Green crabs, native to Europe and North Africa, are a commonly-introduced species in many other marine areas of the world. They have invaded coastal regions from South Africa to Australia to the east and west coasts of North America, and the Atlantic Ocean near South America. In Canadian waters, particularly on the east coast, they may pose a threat to the less-aggressive native crab populations. These highly-resilient species compete for prey and have the potential to upset the overall balance of marine ecosystems.

Green crabs vary quite a bit in colour. They can have a dark green, dark brown, red, yellow or orange shell (depending on a variety of factors including age and stage of reproductive maturity), with yellowish or off-white granules and black spots on their claws. The length of their carapace is roughly equal to their width (typically 1.3 times the length). They have four pairs of walking legs and two claws.