Introduced Species Summary Project
European Green
Crab (Carcinus maenas)
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Common Name: European Green
Crab, Shore Crab, Joe Rocker
Scientific Name: Carcinus maenas
Classification:
Phylum or Division: Arthropoda
Class: Malacostraca
Order: Decapoda
Family: Portunidae
Identification: Green
crabs, although the name indicates otherwise, are not necessarily distinguished
by their shell color. Adults, which measure approximately 3” across, have shells
(carpace) that range from dark green with yellow markings to orange or red. The
underside of the green crab is often bright red or yellow. Do not let these
small creatures trick you though, they can wreak havoc on a grand scale.
Original
Distribution: Along the coast of the North and Baltic Seas.
Current Distribution: Has
invaded numerous coastal shores including South Africa, Australia and both
coasts of North America.
Site and
Date of Introduction: Arrived on the eastern seaboard over 150 years ago,
first finding amenable habitats in coastal areas from New Jersey to Cape Cod
and down in the Chesapeake Bay (1879). In the early 1900s, they began spreading
northwards, up through Maine and all the way to Canada (Cape Sable, Nova
Scotia). First seen in San Francisco Bay in 1989 (Redwood Shores),
the green crab moved northward to Bodega Bay (1993) and southward to Monterey
Bay (1995) Tomales Bay and Humboldt Bay, before crossing over to Coos Bay and
several other estuaries in Oregon (1996). Green crabs were then sighted in
Willapa Bay and Grays Harbor, Washington and on the west coast of Vancouver
Island, in 1998 and 1999.
Mode(s) of
Introduction: There is no general consensus on the means by which the
species was introduced, although there are a number of natural and human-driven
causes. The natural spread of the species is predicated on the fact that the
crab is almost protean in its ability to sustain itself in widely disparate
environmental conditions. Green crab can survive as larvae up to 80 days and
are very adept at establishing new populations as they flow up and down a
coast, eventually spreading themselves over great expanses of shoreline. There
are also a number of human interventions that have the effect of spreading the
species to new areas. Among these is ballast water from incoming ships, seaweed
packed with lobsters, commercial oysters, and bate (particularly from Maine),
bait buckets or boat wells from recreational boaters and through availability
from marine biology supply companies. The original East Coast invasion was
thought to be caused by dry ballast on wooden ships and through the crabs
clinging to mossy crevices of heavily fouled outer hulls, coming from Europe.
Reason(s)
Why it has Become Established: The Green
Crab is an effective forager, adept at opening bivalve shells. Studies have
shown it to be quicker and more dexterous than other crabs and capable of
improving its food gathering skills over time. It preys on a multitude of
organisms, including clams, oysters, mussels, marine worms and small
crustaceans, making it a major potential competitor of the native fish and bird
species. It is highly adaptable and can survive in a wide range of temperatures
and salinities. In addition, the green crab can produce an astounding 200,000
eggs in one reproductive cycle and can, under special circumstances, survive up
to two months out of water. It is theorized that one major way the species
spreads, for example between San Francisco Bay and Bodega Bay, is as crab
larvae that can travel up to five miles a day with the current.
Ecological
Role: They have the potential to restructure the crab population
in ecosystems in which they establish themselves, as they feed on the larvae of
other crab species devastating their near shore nurseries. They like shallow
water, out of range of octopus and other natural predators. Their natural
habitat is under rocks, and in disturbed areas, making it difficult for birds
to capture them. In fact, they pose a direct threat to shorebirds, as they have
similar diets. In total, green crabs eat hundreds of species and are a
voracious predator difficult to stop once they have established a foothold in a
biotope.
Threat(s): At the turn of the century, this
species basically wiped out the soft clam industry of Maine and the surrounding
waterways. More recently, it has taken partial blame as a culprit in the
scallop population decline on Martha’s Vineyard. In California, it has been
estimated to cause the loss of as much as 50 percent of Manila clam stocks and
substantial decreases in other crab populations. In Washington, where there is
a huge shellfish industry, the potential loss to the clam and oyster fisheries
could be astronomical. It is additionally posited that they could compete with
the native Dungeness Crab for resources and thus cause stress to that
population. In addition, the green crab is an intermediate host to
marine worms that could potentially be harmful to local shore birds.
Control
Level Diagnosis: In Washington, they believe they cannot eliminate
the species, but can control the population to the point where it is not
disastrous to the surrounding ecosystem. In other areas, controlling the
population has also been the primary strategy, rather than trying to eliminate
it. Australia, however, is considering using another introduced species to have
a more profound long-term impact on green crabs in their waters.
Control
Method: On the east coast, they are captured in much the same way
as blue crabs, using pyramid shaped wire mesh traps that are baited with fish
and set in the water attached to buoys. In Washington, an Exotic Species work
group was established in 1998 that submitted recommendations on controlling the
species. Among the wide array of recommendations (over 50 in total) were educating
boaters, increased regulation that includes inspecting arriving boats and
setting up volunteer programs with citizens, other agencies and crab fisherman
and oyster growers to catch the crabs with similar traps to those described
above. Bounty programs on the east coast have helped to control the populations
and mollify the affects on scallop fishing, but they have not eradicated the
species. As globalization continues to broaden its reach, shipping traffic will
only increase, introducing green crabs into more and more estuaries and
disabling efforts to control populations where they are currently underway. In
Australia, researchers have suggested introducing the green crab’s natural
European nemesis, the Sacculina carcini barnacle– which pierces the crab’s
exoskeleton and causes sterility. At last check, they were ensuring that the
barnacles would not have adverse affects on native species before introducing
it on a wide scale.
References:
Blair, Tim, “Lateral Thought on the Sea Bed: The fix for a
European pest may be its European foe,” Time, No. 13, March 30, 1998.
Fincham, Michael, “An Endless Invasion? Green Crabs, New
England Intruders Move West,” Maryland Marine Notes, Volume 14: Number 2,
March-April, 1996.
Interview with Scott Smith,
Aquatic Mutant Species Coordinator, Washington Department of Fish &
Wildlife, December 3, 1998.
Online Resources:
Bassett, Zasha, “The European Green Crab....A New
Invader,” Sustainable Ecosystems Institute (www.sei.org), 2000.
Macaulay, Craig, “Farmers Join War Against Seastars, Green
Crabs,” Environmental News Network, December 23, 1998. www.enn.com/enn-news-archive/1998/12/122398/greencrab_767.asp
Smithsonian Environmental Research Center Web Site, 1998. invasions.si.edu/carcinus.htm
Washington Sea Grant Program, Office of Marine Environmental
and Resource Programs (University of Washington) Web site. 2002. www.wsg.washington.edu/newpages/splashanimalt.html
Author: Richard Van Heertum
Last Edited: 02/28/2002
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Project Editor: James A. Danoff-Burg, Columbia University