Tuesday, May 22, 2012

Europe Green Crab and Green Crab

A native of Europe and Northern Africa, the green crab has invaded the Atlantic and Pacific coasts of North America, South Africa, Australia, South America, and Asia. In North America, the distribution of green crabs now extends from Newfoundland to Virginia and from British Columbia to California. Green crabs live up to 4-7 years and can reach a maximum size of 9-10 cm (carapace width). The life cycle alternates between benthic adults and planktonic larvae. Green crabs are efficient larval dispersers, but most invasions have been attributed to anthropogenic transport. The green crab has successfully colonized sheltered coastal and estuarine habitats and semi-exposed rocky coasts. It is commonly found from the high tide level to depths of 5-6m. It is eurythermic, being able to survive temperatures from 0 to over 35oC and reproduce at temperatures between 18 and 26oC. It is euryhaline, tolerating salinities from 4 to 52o/oo. It is reasonably tolerant of low oxygen conditions.

Green crabs prey on a wide variety of marine organisms including commercially important bivalves, gastropods, decapods and fishes. Impacts on prey populations are greater in soft-bottom habitat and in environments sheltered from strong wave action. The species potentially competes for food with many other predators and omnivores. The predominant predators of green crabs include fishes, birds, and larger decapods. The effects of green crabs have been of particular concern to shellfish culture and fishing industries, as well as eel fisheries. Control efforts have included fencing, trapping and poisoning. Commercial fisheries for green crab have reduced its abundance in parts of its native range.

Europe Green Crab

The European green crab or shore crab Carcinus maenas (hereafter, “green crab”) is ranked among the 100 ‘worst alien invasive species’ in the world (Lowe et. al. 2000). In many ways it could be considered a model invader. A native of coastal and estuarine waters of Europe and Northern Africa, it has successfully invaded the Atlantic and Pacific coasts of North and South America, as well as South Africa, Australia, and Asia. It is a voracious omnivore and aggressive competitor with a wide tolerance for salinity, temperature, oxygen, and habitat type. A large number of planktonic larvae are produced, and dispersal occurs at all life history stages (Cohen et al. 1995).

Green crab was first detected in Canadian waters in 1951 when the introduced New England population spread into Passamaquoddy Bay in the Bay of Fundy (Leim 1951). In reference to its arrival, Hart (1955) wrote:

The green crab (Carcinides maenas), which has entered and spread throughout the Bay of Fundy since 1950, has become our most serious clam predator. It destroys adult clams as well as those of seed size. Feeding experiments conducted this year have demonstrated that it will also destroy young oysters and quahaugs. Studies of its spread show that there is serious risk of its extending its range to the Gulf of St. Lawrence where it might do enormous damage.

Europe Green Crab

Subsequently, the green crab did arrive in the Gulf of St. Lawrence as well as western Canadian waters (Jamieson 2000). In all areas where the green crab has invaded, its potential for significant impacts on fisheries, aquaculture, and the ecosystem has caused concern. Numerous studies have shown the potential for green crab to adversely affect many ecosystem components, directly and indirectly, by predation, competition and habitat modification (Grozholz and Ruiz 1996). Because green crab has the ability to modify entire ecosystems, it is considered an “ecosystem engineer”.

Published estimates of the cost of green crabs in Canadian waters are incomplete and of questionable validity. Colautti et al. (2006) used economic losses attributed to 21 other non-indigenous species to propose median (52% loss) and half-quartile (20%) cases as projections of maximum and minimum cost range for any invasive species. Using these projections, the potential economic impact of green crabs on bivalve and crustacean fisheries and aquaculture in the Gulf of St. Lawrence was estimated as $42-$109 million (Colautti et al. 2006).

The only other published estimate of costs of green crab on the Atlantic coast of North America, a value of $44 million, has been shown by Carlton (2001) and Hoagland and Jin (2006) to be based on an incorrect citation in a summary paper by Pimentel (2000). Unfortunately, repeating Pimentel’s error, this estimate has been widely cited in the scientific literature as the actual cost of the green crab invasion of New England and Atlantic Canada. In fact, the $44 million represented an estimate by Lafferty and Kuris (1996) of the potential, not actual, cost of green crab for a hypothetical (at that time) invasion of the west coast up to Puget Sound.


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John Anderson

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