Aquatic Invasive Species

Carcinus maenas (European Green crab)

Animal Crustaceans
Family: Portunidae
Classification: Prohibited

The 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.

In 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.

The 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.

More Information on the European Green Crab

Subject: Carcinus maenas (Linnaeus, 1758)
Taxonomy: Phylum Arthropoda, Class Malacostraca, Order Decapoda, Family Portunidae
Common names: European green crab, green crab, shore crab, European shore crab

Introduction:

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:

• Enacted 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.

Description:

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 width.

Figure 1.

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.

Life History:

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.

Native distribution/habitat:

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

Feeding activities:

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 crab predation.

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.

Studies conducted 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.

Reproduction:

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.

Most European 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.

Invasion:

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.

Observations and Control Effort in Washington:

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

Potential Impacts:

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.

Management (Prevention, Monitoring and Control):

Prevention:

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.

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 232-12-271.)

Live European green crabs may not be imported into Washington without first obtaining written permission from the director of WDFW. (WAC 232-12-01701.)

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.

Monitoring and Control:

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 Control:

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.

Coastal Program:

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.

Puget Sound Program:

In preparation for the potential spread of European green crabs, a monitoring program was launched at the end of 1998 that increases the probability for detection of a European green crab intrusion in Puget Sound, the Strait of Juan de Fuca, and the San Juan Islands. WDFW staff, volunteer groups (via Nakeeta Northwest), tribes, shellfish growers, schools, other government agencies and individual citizens were actively monitoring for the presence of the European green crab. By the end of the 2000 monitoring season, close to 100 volunteers had been trained to identify and survey for the European green crab using WDFW protocols at over 100 different locations throughout the Puget Sound region. While monitoring over-all has decreased, we continue contracting with Nahkeeta Northwest for volunteer training and coordination. These efforts are primarily accomplished by setting baited crayfish traps in areas that have high potential for invasion and/or have habitat characteristics that appear to be favored by European green crab in the Northwest. As part of the cooperative work that is integral to this monitoring program, WDFW has contracted with a non-profit volunteer group to train and coordinate its volunteers and the general public to monitor for the European green crab. By the end of the 2000 monitoring season, close to 100 volunteers had been trained to identify and survey for the European green crab using WDFW protocols at over 100 different locations throughout the Puget Sound region. Although green crabs have never been found in Puget Sound or the Straits, there is an established population on Vancouver Island. We do not know what factors contribute to their lack of ability to establish a successful population in the coastal region, or why none have been found in Puget Sound.

The control/eradication measures found to be most effective on the outer Washington coast populations will be implemented in Puget Sound in the event a European green crab infestation is discovered.

Figures 10 and 11. Puget Sound and Coastal Green Crab Monitoring Sites

Contacts:

Import and transfer applications may be obtained from WDFW's Point Whitney Shellfish Laboratory, phone 360-796-4601.

If you suspect you have found a crab that fits the European green crab description, please contact Russell Rogers at the Point Whitney Shellfish Laboratory at (360) 796-4601 or, Brett Dumbauld/Andrea Randall at the Willapa Bay Field Station at (360) 665-4166.

If you are interested in learning how you can help monitor your local tidelands for the European green crab, please contact the WDFW staff for your area of interest at the phone numbers listed above.

From E.F. Ricketts and J. Calvin 1968 Between Pacific Tides, fourth edition revised by J.W. Hedgpeth,Stanford University Press

Further Reading:

Anonymous. 1997. Non-native species in Puget Sound: a strategy and recommendations. Puget Sound Water Quality Action Team, Sound Waves 12(2): 2.

Aagaard, A., C.G. Warman, and M.H. Depledge. 1995. Tidal and seasonal changes in the temporal and spatial distribution of foraging Carcinus maenas in the weakly tidal littoral zone of Kerteminde Fjord, Denmark. Mar. Ecol. Prog. Ser. 122: 165-172.

Attrill, M.J., and R.M. Thomas. 1996. Long-term distribution patterns of mobile estuarine invertebrates (Ctenophora, Cnidaria, Crustacea: Decapoda) in relation to hydrological parameters. Mar. Ecol. Prog. Ser. 143: 25-36.

Berrill, M. 1982. The life cycle of the green crab Carcinus maenas at the northern end of its range. J. Crust. Biol. 2: 31-39.

Berrill, M., and M. Arsenault. 1982. Mating behavior of the green shore crab Carcinus maenas. Bull. Mar. Sci. 32: 632-638.

Ching, H.L. 1989. Profilicollis botulus (Van Cleave, 1916) from diving ducks and shore crabs of British Columbia. J. Parasitol. 75: 33-37.

Cohen, A.N., J.T. Carlton, and M.C. Fountain. 1995. Introduction, dispersal and potential impacts of the green crab Carcinus maenas in San Francisco Bay, California. Mar. Biol. 122: 225-237.

Dare, P.J., and D.B. Edwards. 1981. Underwater television observations on the intertidal movements of shore crabs, Carcinus maenas, across a mudflat. J. Mar. Biol. Ass. U.K. 61: 107-116.

Dare, P.J., G. Davies, and D.B. Edwards. 1983. Predation on juvenile Pacific oysters (Crassostrea gigas Thunberg) and mussels (Mytilus edulisL.) by shore crabs (Carcinus maenas [L.]). MAFF Direct. Fish. Res., Lowestoft, Fish. Res. Tech. Rep. No. 73, 15 p.

Dawirs, R.R. 1984. Influence of starvation on larval development of Carcinus maenas L. (Decapoda: Portunidae). J. Exp. Mar. Biol. Ecol. 80: 47-66.

Dries, M., and D. Adelung. 1982. The Schlei as a model for the distribution of the green shore crab Carcinus maenas. Helgol. Meeresunters. 35: 65-77.

d'Edekem d'Acoz, C. 1993. Seasonal reproductive activities and size relationships of Carcinus maenas living in the south of North Sea. Cah. Biol. Mar. 35: 1-13.

Dumbauld, B.R., Kauffman, B.E. 1998. The nascent invasion of green crab (Carcinus maenus) in Washington State Coastal Estuaries. 1998 Joint Shellfish Growers Annual Convention. Nanaimo, BC. Canada.

Grosholz, E.D., and G.M. Ruiz. 1996. Predicting the impact of introduced marine species: lessons from the multiple invasions of the European green crab Carcinus maenas. Biol. Conserv. 78: 59-66.

Grosholz, E.D., and G.M. Ruiz. 1995. Spread and potential impact of the recently introduced European green crab, Carcinus maenas, in central California. Mar. Biol. 122: 239-247.

Hartnoll, R.G., and R.G.K. Paul. 1982. The embryonic development of attached and isolated eggs of Carcinus maenas. Int. J. Invertebr. Reprod. 5: 247-252.

Hogarth, P.J. 1983. Crabs in labs: the shore crab (Carcinus maenas) as teaching material. J. Biol. Educ. 17: 105-111.

Hughes, R.N., and R.W. Elner. 1979. Tactics of a predator, Carcinus maenas, and morphological responses of the prey, Nucella lapillus. J. Anim. Ecol. 48: 65-78.

Hughes, P.N., and R.N. Hughes. 1984. Learning of predatory skills by shorecrabs Carcinus maenas feeding on mussels and dogwhelks. Mar. Ecol. Prog. Ser. 16: 21-26.

Jamieson, Glen; Morrison, Jim Pacific Biological Station, Nanaimo, B.C. Canada Pers comm.

Jubb, C.A., R.N. Hughes, and T. ap Rheinallt. 1983. Behavioral mechanisms of size-selection by crabs, Carcinus maenas (L.) feeding on mussels, Mytilus edulisL. J. Exp. Mar. Biol. Ecol. 66: 81-87.

Kaiser, M.J., R.N. Hughes, and R.N. Gibson. 1993. Factors affecting diet selection in the shore crab, Carcinus maenas (L.). Anim. Behav. 45: 83-92.

Kaiser, M.J., R.N. Hughes, and D.G. Reid. 1990. Chelal morphometry, prey-size selection and aggressive competition in green and red forms of Carcinus maenas (L.). J. Exp. Mar. Biol. Ecol. 140: 121-134.

Lee, S.Y., and R. Seed. 1992. Ecological implications of cheliped size in crabs: some data from Carcinus maenas and Liocarcinus holsatus. Mar. Ecol. Prog. Ser. 84: 151-160.

LeRoux, P.J., G.M. Branch, and M.A.P. Joska. 1990. On the distribution, diet and possible impact of the invasive European shore crab Carcinus maenas (L.) along the South African Coast. S. Afr. J. Mar. Sci. 9: 85-93.

Liat, L.B., and A.W. Pike. 1980. The incidence and distribution of Profilicollis botulus (Acanthocephala), in the eider duck, Somateria mollissima, and in its intermediate host the shore crab, Carcinus maenas, in north east Scotland. J. Zool., Lond. 190: 39-51.

McDonald, P.S., Jensen, G.C., Armstrong, D.A. 2000. The competitive and predatory impacts of the nonindigenous crab Carcinus maenas (L.) On the early benthic phase Dungeness crab Cancer magister Dana. School of Fisheries. University of Washington.

McGaw, I.J., M.J. Kaiser, E. Naylor, and R.N. Hughes. 1992. Intraspecific morphological variation related to the moult-cycle in colour forms of the shore crab Carcinus maenas. J. Zool., Lond. 228: 351-359.

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