|
Green
Crab (Carcinus maenas) |
|
|
|
 |
|
(European Green Crab; Photo credit:
Gregory C. Jensen, Washington Department of Fish and Wildlife,
http://www.wa.gov/wdfw/fish/shelfish/greencrb.htm) |
 |
|
DESCRIPTION
The European
Green Crab (Carcinus maenas) is a small shore
crab. Its native distribution is the Atlantic coasts of Europe
and Northern Africa, from Norway and the British isles south to
Mauritania. They are generally found in areas with protected
rocky shores and cobbles to sand flats and tidal marshes. They
can thrive in wide ranges of both salinities and temperature.
Despite its name, the green crab is
usually not green in color. its shell is mottled and varies in
color. During the molting cycle, the green crab's color may
change from green to orange, then red. The abdomen has five
spines on either side, and contains yellow patches. Their pair
of hind walking legs are relatively flat. Their maximum size
ranges from 80 mm (3") to 65 mm (2.5").
The green crab is an able forager.
Studies have shown that the green crab is capable of learning and
improving upon its food gathering skills. This ability has
proven the green crab to be much quicker and more dexterous than
most other crabs. This leas to the crab's remarkable ability
to open bivalve shells in more ways than others.
Green crabs feed upon numerous types
of organisms including clams, oysters, mussels, marine worms, and
small crustaceans. They also will prey on juvenile crabs and
shellfish.
The recent arrival of the green crab
on the U.S. West Coast is cause for concern. The green crab has
already invaded numerous coastal communities outside of its native
range, including South Africa, Australia, and both coasts of North
America. An able colonizer and efficient predator, this small shore
crab has the potential to significantly alter any ecosystem it
invades. It has been blamed for the collapse of the soft-shell clam
industry in Maine.
First seen in San Francisco Bay in
1989, the green crab has been moving northward to Humboldt Bay,
California. Live specimens have been found recently in Coos Bay,
Oregon and Willapa Bay in Washington state.
(Source: Non-Indigeneous
Species Facts: Green Crab, Washington Sea Grant, http://www.wsg.washington.edu/outreach/mas/aquaculture/crab.html)
|
|
|
|
|
IMPACTS 
Voracious predator
The green crab feeds on many
types of organisms, particularly bivalve molluscs (e.g., clams,
oysters, and mussels),
polychaetes, and small crustaceans. For example, Since it can also
prey on juvenile crabs and shellfish, a northward spread to the
Washington coast and Puget Sound could put our Dungeness crab, clam
and oyster fisheries at risk, and the green crab may compete with
native fish and bird species for food. In Bodega Bay, California, a
significant reduction in population abundance of clam and native
shore crab is already evident since the arrival of the green crab in
1993. In addition, the green crab is an intermediate host to a
marine worm that can harm the health of local shore birds.
Feeding habits and tolerance of a wide variety of environmental
conditions
The feeding activity of green crab
greatly impacts populations of mussels (Mytilus edulis),
dogwhelks (Nucella lapillus), and cockles (Cerastoderma
edule). In Scotland, the crab acts as an intermediate host of
the acanthocephalan worm, Profilicollis botulus, which
causes heavy mortalities in eider ducks (Somateria mollissima).
Along the east coast of North America, green
crab preys on quahogs (Mercenaria mercenaria)
and has been implicated in the demise of the Atlantic soft-shell
clam fisheries of the 1950's. In Bodega Harbor, California, a
comparison of long-term (> 10 years) sampling records shows a
significant reduction in clam (Transennella spp.) and
native shore crab (Hemigrapsus oregonensis) population
abundance since the arrival of green crab
in 1993. Furthermore, laboratory studies show that green
crab readily preys on Dungeness crab (Cancer magister)
of equal or smaller size. Dungeness crab spend part of their
juvenile life in the intertidal zone, and may therefore be at risk
from green crab predation.
It is unknown how the crab established itself on
the U.S. west coast. Potential pathways for introducing C.
maenas to Washington include, but are not limited to, the
natural dispersal of larvae from an established population to the
south, discharge of ballast water from transoceanic ships, discard
of seaweed packing materials used in shipping live shellfish, and
the interstate transport of shellfish aquaculture products and
equipment.
(Source:
Non-Indigeneous Species Facts: Green Crab, Washington Sea
Grant, http://www.wsg.washington.edu/outreach/mas/aquaculture/crab.html;
2. WDFW Fact Sheet, Green crabs, Washington Department of Fish and
Wildlife, http://www.wa.gov/wdfw/fish/shelfish/greencrb.htm)
|
|
Top |
|
 |
ORIGIN
The Atlantic coasts of Europe and
northern Africa, from Norway and the British Isles south to
Mauritania. Occupies protected rocky shores and cobbles to sandflats
and tidal marshes. Lives in a wide range of salinities (5-30 ppt)
and temperatures (5-30 C).
Because of its feeding habits and tolerance of a wide variety of
environmental conditions, green crab
has invaded numerous coastal communities outside its native range,
including South Africa, Australia, and both coasts of North America.
The crab was introduced to the western Atlantic coast during the
early 19th century where it occurred between New Jersey
and Cape Cod. By the 1960's, it had spread north through Nova
Scotia. In 1989-1990, green crab
was discovered in San Francisco Bay, California, although anecdotal
reports place the crab in that state as far back as the mid-1970's.
Since its discovery in San Francisco Bay, green
crab has been found as far north as Humboldt Bay,
California (summer 1995), and most recently, Coos Bay, Oregon
(spring 1997).(Source:
WDFW Fact Sheet, Green crabs, Washington Department of Fish and
Wildlife, http://www.wa.gov/wdfw/fish/shelfish/greencrb.htm)
|
|
|
| DISTRIBUTION |
Link to European Green Crab U.S. Distribution Maps
Link
|
|
|
Green Crab
January 2000
Link |
|
|
|
|
Top |
 |
| European
Green Crab
Control
The green crab has made its way to
the coast of Washington State. Because of this the Washington
Department of Fish and Wildlife (WDFW) have taken some measures to
try and stop the crab populations from spreading from Willapa Bay to
Puget Sound. The measures they have taken are:
- Coastal monitoring for presence of
the crab
- Enacting an emergency rule making
the green crab a harmful exotic species and prohibiting the
possession and transportation of any live green crabs
- Enacting an emergency regulation
that prohibits transfer of any shell, shellfish or associated
equipment and vehicles from Willapa bay to any other Washington
waters except with written authorization from WDFW
- Canceling existing transfer
permits and attaching strict conditions to new permits
Another type of control that is being
used is the development of a management program used to track the
spread of the green crab. The management program includes
these guidelines:
- No live aquatic plants and animals
can be released into, or come in contact with the Puget Sound,
without written permission from the director of the WDFW
- Transfer permits are required for
shellfish growers on the coast to move products to portions of
the state from Willapa Bay
- Ship operators should exchange
ballast water outside Puget Sound to avoid introducing the crab
- Seafood handlers, commercial
shellfish growers and boat operators should thoroughly check
their equipment for the presence of green crabs before moving to
crab-free waters
Just as there are management type
controls there are also some biological control proposals for the
green crab. Biologists and scientists have suggested
introducing some parasites as control. One parasite, Sacculina
carcini, apparently inhibits the molting of the crab, suppresses
the development of male crabs, and causes sterilization in female
crabs.
Scientists and biologists think that
this parasite could impact the reproductive output of the green crab
ultimately reducing the crabs abundance in the wild, but before they
could use this parasite, its impact on native ecosystems would have
to be carefully considered because it is a non indigenous species.
(Source:
WDFW Fact Sheet, Green crabs, Washington Department of Fish and
Wildlife, http://www.wa.gov/wdfw/fish/shelfish/greencrb.htm)
|
|
|
Top |
|
 |
|
|
|
|
Credit: Daniel
Smith
Org: Department
of Natural Resources, King county, Washington
(Green crabs can actually vary from dark green to orange and red
with yellow patches) |
|
Credit: Daniel
Smith
Org: Department
of Natural Resources, King county, Washington
(Female green crab is carrying a large mass of eggs on her ventral
surface) |
|
|
|
|
|
Credit:
Todd Huspen & Armand M. Kuris (from Science
News Online by J. Raloff on June 13, 1998.
Org: the University of
California, Santa Barbara (UCSB) |
|
Credit: E.F.
Richard L.etts & J. Calvin
Source: Between Pacific Tides, fourth edition revised
by J.W. Hedgpeth, Stanford University Press, 1968 (Green Crab) |
|
|
 |
|
|
|
|
 |
Help Prevent the Spread of
Aquatic Plants and Animals (IL-IN-SG-98-1,
Free):
Illinois-Indiana Sea Grant
Link http://www.iiseagrant.org/publication/br.htm
Fact
sheet describes how exotic aquatic species are spread by boaters.
Provides easy steps boaters can take to prevent spread of exotics
when transporting watercraft. 4p. |
|
European Green Crab in Oregon:
a preliminary survey of Yaquina, Coos, Winchester and Siletz Bays:
by Sylvia Behrens Yamada and Gary Allison
Link http://ucs.orst.edu/~yamadas/crab/rephome.htm |
|
|
 |
References
related to Green Crab (provided by Washington
Department of Fish and Wildlife)
Link |
|
References (provided by Science
News Online)
Link |
|
