Carcinus maenas tolerates salinities from a few parts per thousand to full seawater and temperatures across roughly zero to thirty degrees Celsius. Adults survive out of water for days in damp conditions. That combination makes them exceptionally good passengers in ship ballast water, hull fouling, packing material for bait and shellfish shipments, all of which have been implicated in introductions.
The ecological damage is mostly through predation. Green crabs consume bivalves, small crustaceans and worms, and populations have been linked to declines in soft-shell clam fisheries on the north-east coast of North America and to damage to eelgrass beds where the crabs dig while foraging. On the Pacific coast of North America the species arrived later, and management responses there focus on trapping and containment.
The invasion has produced one useful correction. Green crabs are not always green; colour varies from green through orange to red with moult stage, and red-phase individuals differ measurably in aggression and physiology. Identification therefore rests on the five marginal spines beside each eye rather than on colour.