European Green Crab
If you think you have discovered an aquatic invasive species, you should:
The European Green Crab is one of the ten most unwanted species in the world. This small coastal crab, which is highly resilient, competes for prey and has the potential to upset the overall balance of the marine ecosystem.
- Adult size: up to 10 cm
- Shell serrated and trapeze-shaped, with three spines between the eyes and five on each side
- Two claws of different sizes
- Tips of its back legs are pointed, slightly flattened and hairy
- Colour variable: green, red or yellow
- Aggressive, territorial and vigorous, can survive out of the water for several days
It has recently been discovered that there are actually two different types of green crabs found in eastern Canada. Looking at their genetic makeup, scientists have learned that the first green crab populations that invaded the Bay of Fundy and Atlantic coast of Nova Scotia (south of Halifax) are different from the green crab populations that arrived in the 1980s and 1990s north of Halifax and in the Gulf of St. Lawrence. The crabs found in the more northern waters are better able to survive in the colder waters because they likely came from a Northern European stock (the North Sea and Scandinavia). Scientists are looking at where these hardier crabs may invade next.
Similar species (native)
Rock crab (Cancer irroratus)
Larger (up to 15 cm), its peach-coloured shell is wide and ovoid with nine smooth notches on either side of the eyes.
Jonah crab (Cancer borealis)
Can be distinguished by its scallop-shaped shell, consisting of nine rounded teeth on either side of the eyes.
Lady crab (Ovalipes ocellatus)
Larger than the green crab. It has five notches, like the green crab, but the tips of its hind legs are oval.
Where it has been found
Green crabs were first found in Canadian waters in 1951 in southwest New Brunswick and have since expanded to many other locations in Atlantic Canada. They entered Nova Scotia waters in 1953/1954, and reached just south of Halifax in 1966. By 1982-1983, green crabs were present along the eastern shore of Nova Scotia. They were seen in Cape Breton and the Bras d’Or Lakes in 1991-1995 and they entered the Gulf of St. Lawrence by 1994, Magdalen Islands in 2004 and Newfoundland in 2007. They are commonly found in southern Gulf of St. Lawrence along New-Brunswick, Prince Edward Island and Gulf shores of Nova Scotia. Their current boundaries include northeastern New-Brunswick and parts of southern Newfoundland.
This species arrived in B.C. likely through larval transport between 1998 and 1999. In B.C., this species is found along the entire West Coast of Vancouver Island from Barkley Sound to Winter Harbour with isolated, potentially ephemeral, populations in the Central Coast. This species has not been observed in the Strait of Georgia.
Found in shallow water, generally on muddy, sandy or pebble bottoms or in vegetation. Prefers sheltered areas. Common in salt marshes, on sandy beaches and on rocky coasts. Can tolerate a wide range of salinities.
The green crab can live four to seven years and can tolerate a wide range of water temperatures and salinities (salt content). Females can release up to 185,000 eggs once or twice per year. They have a long early life (larval stage) of 50 to 80 days when they drift in the ocean current before settling to the bottom. The adult green crab is very hardy and can survive out of the water for five or more days, hiding in fishing gear and equipment or, at the bottom of crates, buckets and boats. It is an aggressive crab and a dominant predator, feeding upon many shellfish species such as clams, mussels, oysters, smaller crabs and other crustaceans and even small fish. The predators of green crabs are other crabs, fish species, birds, mink, otters, seals, etc.
Ecological and economic impacts
Green crab may pose a serious threat to Canadian estuarine and marine ecosystems.
European Green Crab may pose a serious threat to estuarine and marine ecosystems as they are voracious predators feeding on a variety of intertidal animals, including oysters, mussels, clams and juvenile crabs. This species changes the balance between species in the ecosystems and impacts their diversity. Green crab is such an efficient predator that it out-competes native crab species for food. Also, this species is known to disrupt eelgrass beds; productive habitat for many juvenile fish species and can destroy beds of bivalve shellfish. The European Green Crab threatens molluscs, crustaceans and fish, because of its large numbers, its huge appetite and its fierce competition with other species.
Unless controlled, this new aquatic invasive species will have a significant impact on biodiversity and habitat in the Canadian ecosystems.
Harmful to the fishing and aquaculture industries: Green crabs compete with native crabs and lobster for food and shelter, reduces the abundance of species harvested (fish, molluscs and crustaceans). This species harms the marine aquaculture industry because of its huge appetite for molluscs. The European Green Crab impacts the eel fisheries by damaging the eels when they enter the traps. This species primarily feeds on shellfish and other crustaceans, but has been observed eating small and juvenile finfish in eelgrass beds.
Origins and mode of arrival
Native of Europe and North Africa
It is believed to have first arrived in North America around 1817, most likely as adults carried in the holds of wooden ships. Green crabs are now thought to spread mostly during their larval stage through ballast water transfers or drifting on ocean currents.
Arrived in North American waters at the beginning of the 1800s. Observed for the first time in Eastern Canada in the Bay of Fundy, in the 1950s.
Brought to San Francisco Bay in 1989 via packing material. Arrived in British Columbia, probably through larval transport, between 1998 and 1999.
Mode of dissemination
This crab go through a long larval stage (up to 90 days) and it can drift with the currents and settle in new areas.
Ballast water: Given its long larval stage (up to 90 days), the green crab can survive in ballast water tanks for a long time.
Fishing gear movements and discards: Adult green crabs can survive for a long time in fresh water or out of the water. They can also be introduced if fishing gear is moved to a new area or if crabs are intentionally discarded with the bycatch outside their catch area.
Fisheries and Oceans Canada is studying the Green Crab population to improve its understanding of how it reacts and adapts to Canadian conditions.
Once green crabs have started to invade an area though, it is practically impossible to eradicate them, but it is feasible to limit population spread and hence the damage caused by this species
In 2008 and 2009, Fisheries and Oceans Canada collaborated with fish harvesters, the Fish, Food and Allied Workers of Newfoundland, Memorial University of Newfoundland and the provincial Department of Fisheries and Aquaculture to try out various methods to fight the green crab, specifically including trapping and removing these crabs. The results show that, in areas where sustained removal of green crabs took place, the catch rate for these crabs decreased considerably and the native species, the Jonah crab, regained this territory.
In some areas, Fisheries and Oceans Canada distributes nuisance permits to fishermen. Fishermen who have these permits may destroy any green crabs they catch in an effort to reduce the population size.
European Green Crab in Newfoundland
Discovery and Survey Findings
In August 2007, European Green Crab was confirmed in the northern regions of Placentia Bay. This discovery raised significant concerns because of the potential negative impact of this species on biodiversity and habitat in these regions. Following the initial discovery of European Green Crab in North Harbour, Placentia Bay, the Science Branch of Fisheries and Oceans Canada in collaboration with Memorial University of Newfoundland and the provincial Department of Fisheries and Aquaculture, conducted several rapid assessment surveys for aquatic invasive species in Placentia Bay. The largest green crab population was observed in North Harbour. It was also found in smaller numbers in surrounding areas and along the west and southwest coasts of Placentia Bay. This raised concern as small populations may expand rapidly. Green crab was also found on the west coast of Newfoundland in St. George's Bay near Stephenville in 2008, and spread to Bonne Bay by 2010.
For further information
- Canadian Science Advisory Secretariat (CSAS) Publications
- Aquatic Invasive Species Identification Booklet (PDF)
Fisheries and Oceans Canada – March 2013
- European Green Crab in Newfoundland Waters
- Risk Assessment for European Green Crab (Carcinus maenas) in Canadian Waters
Research document – 2008/042 – Fisheries and Oceans Canada
- Use of the Quantitative Biological Risk Assessment Tool (QBRAT) to predict the effects of the green crab, Carcinus maenas, in Atlantic Canada (PDF)
Research document – 2007/077 – Fisheries and Oceans Canada
- A Biological Synopsis of the European Green Crab, Carcinus maenas (PDF)
Canadian Manuscript Report of Fisheries and Aquatic Sciences 2818, 2007 – Fisheries and Oceans Canada
- Ecological Assessment of the Invasive European Green Crab (Carcinus maenas) in Newfoundland 2007-2009
Canadian Science Advisory Secretariat 2010/033 – Fisheries and Oceans Canada
- Keeping the Green Crab at Bay: A Proactive Approach
Feature article, DFO Science, October 8, 2014 – Fisheries and Oceans Canada
- Risk Assessment for European green crab (Carcinus maenas) in Canadian Waters
- Using the Quantitative Biological Risk Assessment Tool (QBRAT) to predict effects of the European green crab, Carcinus maenas, in Atlantic Canada
- Video: Green crab
- Klassen, G. and A. Locke. 2007. A biological synopsis of the European Green Crab, Carcinus maenas. Canadian Manuscript Reports of Fisheries and Aquatic Sciences. no. 2818: vii +75pp.
- McKenzie, C. H., T. Baines, K. Best, R. Boland, E. Dawe, D. Deibel, D. Drover, E. Johnson, S. Kenny, S. Macneill, D. Mouland, R. O'Donnell, L. Park, P. Sargent, C.Vickers, A.Vickerson. 2007. The European Green Crab, Carcinus maenas, in Placentia Bay, Newfoundland. Aquatic Invasive Species Survey 2007. Aquatic Invasive Species Newfoundland Workshop. St. John's, NL. November 16, 2007.
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