European green crab first entered the U.S. in the mid 1800's, coming by sailing
ship to the Cape Cod region. In the early 1900's they spread northwards, arriving
in Maine in the 1950's they are believed to have contributed to the dramatic
declines in the soft shell clam fishery. Soon they had migrated all the way
up to Nova Scotia. In 1989 they were discovered on the West Coast, in San Francisco
Bay. They may have come in the ballast water of ships, they may have been shipped
over hidden in the kelp packing around live main lobsters or Atlantic bait worms.
They found protected embayments filled with molluscs, crustaceans, polychaetes
and green algae for them to eat. Within three years they were well established.
The crabs have a floating larval stage, and by 1993 they had reached Bodega
Harbor, where they soon had an established population. In 1997, helped by strong
El Nino currents, the crab had made it into Oregon, Washington, and British
Columbia Estuaries. Although they have grown rapidly and reproduced, they have
not been able to become well established. This may be partly attributed to rapid
response and control efforts. The fact there have not been additional influxes
coming up from southern waters may also be a contributing factor.
areas where the crab have been able to establish reproducing populations they
have had dramatic impacts on other species, particularly smaller shore crab,
clams, and small oysters. While the crab cannot crack the shell of a mature
oyster, it can prey upon young oysters, and will dig down six inches to find
clams to eat. One green crab can consume 40 half-inch clams a day, as well as
other crabs its own size.
green crab is most often confused with native helmet crabs or hairy shore crab.
The most distinctive feature is not its color – which can vary from reddish
to a dark mottled green – but the five spines or teeth on each side of
the shell. There are three rounded lobes between the eyes, and the last pair
of legs are somewhat flattened. The carapace is broader than it is long, and
seldom exceeds 3.5 to 4 inches across.
The European green crab (Carcinus maenas), considered one of the world's worst invasive species, is poised to enter Washington's inland ecosystems. Washington Sea Grant, the Washington Department of Fish & Wildlife, and partners have teamed up to lead a volunteer-based early detection and monitoring program to improve our understanding of native salt marsh and pocket estuary organisms, and how they could be affected by green crabs.
Learn more about early detection, monitoring and volunteer opportunities here: https://wsg.washington.edu/community-outreach/environmental-threats/invasive-green-crab-volunteer-monitoring/
Information on the European Green Crab
Subject: Carcinus maenas (Linnaeus, 1758)
Taxonomy: Phylum Arthropoda, Class Malacostraca, Order Decapoda, Family
Common names: European green crab, green crab, shore crab, European shore
The European green
crab, a non-native predator that feeds voraciously on shellfish, was discovered
in Washington waters in 1998. Between then and 2003 over 850 adult European
green crabs were captured in Willapa Bay and over 250 in Grays Harbor.
To avoid spreading European green crabs
from Willapa Bay and Grays Harbor to the Puget Sound and other marine areas
via commercial shellfish transfers, the Washington Department of Fish and
Wildlife has implemented the following work:
an emergency regulation prohibiting the transfer of any shells, shellfish
and aquaculture equipment from Willapa Bay or Grays Harbor to any other
Washington waters except with written authorization from WDFW.
- Enacted an emergency regulation making
the European green crab a deleterious exotic species, prohibiting the possession
and transportation of any live green crabs.
- Stepped up coastal monitoring for
presence and control of the crab.
- Developed a comprehensive monitoring
program to detect European green crabs in the Strait of Juan de Fuca and
the Puget Sound.
The European green crab, Carcinus
maenas, is a voracious predator that feeds on many types of organisms,
particularly bivalve molluscs (e.g., clams, oysters, and mussels), polychaetes,
and small crustaceans. Recent information suggests that European green
crabs can out compete Dungeness crabs (Cancer magister) for food
and habitat. The European green crab is capable of learning and can improve
its prey-handling skills while foraging. The European green crab is generally
quicker and more dexterous than native crabs of the Pacific Northwest.
If the European green crab becomes established
in Washington, it may have a significant impact on the state's clam, oyster,
and mussel industries, and possibly the commercially important Dungeness
crab fishery. As an intermediate host of the acanthocephalan worm, Profilicollis
botulus, it may also indirectly impact the health of local shore birds.
Despite its name, the European green
crab occasionally is not green. The dorsal (top) shell or carapace is mottled,
dark brown to dark green in coloration, and has small, yellow patches. Its
ventral surface (underside) color may change from green to orange and then
red during the molting cycle (Figure 1). The most distinctive characteristic
separating it from other Pacific Northwest crabs is the array of 5 spines
on either side of the eyes on the front end of the carapace. The 3 rounded
lobes (bumps) between its eyes may also be used to help identify the European
green crab. An adult European green crab is typically about 2.5 inches long,
but can range up to 4 inches. The last pair of hind walking legs is relatively
flat but not any more so than those on a Dungeness crab.
The European green
crabs captured in Washington have been between 19.0 mm (~0.74" to 90.0 mm (~3.5"). In the European
green crabs native range the maximum size ranges up to 100 mm (~ 4") carapace
There are other native crabs in Washington
State that are frequently green in coloration, and can be mistaken for the
unwelcome invasive European green crab. The most commonly misidentified native
crab species is the helmet crab (Telmessus cheiragonus, Figure 2).
The physical characteristics for the helmet crab are the following: entire
body covered with stiff hairs; 6 spines on either side of the eyes; and a
pair of long, bristly antennae.
|Figure 2 and 3. Comparisons
between the European Green Crab and the Helmet Crab.
The majority of the following life history
information is derived from European green crab populations outside the Pacific
Northwest. Information is taken from studies and publications completed elsewhere
because less is known about European green crab in the Pacific Northwest.
The European green
crab is native to the Atlantic coast of Europe and northern Africa, from
Norway and the British
Isles south to Mauritania. They are found in a variety of habitats, including
protected rocky shores, cobble beaches, sandflats, and tidal marshes. They
can also tolerate wide ranges of salinities (4-54 ppt) and temperatures (0-33 °C).
|Figures 4-5-6-7. Habitats
of Green Crab
In its native range, the feeding activity
of the European green crab greatly impacts populations of mussels (Mytilus spp.),
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 common
eiders (Somateria mollissima). Along the east coast of North America,
the European green crab preys on quahogs (Mercenaria mercenaria),
a hard shell clam, and has been implicated in the demise of the Atlantic
soft-shell clam fisheries of the 1950s. In Bodega Bay, California, there
has been a significant reduction in the populations of native clams (Transennella spp.)
and a shore crab (Hemigrapsus oregonensis) since the arrival of
the European green crab in 1993. Furthermore, laboratory studies show that
European green crabs readily prey on Dungeness crabs (Cancer magister)
of equal or smaller size. Dungeness crabs spend part of their juvenile life
in the intertidal zone, and may therefore be at risk from European green
The European green crab is capable of
learning and can improve prey-handling skills while foraging. The crab is
quicker, more dexterous, and can open shells in more ways than other species
of crabs. Two color varieties exist: red and green. Red-colored European
green crabs prefer larger bivalves and usually dominate green-colored European
green crabs in aggressive disputes over prey. The crusher claws (the larger
of the two claws) of red European green crabs exert more force, on average,
than those of green European green crabs. In Denmark, the foraging activity
of the European green crab is about 20 times higher in summer and fall than
in winter and spring. In summer, large numbers of European green crabs move
up and down the shore with the tides. European green crab usually forage
during high tide, whereas females are active primarily at night, independent
of the tidal phase.
on the European green crab in Europe indicate that when preying on bivalves,
feeding rates generally
decrease with increasing bivalve size and with decreasing crab size. Oysters
are typically "crab-proof" at around 60 mm shell length, whereas mussels
are free of predation at around 45 mm shell length. European green crabs,
with a 25-75 mm carapace width, are capable of eating three oysters up to
60 mm shell length daily; a relatively low number compared to the nearly
three-dozen mussels (up to 45 mm shell length) it is able to eat in the same
period! Mussel populations located high in the intertidal zone tend to survive
predation better than those lower in the intertidal zone.
In the North Sea, mating takes place
after the females molt from April to November, but mainly from June to October.
The size of mating females generally decreases during this period. This is
because most of the females that mate in June are older and have already
bred at least once; the proportion of these females diminishes over time
providing an opportunity for younger females to breed. Furthermore, the size
of females mating for the first time decreases from July to October. Some
small females molt and mate at the beginning of summer then molt and mate
again after August!
In the Baltic Sea, male European green
crabs molt between May and June. Males molt more often and grow larger than
females, and typically mate with females smaller than themselves. When mating,
the male embraces the newly molted female in order to protect her from predation
or cannibalism. This type of guarding behavior may also be a means to ensure
that the male is the sole partner of the female's brood. Because of their
thicker shells and stronger claws, male red European green crabs compete
more successfully for mates than green males. However, this advantage is
gained at the cost of reduced tolerance to the conditions of intertidal life.
For example, male green European green crabs are more tolerant of lower levels
of salinity and oxygen (anoxia) than red males. Although egg-bearing females
can be found from December to August, they are usually scarce after July.
Adult crabs, particularly egg-bearing females, generally migrate to deeper
water during the winter. It is hypothesized that egg-bearing females seek
deeper water to take advantage of stable salinity and temperature conditions.
green crabs extrude their eggs during the spring. Successful embryonic development
occurs at temperatures
between 11 and 25 °C. Appreciable survival of eggs to larval stages occurs
at salinities between 26 and 39 ppt. In the Baltic Sea, when salinities are
unfavorable (< 13 ppt), larval development may be almost completely prevented.
Furthermore, it has been shown that without food, larval development can
be prevented at temperatures below 6°C, even though larvae are well-adapted
to a natural shortage of food in the wild.
The new larvae of European green crab,
or the first stage zoea, aggregate in surface waters during the ebb tide
at night when current velocities are highest. It is hypothesized that this
upward swimming behavior and tidal stream transport are a means to avoid
inshore stranding while enhancing offshore dispersal of the larvae. Conversely,
after a period of growth and development in the open sea (about two weeks),
megalopae, the last larval stage of the crab, aggregate at night in surface
waters during flood. In this way, the European green crab ensures its return
to coastal waters where the megalopae (Figure 8) molt and settle out as juvenile
crabs in the upper intertidal zone.
|Figure 8. Megalopae stage
Along the central coast of Maine, mating
occurs from July to October. Megalopae and early stage juvenile crabs do
not settle until late August, growing to about 6 mm carapace width before
winter. Renewed growth begins the following June, and juveniles grow to 13
to 25 mm carapace width by their second winter. Mature males molt by the
end of July, whereas mature females molt from July to October. The European
green crab reaches maturity within two to three years old, breeds up to three
times, and has a minimum generation time of three years. The maximum life
span of the European green crab in its native range is five years.
The European green crab's feeding habits
and tolerance of a wide variety of environmental conditions has enabled it
to occupy numerous coastal communities outside its native range, including
South Africa, Australia, Japan, 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 1960s, it had spread north through Nova Scotia.
Anecdotal reports place the crab in California as far back as the mid-1970s.
However, the first documented European green crab on the North American West
Coast was in San Francisco Bay, California, in 1989. Since its discovery
in California, the European green crab has spread northward, being observed
in Oregon in 1997, Washington in 1998 and British Columbia in 1999.
Genetic testing has proven that the
European green crabs introduced to San Francisco Bay came from the population
along the east coast of North America. However, the mode of the introduction
is unknown. The general consensus among researchers is that it arrived in
either ballast water or packing material for live bait or Atlantic lobsters.
It is also unknown how the crabs dispersed from San Francisco Bay. Potential
pathways for dispersal include larval transport via ocean currents, ballast
water exchange, and the transfer of live shellfish or aquaculture equipment.
Research data strongly suggest that the introductions to Oregon, Washington
and British Columbia occurred through larval transport via strong ocean currents
associated with an unusually large El Nino event in 1997 and 1998. In addition,
spatial data on captured European green crabs in Willapa Bay and Grays Harbor
suggest that Coriolis forces may have influenced larval transport by deflecting
planktonic European green crab larvae to the south entrance of each bay mouth.
In Washington, European green crabs
have not been observed on rocky shores or cobble beaches but have been exclusively
associated with tidal/salt marshes and oyster beds in Willapa Bay and Grays
Harbor. The initial 1998 invasion of the crab in Willapa Bay and Grays Harbor
was primarily composed of one class size with only one larger crab found
(Figure 9). Trapping efforts resulted in 303 crabs caught in Willapa Bay
in 1998, 343 crabs in 1999, 144 crabs in 2000, 94 crabs in 2001 and in 2002
an increase to156 crabs.
In 2000 and 2001, even though the number
of crabs caught and the catch per unit effort had fallen substantially as
compared to 1998 and 1999, researchers found smaller crabs of a new year
class indicating that further recruitment had occurred in both bays (Figure
9). Whether this recruitment was from local reproduction or from new larval
settlement via ocean currents is unknown. In 2002 the number of crabs caught
had increased but, no new recruits were found and there was a drop in the
number of older crabs, suggesting that the original 1998-99 year classes
were dying out. With the small El Nino event in 2002-03 it will be interesting
to see if another increase in small green crab recruits in Willapa Bay and
Grays Harbor is observed during the 2003-trapping season.
|Figure 9. Concentration and
Trapping Size of Green Crab in Willapa Bay
In Humbolt Bay, California, research
has shown a 40% drop in Manila clam harvest since the European green crab
became established there. Thus far in Washington State, the size of the European
green crab population is small enough as to have no measurable effect on
aquaculture activities. However, commercial shellfish growers may have to
alter their culture methods to reduce losses from predation if the crab becomes
established here. For example, in locations where European green crab already
exists, mesh nets are often used to protect shellfish stocks.
The native Dungeness crab (Cancer
magister) is another important resource in the Pacific Northwest that
may be impacted if European green crab populations are able to expand their
range and increase their numbers. A University of Washington laboratory
study indicates that the European green crab has the potential to out compete
a Dungeness crab of approximately equal size for both food and habitat.
The commercial Dungeness crab fishery is a multi-million dollar industry
in Washington State, and is also an important sport fishery. There is no
known way to mitigate for the potential adverse effects European green
crabs may have on Dungeness crab populations.
Prior to the arrival of the European
green crab in Washington State, work was being done by various government
and private entities to develop a comprehensive management strategy to address
important aquatic nuisance species (ANS) issues, including the threat of
a European green crab invasion. In 1998, the state completed the Aquatic
Nuisance Species Management Plan called for in Section 1204 of the National
Invasive Species Act of 1996. The primary objectives of the state management
plan are to prevent future ANS introductions into Washington's waters through
all known pathways, and develop response mechanisms for the monitoring and
control of invasive species. See the WDFW Washington
State Aquatic Nuisance Species Plan for more information.
As part of the effort to minimize the
potential for a European green crab introduction, WDFW instituted several
measures further regulating all shellfish, aquaculture and other aquatic
invertebrate imports and movements within the state. One such measure was
to place additional restrictions on imports from out-of-state, which included
requiring one-hour chlorine dips for shellfish seed and broodstock from European
green crab infested areas. Commercial shellfish growers and seafood handlers
should routinely inspect their products and equipment for European green
crab before making transfers to crab-free areas.
After European green crabs were found
on the Washington coast, further restrictions were put in place including
declaring it a deleterious species, making it illegal to transport any live
specimens within the state without a special permit.
In recent years, European green crabs have become a popular animal for scientific
study. As a result, it is readily available through biological supply companies.
Studies using the European green crab outside its established range should be
conducted in quarantine or isolation. Under no circumstances should these crabs
or their progeny be released to local marine waters.
It is unlawful for persons
other than the director to plant aquatic plants or release any species,
subspecies, or hybrids of animals, which do not already exist in the wild
in Washington. (WAC
Live European green crabs
may not be imported into Washington without first obtaining written permission
from the director of WDFW. (WAC
Through the subsequent work conducted
by the Puget Sound/Georgia Basin International Task Force, an important recommendation
provided in the ANS Management Plan is to educate Puget Sound residents about
their role in preventing non-indigenous species (NIS) from entering our marine
waters. For example, people should be informed of the importance of removing
marine organisms from boats and trailers before moving them.
One of the most common ways NIS, including
aquatic nuisance species, can enter Puget Sound and the shared waters of
Canada is through ballast water that is discharged from larger ships once
they enter those waters. Ship operators should exchange ballast waters outside
of the Puget Sound region to avoid introducing the European green crab or
other non-native species.
The implication of an invasion of the
European green crab into Washington's bays brought quick response from the
Governor and the Legislature. WDFW received emergency funds from the Governor
(Fund 001-4, Appropriation Code 612-4) in the summer of 1998 in order to
begin a green crab monitoring program and initiate control actions. Peripheral
to this emergency funding, a Zebra Mussel and European Green Crab Task Force
was established in Chapter 153 of the Washington State Laws of 1998. This
task force was charged with developing recommendations for legislative consideration
to prevent or control the spread of these two aquatic nuisance species. In
1999, the Washington State Legislature under ESSB 5180 directed WDFW to develop
a long-term monitoring and control plan for the European green crab. WDFW
has developed these monitoring and control programs as outlined below.
There were two
components to the WDFW European green crab monitoring and control program: a
program for the outer coast (including Willapa Bay and Grays Harbor) and a program
for the Puget Sound, the San Juan Islands, and the Strait of Juan de Fuca (hereafter
collectively referred to as Puget Sound). These two areas are different in terms
of habitat type, ownership, and presence of European green crab; therefore,
different management strategies were developed for each area.
Biological and chemical methods to control
European green crab have been proposed, however basic research for the implementation
of these methods is lacking. Any alternative method to trapping would have
to be carefully considered before its use.
The coastal management
program was developed to track the spread of European green crab in Washington
included both monitoring and control elements. After the first European green
crab was discovered in Willapa Bay, WDFW staff, initiated a monitoring and control
program, starting with a canvas of Willapa Bay and Grays Harbor by setting crayfish
traps at locations where the crab was most likely to be found. Virtually all
European green crabs captured in 1998 were caught in intertidal areas, dominated
by introduced cordgrass (Spartina alterniflora) in Willapa Bay or native
plants such as arrowgrass (Triglochin maritimum) and American threesquare
(Scirpus maritimum) in Grays Harbor. A more detailed and rigorous annual
monitoring program for the coast was designed and implemented in April 1999
with field assistance from the Puget Sound WDFW staff as well as other agencies,
tribes, and local volunteers. Between 1999 and 2002 over 1,100 crabs were removed
from Willapa Bay and Grays Harbor. Of those, 320 were female crabs representing
a potential generation of 80 - 160 million eggs per season (research shows that
a female green crab can produce 250,000 eggs twice in one season). However,
by late 2002 it appeared that juvenile recruitment was not occurring at a level
that would sustain the population, and the coastal program was eliminated.
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