. Crabs eat more in warm water. Feeding behavior is normal down to 7C, is depressed below this, and ceases between 2 and 7°C (Cohen et al. 1995). C. maenas can feed on any shelled animal that is equal or smaller in size than the individual crab's carapace (Ropes 1967). Small animals are frequently eaten whole (Ropes 1967). Intertidal crabs usually eat more plant material, and subtidal crabs usually eat more animals. Small crabs more frequently eat plants and soft-bodied food, and the animals eaten are usually small. Large crabs eat harder-shelled food such as mollusks most of the time. Breeding males and ovigerous females don't feed. The ovigerous females tend to stay buried in sediment and inactive (Ropes 1967). The crabs, in turn, are preyed on by fish such as the European sea-bass Dicentrarchus labrax. Adult bass in particular feed chiefly on crabs, with an emphasis on Carcinus (Kelley 1987).

C. maenas has long been implicated as the selective agent that causes intraspefic morphological differences in several mollusk species (Le Roux et al. 1990). The main mollusks consumed by C. maenas include the blue mussel M. edilus, periwinkle snails such as Littorina littorea, Littorina mariae, and related species, the cockle Cerastoderma edule, and the Atlantic dogwinkle Nucella lapillus. Examination of several decapod predators and their most common prey show that coevolution causes the massiveness of the predator's chela to increase as the prey's armor increases (Juanes 1992). Chelipeds in the crab comprise a large percentage of total body weight. They are important in allowing predators to feed on hard shelled prey (Lee and Seed 1992). In turn, the prey species often evolve to defend themselves from the feeding behavior of the crabs. C. maenas are thought to be a major control on the population of C. edule (Cohen et al. 1995), M. edilus, and N. lapillus (Grosholz and Ruiz 1996).

C. maenas have also been implicated in controlling polychaete annelid distribution by their burrowing (Le Roux et al. 1990). Oligochaetes are known to increase in number with sediment disturbance (Thrush 1986).

In laboratory studies, the crab will not feed on echinoderms such as the urchin Paracentrotus lividus, and studies have found no echinoderm populations affected by the green crab. Other native crab species do prey on the urchins (Grosholz and Ruiz 1996).

The cockle Cerastoderma edule is the most common bivalve in sandy shores and estuaries in Europe. Smaller cockles are a common food for the green crab. Cockles have round, globular shells, and it takes more energy for a crab to crack a cockle shell than a Mytilus edulis shell of corresponding size. Consequently, crabs chose smaller cockles than their chela size would normally allow them to open. Rounder shells for C. edule are an adaptation against predation (Sanchez-Salazar et al 1987).

Gastropods show very distinct changes in morphology due to C. maenas predation. Marine prosobranch gastropods commonly display geographical differences in the morphology of their shells. Wave-swept shore populations generally have small, smooth, thin shells with adaptations for reducing projected surface area. Sheltered shore snails have larger, thicker shells. The large chela on crabs are commonly used for crushing mollusk shells, and their appearance in the fossil record correlates with the disappearance of many open-whorled and thin-shelled mollusk groups, which are most susceptible to breaking. Littorina obtusata snails from sheltered areas have more evidence of crab attacks, which is shown by broken shells or the frequency of repaired injuries during the growth of a shell (Reimchen 1982). The largest, heaviest shells are found where the injury rates are the highest. L. mariae snails are smaller than L. obtusata, and so are more suspectable to crushing. The juvenile stage is the most vulnerable, and L. mariae spend a shorter time as a juvenile then most other snails (Reimchen 1982).

Littorina saxatilis must adapt to both exposure and crab predation. It has a large aperture to have a larger surface area for the foot so it can attach more firmly to the substrate. However, the area immediately behind the hole is narrow to help prevent crab predation (Johannesson 1986).

The dogwinkle N. lapillus shows similar patterns to the Littorinid snails, as snails from moderately sheltered shores have thicker shells with a narrow aperture opening. This relates directly to methods of crab predation on the dogwhelk. C. maenas have three attack methods used on snails: crushing, breaking the columella, and opening the shell whorl (Hughes and Elner 1979). The thinner aperture openings of the sheltered shore snails don't allow the crab to insert its chela and crush the columella. Exposed shore snails are more likely to be broken in this way (Figure 4) (Hughes and Elner 1979). Most adults on sheltered shores are immune to crab attack, lending strong evidence that an elongated growth form with a narrower mouth, thicker shell whorls and a stouter columella is a form evolved to prevent crab predation (Hughes and Elner 1979).

An interesting expansion on the crab's predation on gastropods is its involvement in a parasitic trematode's life cycle. Littorina rudis and Littorina nigrolineata are found in the middle and high zones of the rocky shore in continental Europe. C. maenas prefers smaller prey when there is unlimited abundance, and L. rudis is the smaller of the two gastropod species. Both L. rudis and L. nigrolineata are primary intermediate hosts for the trematode Microphallus similus, and C. maenas is the secondary intermediate host. The final host is the herring gull Larus argentatus. Because the crabs generally prefer smaller prey, L. rudis is more likely to be infected with the parasite, as it is then more likely to pass it to the secondary host (Elner and Raffaelli 1980).

The crab is greatly affected by water temperatures. After colder winters, fewer crabs appear to survive and migrate into coastal waters; and the development of larval stages proceeds more slowly in cold water, as much as four weeks behind the normal time. Differences in selective pressure can often extend into August, allowing bivalves to grow enough to avoid crab predation for that particular year. Bivalve prey species are often much more abundant after a cold winter, because the delayed development and settlement of the crabs allow the bivalves to grow to a size where they are immune to crab predation. Temperature also affects juvenile crabs, who stay in the tidal flats for the winter. Colder winters kill more juveniles, reducing the population for the next year (Beukema 1991).

There are no records of C. maenas found at sea on floating algae or logs (Cohen et al. 1995). Lab studies show the planktonic larval stage ranging from 17 days at 25C to 80 days at 12°C (Cohen et al. 1995). This is too short a time to allow transport of larvae from Europe to North America by ocean currents. This means that invasions are very likely to be caused solely by human means.