Download Carcinus maenas, green crab Background The brachyuran crab

Carcinus maenas, green crab
Background
The brachyuran crab Carcinus maenas (Linnaeus 1758) is officially known by the
common name green crab (McLaughlin et al. 2005), but other vernacular names include
European green crab, shore crab and harbour crab (Williams 1984, Ingle 1983), and it is
sometimes also referred to as the cockroach of the sea (http://www.oagbvg.gc.ca/domino/reports.nsf/html/c20021004se05.html). It is a member of the
swimming crab family Portunidae that also includes the commercially important blue
crab Callinectes sapidus covered elsewhere in this report. The green crab is native to
Europe (McLaughlin et al. 2005), it’s eastern Atlantic distribution ranging from Iceland
to NW Africa (Squires 1990).
This species is a primary example of a rapid successful aquatic invader that has gained a
strong foothold in North America. The green crab has successfully invaded both the
eastern and western coasts of the USA and Canada. While the occurrence on the west
coast is a more recent phenomenon, with rapid northward dispersal from San Franscico in
1989 (http://www.sei.org/crabnews.html) to British Columbia by 1998 (http://www.oagbvg.gc.ca/domino/reports.nsf/html/c20021004se05.html), the Atlantic seaboard was
invaded starting at Cape Cod more than a century ago. From there the crab has since
spread south to Virginia, and north into Canada, where it colonized the Bay of Fundy in
the 1950’s (MacPhail 1953). It has spread along Nova Scotia in the 1980’s up to Cape
Breton and into the southern Gulf of St. Lawrence by the 1990’s (Audet et al. 2003), its
most northern locality in the western Atlantic. There is evidence that crabs in some
localities may have originated from separate introductory events (Cameron and Metaxas
2005).
In many of these areas it has established itself as the most common crab in the intertidal
zone, where it is found on a variety of habitats but with a preference for protected rocky
shores, cobble beaches, sandflats, and tidal marshes (Williams 1984,
http://wdfw.wa.gov/fish/ans/greencrab.htm#distribution).
While the life-cycle of the green crab is temperature and therefore also latitude
dependent, berried females usually appear in spring (Berrill 1982). After hatching, a
planktonic larval phase ensues that includes 5 stages and lasts about 2 months at 15oC
(Rice and Ingle 1975), before becoming a benthic organism. Larval stages are largely
believed to be the vehicle for dispersal of this species, such as by ship ballast water
(http://wdfw.wa.gov/fish/ans/greencrab.htm#invasion). In New England and adjacent
Canadian waters of similar temperature, the green crab matures in 2-3 years, breeds 2-3
times and has a minimum generation time of 3 years (Berrill 1982).
The green crab is edible but the relatively small size (usually less than 8 cm carapace
width) and yield of meat has resulted in apparently little commercial value. Thus, while it
is used as bait (Elner 1985), there presently is no active fishery for this crab. However,
there now are ongoing efforts for a minced-meat experimental fishery (Tremblay et al.
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2006), such as in PEI (http://www.glf.dfo-mpo.gc.ca/sci-sci/crab-crab/crabwiseencrab_2002-e.html#finding_a_use) that may have the added benefit of a population
control measure.
Temperature limits, critical thresholds, vulnerability, and barriers to adaptation
Sea surface temperatures in the current distribution of the green crab range from a
February minimum of -1.8oC to an August maximum of 25.8oC. One of the reasons for
the wide distribution of this crab is its tolerance to a wide range of salinities and
temperatures; for the former, ranging from almost freshwater conditions of 1-3 ppt for
short term exposure, and 4 ppt for longer exposure (Perkins et al. 1969,), to salinities of
54 ppt that are well above typical seawater (http://wdfw.wa.gov/fish/ans/greencrab.
htm#distribution). Temperature tolerance of non-larval stages is equally impressive,
ranging from close to 0°C depending on salinity, to above 30 °C, with critical thermal
maxima, manifested as first signs of heat stress, ranging from 31-36 °C depending on
acclimation conditions (Cuculescu et al. 1998). Not surprisingly, the overall assessment
of thermal sensitivity of the green crab showed it not ranking as a sensitive species, with
above average mobility and eurythermal capacity.
Impact
A 4oC rise in global temperature will impact the future distribution of the green crab in
the western Atlantic. All models predict a loss in the southern range from Chesapeake
Bay to between Delaware and Cape Cod, depending on the model. While most models
predict little or no increase in range with an increase in temperature (green areas), all
models indicate that the crab is already able to expand beyond its present range (crosshatching) into waters of Newfoundland and Labrador without an increase in
environmental temperatures (blue areas).
These areas include rocky shore habitats and prey species preferred by this species, thus
not likely representing a limiting factor to such an expansion. One possible limitation is
the more restricted cold tolerance of eggs and larvae compared to juveniles and adults.
However, evidence is presently not clear in that regard, with successful egg incubation
occurring at 10-17 oC (eggs being killed at 1.4 oC in low salinity; Wear 1974) and
successful larval rearing between 10 and 25 oC (Naragaj 1993). Clearly, there is a need to
understand the survival of these earlier developmental stages at temperatures between 0
and 10 oC. For example, while it is known that low salinities and absence of food at 6 oC
impede or prevent larval development (http://wdfw.wa.gov/fish/ans/greencrab.htm), it is
not clear how larvae would fare at that temperature under more optimal salinity and food
conditions. Nevertheless, even if lower temperatures represent a present barrier for early
developmental stages of the green crab, the predicted scenario of increased temperatures
will increase the likelihood of a northern expansion. Thus a range expansion into more
northern waters with a 4 oC temperature increase is a likely scenario.
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The green crab is an omnivore (Williams 1984) capable of feeding on both animals, e.g.
polychaetes and small crustaceans, and plants, i.e. seaweeds, adapting and varying its diet
depending on local availability (Ropes 1968). However, it is best known as a voracious
predator of molluscs, particularly bivalves of commercial importance (Elner 1981, Miron
et al. 2005), such as soft-shell clams, oysters, quahogs, and mussels. The crab has
negatively affected such fisheries (Congleton et al. 2005, Walton et al. 2002). Thus there
is particular concern as it spreads northward and establishes large enough populations to
affect clam harvests in New Brunswick and Nova Scotia, oyster production in Bras d’Or
Lakes (Tremblay 2006), and the substantial blue mussel industry, as it enters the southern
Gulf of St. Lawrence. Other than molluscs, there is evidence that under certain conditions
the green crab can compete effectively with juveniles of commercial crustaceans, such as
lobster and rock crab (Miron et al. 2005, Williams et al. 2006). There also is evidence
that it is capable learning, such as improving prey-handling skills
(http://wdfw.wa.gov/fish/ans/greencrab.htm#feeding). Considering these characteristics
of the green crab and the abundance of commercial prey species in the areas being or
likely to be invaded, an upward change in temperature will likely result in the crab
affecting commercial fisheries that are presently not yet impacted as this species is
“eating its way up the coast”
(http://www.cbc.ca/canada/story/2002/03/16/green_crabs020316.html).
References
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Geographic expansion of the nonindigenous crab Carcinus maenas (L.) along the
Nova Scotia shore into the southeastern Gulf of St. Lawrence, Canada. Journal of
Shellfish Research 22: 255-262.
Berrill, M. 1982. The life cycle of the green crab Carcinus maenas at the northern end of
its range. Journal of Crustacean Biology 2: 31-39.
Cameron, B., and A. Metaxas. 2005. Invasive green crab, Carcinus maenas, on the
Atlantic coast and in the Bras d’Or Lakes of Nova Scotia, Canada: larval supply and
recruitment. Journal of the Biological Association of the United Kingdom 85: 847855.
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McLaughlin, P.A, D.K. Camp, M.V. Angel, E.L. Bousfield ….. and T.T. Turgeon. 2005.
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