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At a glance
The green crab is a real-life alien invader. This native of Europe and Northern Africa is considered one of the 100 worst alien invasive species in the world. It has surfed ocean currents and hitched rides on the bottoms of ships, pleasure boats and fishing gear—so that today it is found on the Atlantic and Pacific coasts of North America and in the waters of South Africa, Australia, South America and Asia. Its sheer numbers, voracious appetite, and aggressive competition with other species makes the green crab a threat to shellfish and a nuisance for fishing industries.
The green crab has been dubbed the ‘cockroach of the sea' for being seemingly indestructible. Several features set it apart from the rest of the crab world: it can rotate its claws over its back to defend against predators attacking from behind—and it can live out of water for up to a week, in full sun! It can even survive in fresh water for short periods of time.
About the green crab
While it's called the ‘green' crab, this species can be coloured everything from green to yellow and orange. It is a medium-sized crab, wider than it is long. Adults measure up to about six centimetres long and up to nine centimetres wide. The carapace of these crabs have five short ‘teeth' behind each eye, and their hind legs are somewhat flattened, helping them swim at fairly quick speeds.
The green crab is fast and aggressive and has a huge appetite. It eats clams, mussels, oysters, scallops and even small lobster. This crab's eating habits are the undersea equivalent of a ‘scorched earth policy', eating most of what is in its path. The green crab has been blamed for ruining several soft shell clam fisheries around the world.
Lifecycle and reproduction
Green crabs typically live between four and seven years. They produce many offspring: females are capable of spawning up to 185,000 eggs per year. Mating occurs right after a female has shed her shell—a process known as molting. The female carries her eggs on her abdomen until they hatch, usually several months' time. The newly hatched eggs drift in the water column for up to 90 days until they turn into what look like small crabs and settle to the bottom. The timing of breeding can vary greatly by region, but females typically release their eggs during the summer months. Green crabs tend to reach sexual maturity at two or three years of age.
The underwater world of green crabs
The green crab is native to the European and North African coasts—as far as the Baltic Sea in the east, Iceland and central Norway in the west and north, and Morocco and Mauritania in the south. It is one of the most common crabs throughout its native waters. Green crabs were first spotted on the east coast of North America in 1817, and can now be found from Newfoundland to Virginia. They were first detected in waters off Newfoundland in 2007.
In 1989 green crabs appeared on the Pacific coast of the United States, and by 1999 had reached British Columbia. They also moved south in the late 19th century—to Australia. By 1983, they reached South Africa and, in 2003, were spotted on the Atlantic coast of South America.
The green crab is invading Canada's coasts, threatening prime habitats for shellfish stocks and nurseries for juvenile fish. In addition to eating all its neighbours, the green crab burrows into the seabed, damaging the roots of plants that define the habitat, such as eelgrass, and causing established beds to float away. In its native regions, natural predators and parasites keep the green crab in check—fish, birds and larger decapods. This is not the case in other parts of the world.
Green crabs can thrive in a variety of habitats. While they prefer soft bottoms they can also live on rock, amid submerged aquatic vegetation and in salt marshes. The green crab is most commonly found in waters up to six metres deep. The green crab can survive in temperatures from below zero to more than 30 degrees Celsius. Adults typically migrate inshore and offshore with the tides, burying themselves in the bottoms of deeper, warmer coastal waters during winter.
Harvesting green crabs
Green crabs are a common menu item in Europe and are fished commercially there, but in Canada there is no market for them yet. Commercial fisheries for green crabs have reduced the species' abundance in parts of its native range. Green crabs can be caught easily by almost any trapping method, for example with lobster and crab traps and eel-catching devices.
The full impact of the green crab in Canada has yet to be determined. Green crabs can affect shellfish aquaculture and fishing industries—eel fisheries in particular. In some areas, Fisheries and Oceans Canada (DFO) distributes nuisance permits to fisherman. Fishermen who have these permits can destroy any green crabs they catch in an effort to reduce the population size. DFO is studying the green crab population to improve its understanding of how the crab responds and adapts to Canadian conditions. One thing that is known is that completely eradicating green crabs once they invade a region is more or less impossible. Controlling populations of green crabs may, however, be achievable—and a way of limiting the damage they inflict.
Revised: October 2009