The European green crab Carcinus maenas, also called the shore crab, is a voracious predator of the marine environment. Indigenous to the Atlantic coast of Europe and North Africa, it has invaded numerous coastal communities outside its native range, including South Africa, Australia, and both coasts of North America. It was discovered in Cape Town harbour, South Africa, in 1983, and has since invaded the coastal waters of the surrounding Cape Peninsula.
The species has also been recorded 100 kilometres to the north in Saldanha Bay, another large port visited by foreign ships. Should the crab become established there, it could have a devastating impact on the bay’s productive mussel-farming industry, and disrupt the food web of the adjacent Langebaan lagoon, a sensitive ecosystem conserved as part of the West Coast National Park.
Not always green
Despite its name, the European green crab is not always green. While juvenile crabs are typically olive green, in older crabs the colour changes during the moulting cycle from mottled green and black to orange and then red. The crab can be identified by the series of five short spines on either side of the carapace and three rounded lobes between the eyes. The legs are robust, although the last pair is relatively flat. All the legs have flattened but pointed tips.
In its native range – from Norway and the British Isles, south to Mauritania – the crab reaches a maximum size of 8.6 centimetres, but in North America sizes as large as 11 centimetres have been recorded. The crab has a maximum lifespan of five years, and reaches sexual maturity at two to three years of age. Females can breed up to three times per year, producing as many as 200 000 eggs at a time. The eggs may be carried on the pleopods for some months before hatching into planktonic larvae, which are widely dispersed by ocean currents.
After a 17 to 80 day period of growth and development, they settle out as juvenile crabs in the upper intertidal zone of sheltered bays and estuaries. Once settled they tend not to move very far, generally only migrating between the subtidal and intertidal zones with the
tides. However, they are capable of remaining in the upper intertidal zone at low tide by sheltering under seaweed or large boulders, or in rock crevices. They can tolerate a wide range of temperatures and salinities – but not the strong wave action of the open coast – and are normally found in seagrass beds and in unvegetated sand and mud substrates in waters less than 6 metres deep.
A clever predator
European green crabs prey on a variety of organisms, preferring bivalve molluscs such as oysters, clams and mussels but also taking marine worms, insects and crustaceans. Adult crabs are capable of eating 30 to 40 clams or mussels per day. They are quicker and more dexterous than many other crab species, and can even devour crabs as large as themselves. Importantly, they can learn from their experiences and improve prey-handling
techniques while foraging.
It is because of its efficiency as a predator that the European green crab is considered one of the world’s worse invaders. Its huge appetite for bivalves and crabs, as well as its ability to outcompete commercially important crabs for food and habitat, makes it a threat to
shellfish industries. By preying heavily on numerous other organisms, the crab may alter
the structure of marine communities, with ripple effects throughout the food web, while
competing with indigenous fish and birds for the same food sources.
Apart from South Africa, the European green crab has invaded both coasts of North America, Hawaii, Australia, Japan and Sri Lanka. In most cases it is presumed to have been introduced in the ballast of ships, and then spread via larval dispersal. More recently, it may also have been introduced and spread in seaweed packed with bait and live shellfish.
The crab first appeared on the east (Atlantic) coast of North America in the early 1800s, when wooden vessels typically used sand and rocks for ballast. It soon extended its range from Nova Scotia in the north to Maryland in the south, and is believed to have contributed to the collapse of the region’s softshell clam industry in the 1950s.
In 1989 the crab was found on North America’s Pacific coast for the first time, in San Francisco Bay, California. It rapidly invaded coastal embayments to the north and south, and in 1997 was discovered in Oregon, followed by Washington in 1998 and British Columbia in 1999. The crab poses a threat to the region’s commercial and recreational
fisheries for Dungeness crabs, clams and mussels, as well as the United States’ largest oyster mariculture operations in Washington.
In Australia the crab was first discovered in 1902, when it was already abundant in Port Phillip Bay, Victoria. It is believed to have been introduced during the 1850s, when wooden ships visited the bay to offload passengers destined for the gold fields. During the 1900s the crab invaded the entire southern coast of Australia, and in the early 1990s it also colonised the island state of Tasmania.
At present there is no initiative underway to control the European green crab in South Africa. However, the crab’s distribution may be naturally restricted by the high-energy
wave action along the country’s rugged coastline, which has relatively few sheltered bays.
In other parts of the world, the only control method currently implemented is physical removal of the crab. In Australia and the Pacific coast of North America, baited traps are set in areas where the crab is most likely to occur. In addition, the Washington Department of Fish and Wildlife in the United States has instituted several measures to regulate all shellfish, aquaculture and other aquatic invertebrate imports and movements within the state to prevent the species from spreading further. American and Australian scientists are also investigating the host-specificity of a potential biological control agent. In its native range, the crab is parasitized by a rhizocephalan barnacle Sacculina carcini, the larvae of which bore into the crab and develop in its tissues. The parasite blocks moulting and effectively castrates the crab, causing female sterility and male feminisation. Should the parasite be found to pose no risk to indigenous species, it may be considered for future release.
Reference: Matthews S. & Brandt K. Africa Invaded: The growing danger of invasive alien species Global Invasive Species Programme 2004