Carcinus maenas (European shore crab)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Biology and Ecology
- Water Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Impact Summary
- Economic Impact
- Environmental Impact
- Social Impact
- Risk and Impact Factors
- Uses List
- Detection and Inspection
- Similarities to Other Species/Conditions
- Prevention and Control
- Gaps in Knowledge/Research Needs
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Carcinus maenas Linnaeus, 1758
Preferred Common Name
- European shore crab
Other Scientific Names
- Cancer maenas
- Carcinides maenas
- Carcinoides maenas
International Common Names
- English: European green crab; green crab; harbour crab; shore crab
- Spanish: cámbaro; cañeta; cangrejo de mar
- French: le crabe enragé; le crabe vert; le crabe vert europeén
Local Common Names
- Germany: Strandkrabbe
- Netherlands: strandkrab
Summary of InvasivenessTop of page
C. maenas is considered one of the worst alien invasive species in the world, native to Atlantic Europe, the western Baltic and west Africa to Mauritania, but widely introduced to North America and Australia, and more recently to South Africa, South America, East Asia and elsewhere, It is an omnivore, which can consume species from at least 104 families, 158 genera including phyla of animals, plants and protist, so food is usually not a limiting agent. C. maenas possesses characteristics that enable it to be transported by a growing number of vectors, which helps explain why the species has obtained its extensive global invasive range that includes parts of all non-polar continents. As a generalist, it can survive in many of the places it is transported to, and once established C. maenas can negatively affect many species by predation and competition.
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Metazoa
- Phylum: Arthropoda
- Subphylum: Crustacea
- Class: Malacostraca
- Subclass: Eumalacostraca
- Order: Decapoda
- Family: Portunidae
- Genus: Carcinus
- Species: Carcinus maenas
Notes on Taxonomy and NomenclatureTop of page
Carcinus maenas is closely related to a ‘sister species’ Carcinus aestuarii, with which it hybridizes, and introduced populations in South Africa and Japan are comprised of the hybrid form (Gellar et al., 1997).
DescriptionTop of page
C. maenas is a shore crab that can range in size from a carapace width of a 1-2 cm to 9-10 cm (Grosholz and Ruiz, 1996) and is wider than it is long (Klassen and Locke, 2007). Its colour is highly variable and therefore not a good characteristic for identification as it can be brown to green, to orange and even red in color. The easiest clue that you possibly have collected C. maenas is that it has 5 antero-lateral teeth or spines on each side of the crab and three rounded lobes between the eyes.
It can live up to 6 years along the east coast of North America (Berrill, 1982). However, on the west coast and in its native range, such as along the coast of Belgium, it grows faster and has a maximum life span of only 4 years (d'Udekem d'Acoz, 1993; Yamada et al., 2001). Females usually live for three years whereas males live for 5 years. Eggs develop attached to the female under her abdominal flap. The free swimming planktonically dispersed larva is transported by surface currents as it undergoes 4 zoeal stages before becoming megalopa, which metamorphose into benthic juveniles before maturing into adult organisms (NIMPIS, 2002; Klassen and Locke, 2007). The duration of each zoeal stage is roughly 5 to 7 days before an 8 day megalopa stage (Webster and Dircksen, 1991). The amount of time spent in any of these stages is temperature-dependent (Dawirs, 1982) and can range from approximately 2.5 to 13.5 days for zoeal stages and 5.5 to 26 days for the megalopal stage (Dawirs and Dietrich, 1986). It can be up to 90 days before C. maenas develops into benthic-dwelling shore crab (Grosholz and Ruiz, 2002). C. maenas can reach a carapace width over 9 cm although they are more commonly only a few cm wide.
DistributionTop of page
The native distribution of C. maenas includes coastal parts of Europe and Africa, from Iceland and Norway in the north, to Mauritania in the south, and the Baltic Sea in the east.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 14 Dec 2020
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Egypt||Absent, Formerly present||First reported: early 1800s|
|Madagascar||Absent, Formerly present||1922|
|South Africa||Present, Localized||Introduced||Hybrid with C. aestuarii|
|Western Sahara||Present, Widespread||Native|
|India||Absent, Unconfirmed presence record(s)||Carlton and Cohen (2003) raise doubt on Alcock's record of C. maenas in India|
|Japan||Present||Introduced||Hybrid with C. aestuarii|
|Myanmar||Absent, Formerly present||1933|
|Pakistan||Absent, Formerly present||1971|
|Sri Lanka||Absent, Formerly present||First reported: 1866 or 1867|
|Faroe Islands||Present, Widespread||Native|
|United Kingdom||Present, Widespread||Native|
|Canada||Present||Present based on regional distribution.|
|-British Columbia||Present, Localized||Introduced|
|-New Brunswick||Present, Widespread||Introduced||1951||Invasive|
|-Newfoundland and Labrador||Present, Localized||Introduced||First detected in August 2007 in North Harbour, Placentia Bay but was well established. In September, after expanded monitoring it had been discovered in Davis Cove, Swift Current, Goose Cove, Come-By-Chance, Arnold's Cove, Southern Harbour and Black River; Original citation: Klassen and Locke (2007)|
|-Nova Scotia||Present, Widespread||Introduced||Invasive||Original citation: Klassen and Locke (2007)|
|-Prince Edward Island||Present, Localized||Introduced||Invasive||Original citation: Klassen and Locke (2007)|
|-Quebec||Present, Localized||Introduced||Found on the Magdalen Islands but not yet found on the mainland; Original citation: Klassen and Locke (2007)|
|Panama||Absent, Formerly present||1866||Specimen (PMNH 9535) “from the Bay of Panama”|
|United States||Present, Widespread||Introduced||1817||Invasive|
|-Hawaii||Absent, Formerly present|
|-New Hampshire||Present, Widespread||Introduced||Invasive|
|-New Jersey||Present, Widespread||Introduced||Invasive|
|-New York||Present, Widespread||Introduced||Invasive|
|-Rhode Island||Present, Widespread||Introduced||Invasive|
|-New South Wales||Present, Localized||Introduced||Invasive|
|-Western Australia||Absent, Formerly present|
|Atlantic - Eastern Central||Present, Localized||Introduced||Invasive|
|Atlantic - Northeast||Present, Widespread||Introduced||Invasive|
|Atlantic - Northwest||Present, Widespread||Introduced||Invasive|
|Atlantic - Southeast||Present, Localized||Introduced||Invasive|
|Atlantic - Southwest||Present, Localized||Introduced||Invasive|
|Atlantic - Western Central||Present, Widespread||Native|
|Indian Ocean - Antarctic||Absent, Formerly present|
|Indian Ocean - Western||Absent, Formerly present|
|Pacific - Northeast||Present, Localized||Introduced||Invasive|
|Pacific - Northwest||Present, Localized||Introduced||Invasive|
|Argentina||Present, Localized||Introduced||Caleta Caroline in Camarones Bay and Caleta Sara|
|Brazil||Absent, Formerly present|
|-Pernambuco||Present, Localized||Introduced||Alcock did not record the date of collection or exact location (Carlton and Cohen, 2003)|
|-Rio de Janeiro||Present, Localized||Introduced||1857|
History of Introduction and SpreadTop of page
The episodic global dispersal of C. maenas (Carlton and Cohen, 2003) and establishment began when it was first detected in the northeast USA in the early 1800s (Say, 1817). Although the exact location was not noted, it was somewhere along the coast of New York or New Jersey (Carlton and Cohen, 2003). It expanded south to Maryland, and north, reaching Canadian waters by 1951 (Leim, 1951). Well established populations of C. maenas were also detected in 2007 in Placentia Bay, Newfoundland, Canada (Klassen and Locke, 2007). Molecular studies confirmed that C. maenas from the east coast of the USA was the source population for the introduction and its established population on the west coast of North America (Geller et al., 1997), where it was detected in Estero Americano and San Francisco Bay, California in 1989 or 1990, and it is now present up the Pacific coast (Carlton and Cohen, 2003), now as far as British Columbia, Canada (Gillespie et al., 2007).
IntroductionsTop of page
|Introduced to||Introduced from||Year||Reason||Introduced by||Established in wild through||References||Notes|
|Natural reproduction||Continuous restocking|
|USA||Europe||1817||Yes||Carlton and Cohen (2003); Say (1817)||Accidental introduction to the east coast of the USA via solid ballast|
|USA||USA||1989||Live food or feed trade (pathway cause)||Geller et al. (1997)||Accidental introduction from east to west coast on USA probably via packing material from live food trade|
Risk of IntroductionTop of page
C. maenas is already established in areas of all non-polar continents (Carlton and Cohen, 2003; Hidalgo et al., 2005). Given the breadth of its abiotic tolerances, it could expand its distributions beyond already colonized regions. Introductions will probably be continued by the same vectors that have been utilized previously, i.e. mainly by ballast water, but could include hull fouling, and indirectly by activities of the live food and bait industries (Cohen et al., 1995; Carlton and Cohen, 2003). The chance that there would be intentional release is possible but minimal. Given that ballast water and larval drift, among other vectors such as aquaculture, are readily available, continued primary and secondary spread is likely to occur in the future (Carlton and Cohen, 2003; Carlton, 2005). Further expansion north on the Pacific coast of North America is predicted to lead to significant economic costs to the shellfish industry, as the habitat is suitable into Alaska (Cohen and Carlton, 2003).
HabitatTop of page
C. maenas is a generalist and can survive in soft and hard substrates in most marine and brackish waters; however, its habitat usage varies between introduced ranges (Grosholz and Ruiz, 1996). C. maenas can be found in protected and semi-protected coastline habitats such as rocky intertidal, mudflats, estuaries, eelgrass and cordgrass marshes, manmade structures and shallow subtidal areas (Cohen and Carlton, 1995; Cohen et al., 1995; Grosholz and Ruiz, 2002). On the west coast of North America, it has not yet colonized rocky habitat as it is along the coast of South Africa, the east coast of the continent and its native range where it has been established for much longer periods (Grosholz and Ruiz, 1996). It can be found on man-made structures such as docks and aquaculture equipment. C. maenas is a crab that mainly inhabits the intertidal zone but it has been found at depths of 60 m (Crothers, 1968).
Habitat ListTop of page
|Brackish||Inland saline areas||Principal habitat||Harmful (pest or invasive)|
|Marine||Sea caves||Principal habitat||Harmful (pest or invasive)|
|Littoral||Mud flats||Principal habitat||Harmful (pest or invasive)|
|Littoral||Intertidal zone||Principal habitat||Harmful (pest or invasive)|
|Littoral||Salt marshes||Principal habitat||Harmful (pest or invasive)|
|Brackish||Estuaries||Principal habitat||Harmful (pest or invasive)|
|Brackish||Lagoons||Principal habitat||Harmful (pest or invasive)|
|Marine||Inshore marine||Principal habitat||Harmful (pest or invasive)|
|Marine||Pelagic zone (offshore)||Secondary/tolerated habitat||Harmful (pest or invasive)|
|Marine||Benthic zone||Principal habitat||Harmful (pest or invasive)|
Biology and EcologyTop of page
Although C. maenas is one of the most studied crabs after the blue crab (Callinectes sapidus) for its biology and ecology, knowledge of genetics for C. maenas is still lacking in many regards but is an area of current research, especially with the aim of this work is to identify the source population for an introduction (Gellar et al., 1997; Roman and Palumbi, 2004; Roman, 2006). Genetic tools have already determined that the donor region for the invasion of C. maenas on the west coast was the east coast of North America rather than from its native range in Europe (Gellar et al., 1997), and that hybridization between C. maenas and its sibling species C. aestuarii in South Africa and Japan (Geller et al., 1997). Molecular techniques can also be used to help detect the presence of C. maenas larvae in ballast tanks (Darling et al., 2007).
C. maenas starts its life as part of the zooplankton community. It is a highly fecund species that reaches maturity quickly, and females are reproductively mature after one to three years. C. maenas is an iteroparous species and female crabs can mate multiple times during a breeding season but probably only produce one clutch of eggs per year (Grosholz and Ruiz, 2002). A maximum clutch size ranges from 185,000 to 200,000 fertilized eggs (Cohen and Carlton, 1995; NIMPIS, 2002). The exact timing of the breeding season varies between geographic regions (Klassen and Locke, 2007) but usually occurs between April to November (WDFW, 2008). The female carries the eggs in an egg sac (plug) under her abdominal flap (NIMPIS, 2002). While the females are gravid, it is hypothesized that they live in deeper water to take advantage of more stable conditions of salinity and temperature (WDFW, 2008). Eggs hatch into free swimming planktonic larvae that live in the water column for 17-80 days, depending on temperature (NIMPIS, 2002).
Physiology and Phenology
C. maenas has a carapace that offers many benefits, mainly protection from predators, but can also undergo autotomy in dangerous or stressful conditions (Davis et al., 2005), and it reduces the probability of desiccation. To further reduce the risk of desiccation and predation, it hides under rocks and macrophytes during low tide.
C. maenas is an omnivore that has been documented to prey on species from at least 104 families, 158 genera in 5 plant and protist and 14 animal phyla (Cohen et al., 1995). Its prey preference has been relatively similar in its multiple introduced regions, as C. maenas consistently prefers shellfish but does not consume echinoderms (Grosholz and Ruiz, 1996). It also feeds on algae but to lesser extent than the Asian shore crab, Hemigrapsus sanguineus, which is another crab introduced to the east coast of the USA (Ropes, 1968; Elner, 1981; Tyrrell and Harris, 1999; Bourdeau and O’Connor, 2003; Brousseau and Baglivo, 2005; Griffen and Delaney, 2008).
As a shore crab that has been introduced to sections of all non-polar continents, it inhabits the same habitat as many intertidal, subtidal, and estuarine organisms (see also Notes on Natural Enemies).
C. maenas is an excellent osmoregulator, therefore it is a euryhaline species. It can tolerate a wide range of salinities and temperatures, which allows it to inhabit a diverse range of marine ecosystems, ranging from intertidal habitats of estuaries, to the coast and to the ocean. Adults can tolerate salinities from as low as 4 parts per thousand (ppt) up to 52 ppt (Cohen and Carlton, 1995) and can also tolerate a range of temperatures from -2°C to 35°C (Eriksson and Edlund, 1977; Spaargaren, 1984; Carlton and Cohen, 1995; Cohen et al., 1995; Hidalgo et al., 2005), but prefers a temperature range of 3°C to 26°C and salinities between 10 and 33 ppt (Grosholz and Ruiz, 2002). Given that C. maenas is an omnivore, food is usually not a limiting agent (Grosholz and Ruiz, 1996), and normal feeding can occur down to 6-7°C but molting and growth is suppressed or stops at temperatures below 10°C (Eriksson and Edlund, 1977; Cohen et al., 1995; Klassen and Locke, 2007). Furthermore, C. maenas can endure relatively low levels of dissolved oxygen (i.e. 1-1.5 mg oxygen/l) and therefore hypoxia is not as important as it is to other species (Legeay and Massabuau, 2000). Larvae are less tolerant of a wide range of abiotic conditions than adults and survive in temperatures of 9°C to 22°C and salinities of 20 ppt and greater (Dawirs et al., 1986; Grosholz and Ruiz, 2002). Therefore, the environmental tolerances of larvae may be more limiting and thus more important for determining suitable habitats and understanding its ability to spread and establish (Williams, 1984; Dawirs, 1985; Klassen and Locke, 2007).
Water TolerancesTop of page
|Parameter||Minimum Value||Maximum Value||Typical Value||Status||Life Stage||Notes|
|Depth (m b.s.l.)||<10||Optimum||60 tolerated|
|Dissolved oxygen (mg/l)||>2||Optimum||1-1.5 tolerated|
|Salinity (part per thousand)||20||35||Optimum||4-52 tolerated. For larvae 20-35 preferred; 4-52 tolerated|
|Water temperature (ºC temperature)||3||26||Optimum||-2-35 tolerated. For larvae 9-22.5 preferred; 5-22.5 tolerated|
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Carcinus maenas||Predator||Adult; Larval|
|Crangon crangon||Predator||Fry; Larval|
|Sacculina carcini||Parasite||Adult||not specific|
Notes on Natural EnemiesTop of page
The main predators of C. maenas are species of shore birds (Moreira, 1999), such as herring gulls (e.g. Larus argentatus) along the coast of the Atlantic Ocean (Sibly and MacCleery, 1982; Dumas and Witman, 1993). Various species of fish, such as the European sea bass (Dicentrarchus labrax), consume C. maenas (Kelley, 1987). C. maenas is cannibalistic and predates on smaller C. maenas. Two species of shrimps (Crangon crangon and Palaemon elegan) are also known to be significant predators of juvenile C. maenas (Moksnes, 2002). Also in its native range, it is parasitized by multiple species such as rhizocephalan parasitic castrator, Sacculina carcini (Kuris et al., 2003). In the native range, C. maenas is parasitized, but parasite loads are reduced in their introduced ranges, which leads to increased fitness and larger sizes (Grosholz and Ruiz, 1996; Torchin et al., 2001; Torchin et al., 2003).
Means of Movement and DispersalTop of page
There are multiple vectors for the spread of C. maenas and they have greatly increased over time as global trade and its technologies have increased (Carlton and Cohen, 2003). Initially the major vectors were solid ballast and hull fouling (Carlton 2005). Then ballast water became a common and important vector for primary spread; and it can even be important for secondary spread. Other vectors for spread are dispersal by larval drift and unintentional transfer from activities such as aquaculture, live food and bait trade (Cohen et al., 1995; Carlton and Cohen, 2003). As the speed of ships has increased so has the overall survivorship of the invader and its hitchhikers (Sassi et al., 2005).Natural Dispersal (Non-Biotic)
Oceanographic dispersal occurs when C. maenas is in its planktonic larval stage; the length of this stage being temperature-dependent but is usually between 17 and 80 days (Dawirs, 1982; NIMPIS, 2002; Grosholz and Ruiz, 2002; Klassen and Locke, 2007). This allows for secondary spread that can allow dispersal for hundreds of kilometers by surface currents. Also, adult migrations may play a small role in secondary spread, but are usually mainly vertically with the tidal cycle (Klassen and Locke, 2007) and therefore probably not important for large-scale spread.Vector Transmission (Biotic)
No spread on live animal vectors that are not transported commercially is recorded.
All the introductions of C. maenas have been accidental, but due to multiple vectors. It was first accidentally introduced to east coast of North America in the early 1800s via solid ballast (Carlton and Cohen, 2003). Then over a hundred years later in the early-mid 1900s, it was probably introduced into the west coast of the continent by accidentally being transported in the seaweed, usually Ascophyllum nodosum, which is used to pack New England baitworms (Glycera dibranchiata and Nereis virens) and live sea food such as lobsters (Homarus americanus) during transport (Cohen et al., 1995; Cohen and Carlton, 2003).
Pathway CausesTop of page
|Hitchhiker||Yes||Yes||Cohen and Carlton (1995)|
|Live food or feed trade||Brought C. maenas from east to west coast of USA||Yes||Cohen and Carlton (1995)|
|Pet trade||low probability||Yes||Yes|
|Research||Is a possibility as they are shipped around the world for research and education||Yes||Yes|
Pathway VectorsTop of page
|Aquaculture stock||Proportional to the amount of aquaculture for a site but in these areas quite common||Yes||Yes|
|Bait||Yes||Cohen et al. (1995)|
|Live seafood||Indirectly in the packing of the actual seafood C. maenas can be transported||Yes||Yes||Cohen et al. (1995)|
|Machinery and equipment||Movement of oil rigs, docks and other marine structures||Yes||Yes||Le et al. (1990)|
|Pets and aquarium species||low probability||Yes||Yes||Green Crab Control Committee (GCCC) (2002)|
|Ship ballast water and sediment||most common vector||Yes||Yes||Carlton and Cohen (2003)|
|Ship hull fouling||Yes||Yes|
|Ship structures above the water line||Lower probability than ballast water but can survive out of water for up to 5 days||Yes||Klassen and Locke (2007)|
|Soil, sand and gravel||Possible if marine sediments are those such as found in ballast tanks||Yes||Yes||Klassen and Locke (2007)|
|Water||Larvae are dispersed by wind-driven surface currents||Yes||Yes|
Impact SummaryTop of page
|Environment (generally)||Positive and negative|
ImpactTop of page
Impacts are a hard thing to objectively quantify as they are scale-dependent and vary between geographical regions. In general ecological and economical terms, C. maenas will affect marine biodiversity. The addition of the species itself affects marine diversity but given that it is an opportunistic predator and fierce competitor it is likely to impact species on various trophic levels. Given that it is a generalist, it can broadly affect marine ecosystems and their biodiversity, including economically important species, such as shellfish and crabs.
Economic ImpactTop of page
C. maenas can greatly impact crab and shellfish industries. For example, one crab can consume up to 40 half inch (12 mm) clams per day (WDFW, 2008) and the species has been implicated in the demise of New England (north-eastern USA) shellfish industry in the 1950s (Glude, 1955; Hart, 1955; Cohen et al., 1995; Smith et al., 1955). C. maenas has the potential to negatively affect not only shellfish but possibly other economic species such as crabs (e.g. Cancer magister). Quantifying the economical effects can be challenging and attempts to predict the potential cost of the damage caused by the invasion of C. maenas are limited, and of questionable validity (Hoagland and Jin, 1996; Klassen and Locke, 2007). Lafferty and Kuris (1996) estimated that the species could possibly cause US $44 million of damage for the then potential and now on-going invasion of the west coast of the USA (Klassen and Locke, 2007). More informed and realistic values from Lovell et al. (2007) found that on the east coast of the USA, C. maenas causes US $22.6 million of damage per year in predation on shellfish alone and that on the west coast of North America, the damages are negligible but estimated to increase to at least US $0.84 million per year if C. maenas continues its spread north. This further expansion and this predicted economic loss is likely to occur because the habitat is suitable north into Alaska (Cohen and Carlton, 2003).
Environmental ImpactTop of page
C. maenas can degrade habitats. Its omnivorous diet can significantly impact the structure of intertidal and shallow subtidal communities (Cohen et al., 1995). Klassen and Locke (2007) suggest that C. maenas and other factors have decreased the abundance of eelgrass (Zostera marina) in the southern Gulf of St. Lawrence, Canada, but further studies are needed to better document and understand these interactions and impacts on the survivorship of Z. marina (Davis and Short, 1997; Davis et al., 1998) and degradation to other habitats. C. maenas is a voracious omnivore that will affect biodiversity, given that it feeds on species from at least 104 families, 158 genera in 5 plant and protist and 14 animal phyla, it could possibly cause extinctions, extirpations and reductions in abundance of native species (Cohen et al., 1995).
Social ImpactTop of page
Reduced abundance of shellfish due to the predation of C. maenas will reduce the enjoyment, job security and revenue that recreational and commercial crab and shellfish harvesting provides.
Risk and Impact FactorsTop of page
- Proved invasive outside its native range
- Has a broad native range
- Abundant in its native range
- Highly adaptable to different environments
- Is a habitat generalist
- Capable of securing and ingesting a wide range of food
- Highly mobile locally
- Benefits from human association (i.e. it is a human commensal)
- Fast growing
- Has high reproductive potential
- Has high genetic variability
- Altered trophic level
- Changed gene pool/ selective loss of genotypes
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Increases vulnerability to invasions
- Modification of natural benthic communities
- Negatively impacts agriculture
- Negatively impacts human health
- Negatively impacts livelihoods
- Negatively impacts aquaculture/fisheries
- Reduced native biodiversity
- Threat to/ loss of native species
- Competition - monopolizing resources
- Competition (unspecified)
- Interaction with other invasive species
- Highly likely to be transported internationally accidentally
- Difficult/costly to control
UsesTop of page
Uses ListTop of page
Animal feed, fodder, forage
- Pet/aquarium trade
- Research model
Human food and beverage
- Meat/fat/offal/blood/bone (whole, cut, fresh, frozen, canned, cured, processed or smoked)
- Green manure
Detection and InspectionTop of page
Detection and inspection programmes are very important but often insufficient and sometimes non-existent. The areas of greatest concern would be to survey ballast water for larvae of C. maenas, its sediments and the hulls of ships for the presence of adults and juveniles. Also surveys of ports and surrounding areas, especially estuaries, bays and protected rocky coasts, which have habitats that could support C. maenas, should be conducted. Given its susceptibility and economic importance, aquaculture sites should be intensely monitored. Lastly, aquaculture gear, boats, research equipment, dock, oil rigs, sea weeds, sediments, traps and all material that is transported from one geographic region to another should be inspected for the presence of C. maenas and other hitchhiking species (Grosholz and Ruiz, 2002). In general, any material that has entered infested brackish or marine waters should be inspected, if not quarantined or treated, before it is allowed to be put in uninfected waters. New genetics tools might greatly increase our ability for early detection. Darling et al. (2007) are currently developing tests that can detect if a single larva of C. maenas is present in ballast water.
Similarities to Other Species/ConditionsTop of page
C. maenas has a sibling species, C. aestuarii, which has also been referred to as C. mediterraneus (Klassen and Locke, 2007). Geller et al. (1997) state that both species are very similar in morphology except for the shape and degree of setation of the male’s pleopods. C. aestuarii inhabits the shores of the Mediterranean Sea (Roman and Palumbi, 2004). For more in depth description of the two species and how to distinguish between them, please see Yamada and Hauck (2001). Genetic evidence supports that these are two different species (Roman and Palumbi, 2004). To date, C. maenas has been more widely introduced than C. aestuarii (Carlton and Cohen, 2003).
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.Prevention
Prevention is the most effective and cost-efficient way to deal with invasive species (Leung et al., 2002). Yet it is challenging, because the global economy grows quickly, policy reacts slowly, the vectors are great, and the area that needs to be searched is huge. Prevention is extremely important but by no means a fool-proof solution and is not always feasible.
Early warning systems
Early warning systems are very important but is usually only possible during a small period of time when the populations of the introduced species are localized and at low densities. Unfortunately, given the challenges of successful early detection and the lack of warning systems, rapid dissemination occurs and rapid response does not usually occur in time. The actual monitoring for early detection varies from region to region but is quite limited. If an introduced species is detected in the USA and reported to the United States Geological Survey, its Nonindigenous Aquatic Species Alert system (http://nas.er.usgs.gov/AlertSystem/default.asp), they will email the information to registered members. Anyone can subscribe for informational updates provided by this free service on a species (e.g. C. maenas), taxonomic group (e.g. crabs) or state (e.g. New York). This type of service is limited yet has great potential as a framework for early detection systems but requires further development and funding to allow it to move beyond its national boundaries, as introduced species is a global problem.
Rapid response is vitally important for the success of eradication but improved tools and greater funding is needed to make it feasible. The Washington (state) Department of Fish and Wildlife, USA in the summer of 1998 received emergency funds from the Governor to monitor and control C. maenas (WDFW, 2008). Government agencies should ideally have a prepared and peer-reviewed rapid response plan before C. maenas is ever detected. Also increased monitoring is needed so C. maenas can be detected during the early stage of the invasion, when eradication is still feasible, because the populations of the invader are localized and at low densities. Rapid response needs to be linked with increased monitoring that will lead to increased probability of early detection and successful eradication (Myers et al., 2000; Bax et al., 2001; Rejmanek and Pitcairn, 2002).
There is a growing and important need to increase public awareness and involvement in detecting, controlling and eradicating invasive species. The public can be trained very easily to monitor for C. maenas with high levels of accuracy (Delaney et al., 2008). The public could increase the monitoring effort and fill in areas where scientists cannot monitor themselves. In North America, various programs are currently executing this type of work including Invasive Tracers (www.InvasiveTracers.com) and the Coastal Habitat for Invasives Monitoring Program (http://www.salemsound.org/chimp.htm). On the west coast of the US, programs have also been conducted. For example, Washington Department of Fish and Wildlife started a non-profit volunteer group to raise awareness of the invasion of C. maenas and to monitor for its presence along the coast of the state (WDFW, 2008). These efforts should be continued and expanded by increasing available funding for programs that raise public awareness and sustain increased monitoring by training and incorporating citizen scientists.
Eradication has been attempted for C. maenas but has never been successfully achieved (Grosholz and Ruiz, 2002). In terrestrial eradication attempts, the probability of success is negatively correlated with the size of the infestation (Myers et al., 2000; Rejmanek and Pitcairn, 2002) and this has been suggested to also be the case in aquatic systems (Bax et al., 2001). Unfortunately, usually the invader is well established, its densities are large and its populations are not localized, which make eradication challenging, if not impossible. Early detection, warning systems, rapid response plans and action are probably prerequisites for eradication to be successful.
Control is a measure that does not need early detection and therefore has been heavily used in various regions around the world. It is mainly used in shellfish aquaculture sites given their great economic importance. Various methods have been explored but the majority of current approaches involve trapping and poisoning with varying success (Cohen and Carlton, 1995). Control is challenging and expensive but could possibly be achieved on small-scales such as within an aquaculture lease or an isolated bay or estuary (Grosholz and Ruiz, 2002).
The most successful and widely used approach for physical control is trapping. Given the high fecundity of C. maenas, trapping is usually limited in its ability to control the density of the invader (Walton et al., 1999; Walton and Walton, 2001).
Movement control has only been effective with adults in aquaculture sites. Physical barriers are used to prevent or minimize adult C. maenas from reaching and predating on shellfish. Movement control is difficult in other contexts since dispersal in mainly done by larvae transported by currents and cargo ships’ ballast water. Vector ecology and management is a promising area of current research to deal with movement of propagules for C. maenas and other species using the same means of transport (Ruiz and Carlton, 2003). Various options are currently being researched (Sassi et al., 2005). Also C. maenas can be moved via the bait and live food trades and these vectors should be inspected to prevent or at least reduce the amount of propagules being transported.
A proposed biological control agent of C. maenas is the castrating parasite, Sacculina carcini, which infects C. maenas in its natives range, but like most native parasites it did not get introduced with its host to the non-native range. Preliminary results from host specificity trials, however, indicate the parasite would parasitize both C. maenas and non-target species (Goddard et al., 2005). This area could be further explored to find a biological control agent that is species-specific, and extensive study will be needed to properly examine the specificity of the parasite before its utilized (Kuris et al., 2003).
Chemical control could be used on different scales for eradication attempts and control efforts. On a large-scale Carbaryl a broad-spectrum organocarbamate can be applied by helicopters (McEnnulty et al., 2001). Carr and Dumbauld (1999) predict that Carbonyl would be an effective chemical control agent for C. maenas, which has been used to minimize the impacts of burrowing thalassinid shrimps on oysters in various bays of Washington state and would probably be quite effective on crabs as it has been used in terrestrial species and environments, for example, to control insects in agricultural fields (McEnnulty et al., 2001; Grosholz and Ruiz, 2002). Before wide-scale application of Carbaryl or any other pesticide, extensive research on impacts of non-target species, including humans and environmental review must be conducted (WDFW, 2008). Another avenue for chemical control is the use of poisoned bait, which was used on the east coast of North America with some success (Hanks, 1961). The poisons previously used on the east coast are more toxic than the ones currently being considered for controlling C. maenas on the west coast of the USA (Grosholz and Ruiz, 2002). The advantage of using the poison, in the form of bait soaked in the chemical, is that it is cheaper, easier to implement on a larger scale and is not as size selective as traps are, but it can also impact non-target species (McEnnulty et al., 2001; Grosholz and Ruiz, 2002). Environmental, legal and permitting issues must be considered and addressed (McEnnulty et al., 2001; Grosholz and Ruiz, 2002) before C. maenas is ever detected, as the window for eradication is quite limited.
Control by utilization
Control by utilization is an area that is surrounded by much uncertainty. The crab is harvested for consumption in parts of Europe (Gomes, 1991; Cohen and Carlton, 1995). This has lead to declines due to the intense and commercially motivated harvesting activities. Control efforts, mainly harvesting by trapping, have been attempted on the east coast of North America but have not led to similar declines in C. maenas populations. A few oyster growers in Massachusetts, USA are considering harvesting and selling C. maenas as a way to offset management costs, but success has been varied and usually limited (Cohen et al., 1995; Walton, 2000). Nevertheless, if enough resources are available, control can be effective by intense and prolonged harvesting, which may only be feasible by industrial harvesting for commercial sale. Programmes that create economic incentives for harvesting of aquatic introduced species, however, can be risky and this option is currently not recommended by managers on the west coast of the USA (Grosholz and Ruiz, 2002). It is nonetheless a challenging and complex question that deserves further debate and consideration. If control by utilization is going to be attempted, it needs to be intensely implemented, which may only occur if the harvest is for commercial purposes. This has been examined to determine if this is a feasible means to control C. maenas population on Prince Edward Island, Canada (JCB, 2002). Harvesting C. maenas should be further investigated to determine if it would be an economically and logistically feasible management strategy to develop a commercial market to harvest and sell C. maenas.
Monitoring and surveillance
Monitoring for early detection is limited in scale and intensity and is usually conducted using a suboptimal sampling technique. Monitoring is important for early detection and improves chances of rapid response, which increases the probability of successful eradication.
When the populations are well established and self-sustaining, eradication is often no longer an option; but mitigation combined with control can be used to reduce costs for shellfish aquaculture (Grosholz and Ruiz, 2002). Physical barriers, such as predator netting, cages, or putting small clams in seed bags or wracks, can reduce predation pressure of C. maenas (Grosholz and Ruiz, 2002). Furthermore, selective harvesting and changing strategies to minimize the amount of time that shellfish of a vulnerable size are exposed to the C. maenas by changing the density, seed size and timing of planting the seed has reduced the impacts of C. maenas (Smith, 1954; Walne and Dean, 1972; Dare and Edwards, 1976; Castagna and Kraueter, 1977; Eldridge et al., 1979; Kraueter and Castagna, 1980; Arnold, 1984; Walker, 1984, Eggleston et al., 1992; Peterson et al., 1995; Procter, 1997, Beal, 1998; Walton, 2000; Grosholz and Ruiz, 2002).
Gaps in Knowledge/Research NeedsTop of page
C. maenas is relatively well studied compared to other marine introduced species, as it is large, easily collected and tagged, and it has both a large native and introduced range. The major gap in the research is determining the most effective and cost-efficient way to detect and control the invader and developing models that identify what geographic areas are at greatest risk for spread of C. maenas, which will be important for determining areas to monitor and optimally allocate limited resources such as personnel and equipment.
ReferencesTop of page
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OrganizationsTop of page
ContributorsTop of page
29/05/08 Original text by:
David Delaney, McGill University, Department of Biology, 1205 Docteur Penfield, Montreal, Quebec, H3A 1B1, Canada
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