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  A Literature Review of the Aquatic Invader Eriocheir sinensis, the Chinese
                                Mitten Crab
                                          Daniel Weller

Introduced biota is one of the major problems of the 21st century due to the exponential increases
in human population and trade interaction in the 20th century, which increased the movement of
biota on a global scale (Carlton and Cohen 1997). This increased movement afforded generalist
and opportunistic species the opportunity to colonize and become established in novel
environments where they have deleterious impacts on the native ecosystem, local economy and
human health.

Although the Chinese mitten crab, Eriocheir sinensis, is considered one of the world’s worst
invaders little is known about the impacts of the Chinese mitten crab as little is known about the
Chinese mitten crab within its native range. Currently, the Chinese mitten crab, a native of East
Asia, has colonized continental Europe, the United Kingdom and the Pacific Coast of North
America. The Chinese mitten crab is continuing to expand its range in all areas it has been
introduced. As a generalist with both a wide range of physiological tolerances and high
phenotypic plasticity it seems likely that the Chinese mitten crab will continue to spread. Its
feeding habits and interspecific interactions within its introduced ranges suggest that it poses a
direct threat to the health of local ecosystems and economies. Relatively little has been done to
manage mitten crab populations after the species has become established. The immediate, global
threat posed by the Chinese mitten crab to estuarine and riverine systems makes research on this
species of tantamount importance.

The taxonomy of the genus Eriocheir, commonly referred to as the mitten crabs, is currently
under debate. Six different species within the genera Eriocheir have been classified by different
authors (Tang et al 2003, Veilleux and Lafontaine 2007,). However, these same species have also
been divided into three seperate genera by Tang et al (2003). Morphologically and genetically
the species currently known as Eriocheir japonica and Eriocheir hepuensis are so closely related
to E. sinensis that Tang et al. (2003) recommended designating them a subspecies of E. japonica.
The disagreement between these studies demonstrates the need for further research to clarify the
taxonomic status of the Chinese mitten crab so we can better understand its invasive potential. If
the mitten crab is a subspecies it has the potential to draw on the larger gene bank and tolerances
of the other Eriocheir subspecies. This would increase the invasion potential of the mitten crab
and alter current predictions of its impact. The need to determine the potential for hybridization
between E. sinensis and other Eriocheir subspecies is especially acute as several of the species E.

sinensis maybe related to, including E. japonica, are also extremely successful invaders (Anger

 Native Range:

   The Chinese mitten crab is native to the river systems and estuarine regions of China, Korea,
   and Japan. The northern extent of its native range is 40 ° N and the southern extent is 26 ° N
   (Veilleux and Lafontaine 2007, Clark et al. 1998). The Yangtze River is the longest river
   within its native range and it can be found as far as 1500 km upriver (Chin Mitt Crab Control
   Comm 2002).

 Introduced Range:


   The Chinese mitten crab was first reported in the Aller River, Germany in 1912 and by 1914
   it had spread to the Elbe River (Panning 1939 as cited in Veilleux and Lafontaine 2007,
   Clark et al. 1998). Between 1914 and 1926 it used the Elbe River to reach the Baltic Sea; by
   1933 it was found throughout the Baltic and its adjacent rivers as far as the Vyborg River in
   Russia (Veilleux and Lafontaine 2007). Over the next seven years the crab spread west along
   the coasts of the Netherlands, Belgium and Northern France (Clark et al 1998, Veilleux and
   Lafontaine 2007). At the same time the crab was spreading west it had started moving inland
   and by 1940 it had reached Prague, 700 km inland (Clark et al 1998).

   A separate series of introductions to the Atlantic coast of France is believed to account for
   Chinese mitten crab introductions in Southern France, the Iberian Peninsula, the
   Mediterranean and Asia Minor (Herborg et al 2007). The year of the initial introduction is
   unknown but by 1956 the mitten crab had become established throughout France and the
   Western Mediterranean (Veilleux and Lafontaine 2007).

   Mitten crab populations in the United Kingdom (UK) are the result of a third series of
   introductions to the Thames River system beginning in 1935; by 1949 the crab had spread to
   along coast to Yorkshire (Clark et al. 1998, Herborg et al 2003, Veilleux and Lafontaine
   2007). The Chinese mitten crab is well-established in both areas and in 1992 a sudden
   increase in the crab population resulted in further rapid range expansion throughout the UK
   (Clark et al. 1998, Veilleux and Lafontaine 2007).

   To date the Chinese mitten crab is found throughout the Baltic, in coastal and estuarine
   regions of France, Belgium, the Netherlands, Portugal and Spain, the Thames Catchment
   Basin and selected estuarine habitats in the United Kingdom and the river systems adjacent to
   these areas (Clark et al. 1998, Herborg et al. 2006, Normant et al. 2000, Rudnick 2002,
   Veilleux and Lafontaine 2007). The crab is continuing to extend its range into the

   Mediterranean and west Asia; recently the crab has been reported and may or may not be
   established in Serbia, Northern Italy, Archangel Bay, the Azov Sea, the Gulf of Riga, the
   Black Sea, the Caspian Sea and the Persian Gulf (Ojaveer et al. 2006, Ruiz et al. 2006,
   Veilleux and Lafontaine 2007).

   North America

   The Chinese mitten crab was first reported in the San Francisco Bay in 1992 (Rudnick et al.
   2000). By 1998 the population had exploded and high propagule pressure was causing rapid
   colonization of estuarine and river systems throughout Northern California (Veilleux and
   Lafontaine 2007, Rudnick et al 2000, Rudnick 2002). There is no other confirmed population
   within North America however specimens have been caught in the Columbia River (1997),
   the Chesapeake Bay (2005), Delaware Bay (2007), Hudson River (2007), the Mississippi
   River Delta (1987), the Great Lakes (1965), and the Laurentian Lakes/St. Lawrence River
   (11 times between 1973-1996) (Chin Mitt Crab Control Comm 2002, Rudnick 2002, Ruiz et
   al. 2006, Veilleux and Lafontaine 2007).

Life History:
The Chinese mitten crab is a euryhaline species with a catadramous life cycle (Jin et al 1991,
Rudnick et al 2000). Upon hatching, the mitten crab spends 1 to 2 month as a planktonic larvae
(Anger 1991). During this marine free-swimming phase the larvae pass through five zoeal stages
and one megalopeal stage (Anger 1991, Chin Mitt Crab Control Comm 2002, Veilleux and
Lafontaine 2007). Under harsh conditions, such as low temperature or salinity, the larvae adapt
by undergoing a sixth zoeal and/or second megalopeal phase (Anger 1991, Veilleux and
Lafontaine 2007). In late summer or early fall the larvae metamorphasizes into juveniles within
the estuary, initiating the crab’s upstream migration into freshwater rivers where they complete
their maturation (Rudnick et al 2005).

The crab spends the entirety of its developmental stage in freshwater but it needs saltwater to
reproduce (Veldhuzien and Stanish 1999). During the migration downstream the gonads develop
(Rudnick et al. 2005, Veldhuzien and Stanish 1999). The conditions causing gonad development
and downstream migration are not yet understood and should be further researched as it may
provide insight into the best management strategy. Once the gonads are developed the crab
enters its adult stage; as an adult the crab is highly tolerant of salinity (0-35 PSU) and
temperature (4-32 ° Celsius) variations (Anger 1991, Veilleux and Lafontaine 2007). This
physiological tolerance allows adult mitten crabs to enter and maintain high densities in
ecosystems where reproduction cannot occur (Ojaveer et al 2006,Veilleux and Lafontaine 2007).

Mating lasts several days depending on environmental conditions; it seems that egg release is
tied to the tidal cycle, which maximizes larval retention within the estuary (Herborg 2004).
Successful mating can only occur at salinities greater than 15 PSU and produces 250,000 to a
1,000,000 eggs, which are attached to the abdomen of the female and thus protected (Cohen and

Carlton 1997). In northern climates, the female stays dormant in deeper waters during the winter
during which embryo development slows (Ingle 1986). In the spring the females return to the
estuaries where the eggs hatch (Ingle 1986). This adaption may allow the mitten crab to invade
climates farther north than previously thought.

The mitten crab lives one to five years depending on ambient environmental conditions (Veilleux
and Lafontaine 2007). Under experimental conditions juveniles collected from the European
population stopped maturing below 7 ° Celsius, however observations within the native range
showed failure to molt at temperatures below 10 ° Celsius (Hymanson et al 1999, Jin et al 2001).
Within both populations the greatest rate of growth occurred between 20 and 30 ° Celsius; at 30 °
Celsius growth stopped (Hymanson et al 1999). While the optimal temperature seems to be
constant for all populations the minimum temperature at which molting can occur should be the
subject of more study. If the difference in molting is a result of adaption within the European
population it would suggest that the Chinese mitten crab can adapt relatively quickly to new
environmental conditions. Cool temperatures therefore might not limit mitten crab range
expansion in the long term, increasing the number of ecosystem’s susceptible to mitten crab
invasion and generally increasing the crab’s invasive potential.

The complex life cycle of the Chinese mitten crab allows it to tolerate a wide range of
conditions; the only real limiting factors being salinity and temperature (Jin et al 2001).
Temperature tolerance varies with life stage but the species is generally abundant in temperatures
above 5 ° Celsius and sea surface temperatures above 0 ° Celsius. The presence of crabs in parts
of the Baltic with temperatures and salinities below this may mean the mitten crab is more
tolerant of salinity and temperature extremes then previously thought, if this were true this would
greatly increase the area susceptible to mitten crab invasion (Ojaveer et al 2006).

The crab is well-adapted to areas with variable water levels. It is highly resistant to desiccation,
capable of walking on land and able to survive 35 days in a moist terrestrial environment and 10
days in a dry burrow (Rudnick and Resh 2000, Velduizien and Stanish 1999). The crab’s
burrowing behavior is an adaption to areas with inconsistent or low rainfall (Rudnick and Resh
2000). The species appears to be relatively tolerant of poor water quality; populations in France
did decline following an increase in water pollution, although this seems to be a result of other
stressors weakening the crab’s tolerance to poor water quality (Gollasch 1999, Hymanson et al.
1999, Rudnick et al. 2003).

It is unlikely that reported differences in adaptive or physiological traits can be accounted for by
genetic differentiation between populations and are likely a result of phenotypic plasticity rather
than genetic divergence (Sui et al 2009). Such low levels of genetic differentiation are found
between populations of mitten crab that genetic drift and isolation are probably not having major
impacts on the species; it is probably due to the continuous introduction of new individuals and
genes into the population (Sui et al 2009). The high genetic diversity of all three mitten crab
centers, East Asia, Europe and San Francisco suggests that initial bottlenecks had little effect

(Veillieux and Lafontaine 2007). If the adaptive traits and physiological tolerance of the mitten
crab is a result of high phenotypic plasticity rather than genetic divergence the mitten crab may
be able to invade a greater area than previously thought. However, certain differences between
mitten crab populations, such as the difference in temperature tolerance between native and
Baltic populations, maybe a result of genetic adaption and evolution within the introduced

The general habitat requirements for the Chinese mitten crab are access to an estuarine and a
freshwater system in a temperate or subtropical climates; the specifics of these systems is highly
variable (Rudnick et al 2002). In terms of estuary requirements, the mitten crab does best in
estuaries with a large area of deep, saline or brackish water for embryonic and larval
development, a significant amount of shallow, productive water for juvenile growth and an
adjacent freshwater drainage system for the sexual maturation phase (Cohen and Carlton 1997).

The adult Chinese mitten crab appears to do best in freshwater drainages composed of numerous,
small channels with soft-bottom sediments, steep banks and deep pools of slow-moving water
lined with emergent macrophytes (Ingle 1986, Hymanson et al. 1999, Rudnick 2000, Rudnick
2002). Areas with large amounts of aquatic vegetation, shallow, open waters, muddy banks, fast-
flowing constrained channels and/or vegetated, cobbled banks may provide an alternative habitat
whose intrinsic nature prevents desiccation, and allows crab populations to reach high densities
in environments with less than ideal traits (Ingle 1986, Rudnick 2000, Rudnick 2002).

The juvenile Chinese mitten crab does best in low salinity systems with steep, clay banks lined
with emergents (Rudnick 2000). However, the crab can be found at lower densities in concrete-
lined channels, although this is probably a result of high propagule pressure from adjacent
systems forcing individuals into these less than ideal habitats (Ingle 1986, Rudnick 2000).

The Chinese mitten crab can also survive and reach high densities in highly modified and/or
variable environments (Ingle 1986, Rudnick 2003). This allows the crab to take advantage of
high disturbance environments especially those with frequent drought, and high levels of water
pollution (Ingle 1986). The crab may be able to take advantage of these highly disturbed
ecosystems because increases in pollution, increases in salinity and decreases in water levels
decrease the abundance of fish and other organisms that compete with and/or prey on the crab
(Ingle 1986). However, decreases in prey populations and the corresponding increases in
cannibalism because of increased disturbance will cause some decrease in crab populations
eventually (Chin Mitt Crab Control Comm 2002). In some areas the mitten crab appears
intolerant to poor water quality, this is may be due to additional stressors reducing the crab’s
invasive capability.

Based on the crabs ability to survive in less than ideal habitats it appears that once the crab is
introduced and becomes established it is there to stay. Abiotic and biotic factors appear to

provide a given ecosystem little resistance to invasion by the Chinese mitten crab. Although the
Chinese mitten crab seems to be able to survive almost anywhere, recent studies show that
estuaries with limited salinity intrusion and, “short flushing rates,” have a reduced risk of mitten
crab invasion (Hanson and Sytsma 2005).

Interspecific Interactions
Within all areas of interspecific interactions more research is needed especially within the
Chinese mitten crab’s native habitat.

 Feeding and Diet

   The Chinese mitten crab is omnivorous and opportunistic suggesting that feeding habits will
   not be a limiting factor in its spread (Koyabashi 2008). The feeding habits of the Chinese
   mitten crab are highly variable and they shift throughout its life cycle (Rudnick 2000,
   Hymanson et al. 1999). According to Hymanson et al (1999) larvae feed on plankton,
   juveniles eat aquatic plants (Velduhizen and Stannish 2002). As juveniles grow they shift
   more towards detrivory and carnivory although algae and detritus remain the basis of the
   adult crab’s diet (Rudnick and Resh 2005, Velduhizen and Stannish 2002). The lime needed
   to harden the shell after molting is provided by consumption of algae-associated
   invertebrates, mainly gastropods and amphipods (Rudnick and Resh 2005). European studies
   show that the adult crab feeds on worms, clams, snails, shrimp, other crustaceans, water
   insects, and insect larvae as well as dead and injured fish (Chin Mitt Crab Control Comm
   2002, Veilleux and Lafontaine 2007, Rudnick 2000). Gut analysis of E. japonica, a close
   relative the mitten crab, showed large amounts of animal materials, including fish, bivalve
   and crustacean; since the Chinese mitten crab maybe a subspecies of E. japonica it may
   actively prey on fish, bivalves and crustaceans as well (Kobayashi 2008). Adult mitten
   crabs show little tendency to attack healthy fish or amphibians in their native range (Rudnick
   2000), however data on crab-fish interactions is minimal and could benefit from further
   investigation. The adult crabs have been known to engage in cannibalism when food is
   scarce (Chin Mitt Crab Control Comm 2002).


   Several studies suggest that the Chinese mitten crab actively competes with other benthic
   invertebrates for food and shelter (Rudnick 2000). Crabs and crayfish have been seen feeding
   simultaneously on dead fish; the crayfish backed away almost every time a crab came up but
   crabs never retreated when a crayfish approached suggesting crabs may exhibit dominance
   within the community hierarchy (Rudnick 2000) This would make the crab a major
   competitor with the native crayfish for food. Crab-crayfish competition seemed to have
   limited impact on crayfish abundance when food was plentiful but when food was scarce
   there may be a noticeable impact (Rudnick 2000). In laboratory studies the Chinese mitten
   crab excluded other crustaceans from shelter (Rudnick 2000). The effect of competitive

 interactions on native crayfish seems to be amplified during periods of drought and increased
 pollution as it increases stress on the more intolerant native species (Clark et al 1998).
 Rudnick and Resh (2005) speculate that a native crayfish Camabrus nigrescens was
 extirpated in the San Francisco Bay region by the Chinese mitten crab invasion through


 Little is known about native predators of the Chinese mitten crab or predators within Europe.
 Predation by bullfrogs, loons, egrets and several species of fish including white bass and
 catfish has been reported in San Francisco (Veldhuzien and Stanish 2002). Raccoons, otters,
 wading birds, and other predatory fish are may also be potential predators (Veldhuzien and
 Stanish 2002). The nocturnal habits of the Chinese mitten crab is an adaption that may
 minimize potential predation and reduce the role these interactions play in limiting
 population levels (Veldhuzien and Stanish 2002). According to Rudnick (2003) release from
 predation in San Francisco is what allows the Chinese mitten crab population to explode in
 highly modified and polluted systems. Thus, predation within introduced ranges could
 significantly reduce mitten crab population levels (Ingle 1986).

 Predation on the juveniles of related Eriocheir species by pelagic fish have been shown to
 cause evolution within introduced populations of crabs (Anger 1995). Although this response
 to predation has not been seen in the Chinese mitten crab, its occurrence in related species
 suggests there is potential.

Parasitic and Pathogenic Interactions

 The only pathogen known to attack the mitten crab is a spiroplasma caused-tremor disease
 (Wang et al 2004). The disease first appeared in 1994 and spread rapidly throughout the
 crab’s native range causing 30%-90% mortality (Wang et al 2004). While the disease only
 appears to be effective at killing the crabs at higher temperatures, it presents the possibility of
 biocontrol. Since the species specificity of the tremor disease and its effectiveness at lower
 temperatures is unknown, further research is needed to determine if it will be useful as a
 biocontrol agent. Studies on the biology and ecology of the spiroplasma causing the disease
 could reveal insights into how to control tremor disease in other organisms (Wang et al

Invasion Facilitation

 The migratory habits of the mitten crab allows it to transport native and non-native species to
 new habitats, allowing it to act as a potential vector for other invasive species (Normant et al.
 2007). Mitten crabs can be a vector for introduction of other invasive species as numerous
 organisms inhabit the setae on the mitten crab claws (Normant et al. 2007). The majority of
 species found within the setae are juvenile forms of bivalvia, gastropoda, oligochaeta,

   gammaridae and chironomidae (Normant et al. 2007). Algae, seaweed and emergent plant
   propagules have been found in association with mitten crab setae (Cohen and Carlton 1997,
   Normant et al. 2007). Successful spread of the epibiota that inhabit mitten crab setae has not
   yet been documented.

   The Chinese mitten crab also appears to facilitate further invasion by introduced crayfish and
   serpulid worms, which have been found in conjunction with mitten crab burrows in San
   Francisco (Cohen and Carlton 1997). It appears that burrows may offer shelter for these non-
   indigenous species, decreasing the impact of desiccation and other stressors on the species.

   The crab is unaffected by but has the potential to transport the North American crayfish
   disease, which has the ability decimate crayfish populations. The mitten crab could carry the
   disease long distances during migration or in ballast, infecting previously uninfected
   populations in North America and Europe. If the disease arrives in Europe or other
   uninfected areas its ability to impact native crayfish populations is unknown.

Dispersal and Establishment Trends
According to Cohen and Carlton (1997) there are ten potential mechanisms for the long distance
dispersal of the Chinese Mitten Crab (1) transport of adults crabs within cargo (2) transport on
slow-moving vessels (3) transport of larvae or juveniles within ballast (4) transport with fisheries
products or stock (5) escape from research, or aquaria (6) intentional introduction to establish
aquaculture (7) dispersal of larvae by the currents (8) dispersal on floating material (9) transport
via ship fouling. The relatively short larval stage of the mitten crab, and its complex,
catadramous life cycle makes it unlikely that the mitten crab reached North America or Europe
by natural means, on floating debris or currents; these mechanisms could account for range
expansion once the crab is established in a given area (Cohen and Carlton 1997). The most
likely transport mechanism for the mitten crab’s introduction was in ballast water except in San
Francisco where genetic data suggests that the introduction was the result of an attempt to
establish aquaculture (Veilleux and Lafontaine 2007). In Southern France the movement of
fisheries products and stock is a more likely mechanism for the introduction of the mitten crab
(Herborg et al 2005).

Based on shipping and consumption patterns Europe is an unlikely source for introduction of the
mitten crab to the San Francisco Bay but could be the source of the crabs being found along the
East coast of North America (Cohen and Carlton 1997, Veilleux and Lafontaine 2007).
However, high levels of genetic diversity within invasive populations of mitten crab suggest
multiple sources of introduction and repeated introductions into Europe, the St. Lawrence, San
Francisco, and the East Coast (Veilleux and Lafontaine 2007). Determination of the source
populations for the invasive mitten crab populations through genetic, morphological or
behavioral studies may allow for the determination of a pattern which would help it the
development of management strategies.

Non-native species are most likely to become established when there is a high-level of propagule
pressure and high likelihood of survival (Jerde and Lewis 2007). Since the Chinese mitten crab is
a generalist species with high phenotypic plasticity, it has a high likelihood of survival in most
ecosystems. The large volume of shipping between areas with and without Chinese mitten crab
populations means propagule pressure is consistently high. The Chinese mitten crab therefore
matches both of the criterion established by Jerde and Lewis (2007) for successful establishment
of a non-native, suggesting that once introduced to an area there is a high probability the Chinese
mitten crab will become established.

After introduction the mitten crab appears to go through an establishment phase of about 40
years, followed by a sudden population explosion brought on by a short-term change in
environmental conditions such as drought (Veilleux and Lafontaine 2007, Clark et al 1998,
Herborg et al 2005). The length of this lag phase is dependent on the amount of propagule
pressure and individual probability of survival; in areas with low propagule pressure the lag
phase appears to last longer than 40 years, in areas with high propagule pressure it can be as
short as six (Herborg et al 2005, Jerde and Lewis 2007, Rudnick et al 2000) Until conditions
change in favor of the mitten crab it survives in mircohabitats that act as incubators; presence of
these microhabitats increase the invasibility of an estuarine or river system (Herborg et al 2005).
An important characteristic of these microhabitats seems to be deep, saline waters (Herborg et al
2005). Once the crab population explodes the crabs undergo range expansion as the population
surpasses carrying capacity of the current habitat (Carlton and Cohen 1997).

The mitten crab population can undergo rapid range expansions due to its catadramous life cycle
and physiological tolerance. Mitten crabs can spread between 78 and 448 km per year (8 to 12
km per day) depending on temperature, current patterns and hydrology (Ingle, Rudnick 2000, Sui
et al 2009). Where canals are present range expansion is more rapid as canals facilitate the
spread of mitten crabs between river systems and farther inland. The rapidity of mitten crab
spread is not inhibited by natural or man-made barriers as the crabs are resistant to desiccation
and can walk on land (Herborg 2003, Sui et al 2009).

Impacts of Invasion

       Individual and Species Level Impacts

       The Chinese mitten crab has a high potential to greatly alter native ecosystems. The
       majority of research has been on potential impacts of mitten crabs on native benthic
       invertebrates. These studies suggest that native crayfish could be outcompeted for food
       and habitat, however past invasions by other species suggest that native extinctions rarely
       result from competitive exclusion by invasives (Veilleux and Lafontaine 2007, Fridley et
       al. 2007). Predation is a more likely cause of native extinction than competition
       (Gurevitch and Padilla 2004). Since native extinction is often a result of predation,

predation by the mitten crab on the eggs of aquatic vertebrates and wading birds is of
particular concern (Fridley et al. 2007, Veilleux and Lafontaine 2007). Since eggs make
up an extremely small proportion of the crab’s diet it is unlikely that crab predation alone
would cause species’ extinction, other factors would also have to be stressing the aquatic
organism (Veilleux and Lafontaine 2007). Predation by the mitten crab also has been
shown to cause local extinctions of several gastropods and bivalve species within its
European range (Chin Mitt Crab Control Comm 2002). Within its European range it has
been shown to compete with and depress benthic fish populations suggesting that the
mitten crab has the potential to significantly affect native species and potentially disrupt
ecosystem composition and structure even if it does not cause extinction (Chin Mitt Crab
Control Comm 2002).

Community Level Impacts

The crab’s movement habits means that a great variety of ecosystems have the potential
to be disrupted. Since larval and juvenile crabs move vertically within the water column
as they develop, they have a greater ability to impact pelagic and planktonic communities
within estuaries (Hanson 2005). This catadromous habit of that mitten crabs allows the
crab to significantly impacting energy and nutrient availability within aquatic systems by
consuming large amounts of biomass within a given aquatic system and then transporting
it out of said system (Rudnick and Resh 2005). This effect is most likely greatest on the
estuary in systems composed of small estuaries and extensive river systems; in systems
composed of short river systems and large estuaries the reverse is probably true. Benthic
food webs are especially impacted as the basis of this food web is detritus and the crab
has the potential to remove much of this (Rudnick and Resh 2005).

A recent study by Rudnick and Resh (2005) suggests that the Chinese mitten crab feeding
behavior could cause shifts in community composition. Direct competition with algal-
associated invertebrates and shallow-dwelling invertebrates could cause a shift towards
deeper sediment-dwelling species, which would explain its facilitation of the invasion of
the serpulid worm (Carlton and Cohen 1997).

River Integrity

The burrowing behavior of the Chinese mitten crab presents a clear threat to river bank
and bed integrity. Mitten crab burrowing stresses and destroys bank vegetation increasing
erosion rates (Rudnick 2002, Rudnick et al 2005). Burrowing itself can remove as much
as 90 km3 of sediment within a 2 km segment of stream causing bank slumping and
collapse (Rudnick 2002, Rudnick et al 2005). Areas with high crab densities, intertidal
areas, areas with lots of wave action, areas of silt-sand sediment and areas with steep
banks are at greatest risk of bank weakening and collapse (Rudnick 2002, Rudnick et al
2005). The high volume of sediment removed by the mitten crab has the ability to highly

   modify bottom substrates, increase turbidity and decrease water quality, which would
   have a cascade effect throughout the river or lake ecosystem (Rudnick 2000, Rudnick et
   al 2005).

Economic Impacts

   Impacts on Fish Salvage Operations and Industry

   Both European and North American mitten crab populations have hampered water intake
   by industry and decreased the effectiveness of fish salvage operations. During crab
   migration densities reach extremely high numbers causing crabs to swim into intake
   pipes, fish salvages and other water associated infrastructure creating clogs and breaking
   equipment (Rudnick 2000). When the daily crab count got above 1000 crabs per day in
   the San Francisco Bay area the crabs caused high fish mortality during salvage operations
   (Veldhuzien and Foss 2001) and in the UK mitten crabs have been known to clog power
   station intakes (Clark et al 1998). The exact cost of mitten crabs to industry and fish
   salvage operations has not yet been calculated although yearly predictions exceed one
   million dollars.

   Impacts on Fisheries

   The Chinese mitten crab has the potential to disrupt fishery operations in Europe and
   North America. Several studies have suggested that the Chinese mitten crab poses a
   threat to fisheries by reducing target species populations, reducing catch size, damaging
   equipment and bait stealing. According to Rudnick and Resh (2002) studies conducted in
   concert with local fisherman in the San Francisco Bay reported that mitten crab impact
   was in inverse relationship to speed of the trawler with low-speed trawlers catching the
   most crabs, thereby reducing catch size. This means that shrimp and other fisheries using
   low-speed trawls would be the most impacted (Rudnick and Resh 2002). Suggested
   correlations between increases in mitten crab and decreases in the other crab species such
   as the Dungeness, bivalve species and crayfish species means the crab could also pose a
   threat to these fisheries (Rudnick and Resh 2002, 16). In Portugal the mitten crab is
   widely viewed as a pest responsible for significantly reducing eel fishery yields through
   bait stealing, equipment damage and predation on eels in traps, although the negative
   economic impacts on the eel fishery have been mitigated by establishment of a mitten
   crab fishery (Cabral and Costa 1999).

   The claim by European fisherman that the Chinese mitten crab directly consumes fish
   and reduces population levels is highly unlikely. Predation on healthy fish is unlikely to
   reduce fish stocks because the crab is too slow to catch or harm healthy adult fish
   (Rudnick and Resh 2002, Chin Mitt Crab Control Comm 2002, Velduzien eand Stanish
   2002). Crab predation on fish eggs, however could be a problem especially for species
   already stressed by external factors (Veilleux and Lafontaine 2007).

       The Chinese mitten crab has the potential to severely disrupt recreational fishing. The
       mitten crab interferes with recreational fishing by stealing bait so that some areas within
       San Francisco Bay have become unfishable because of the rate of bait stealing (Chin Mitt
       Crab Control Comm 2002).

       Agricultural and Urban Impacts

       Little is known about the crab’s impact on agriculture and more research is definitely
       needed in these areas. Damage to rice crops through feeding on young shoots in San
       Francisco (Chin Mitt Crab Control Comm 2002). It has also been cited as a potential
       cause of decreased crop yields in China and Korea (Chin Mitt Crab Control Comm
       2002). The crab’s ability to survive and rapidly multiply within highly modified
       environments, presents a severe threat to human earthworks, irrigation canals and levees
       along banks (Rudnick 2002). This has the potential to magnify the effects of flooding and
       decrease crop yields by increasing water stress.

 Impacts on Human Health

       The only known threat to human health posed by the mitten crab is its potential to act as a
       vector for introduction of the Oriental lung fluke, Paragonimus westermani. The Chinese
       mitten crab and other crustacean species are the second intermediate hosts of the Oriental
       lung fluke and consumption of the mitten crab is an important source of infection in
       China and Korea (Cohen 2003). Since twelve decapods species within the United States
       have been found to be a potential host of the lung fluke, since several species of snails
       within the US could act as primary hosts and since lung fluke has been found in
       introduced crabs, establishment of a lung fluke population in North America is likely. No
       research is available on the potential for establishment of a population in Europe, studies
       should be done to determine if there are primary and secondary intermediate hosts
       available there.

Potential Range Expansions:
Currently there are four global population centers from which the Chinese mitten can spread to
colonize suitable river systems (Cohen and Carlton 1997). Due to the increase in global ship
traffic the movement of mitten crabs to new areas is highly likely (Sui et al 2009).

Repeated reports of Chinese mittens crabs throughout the east coast of the United States (US)
makes eventual expansion by the Chinese mitten crab there is likely. The St. Lawrence River is
at high risk of mitten crab invasion as it offers an ideal habitat; the Laurentian and Great Lakes
are at moderate risk of invasion (Lafontaine et al 2008). In the Northeast establishment is also
likely within the Hudson River system due to high level of shipping, and large amount of
suitable habitat (Chin Mitt Crab Control Comm 2002). In the Southeast establishment of the
mitten crab is most likely in the lower section of the Mississippi River, and the adjacent Gulf

Coast or the Chesapeake or Delaware Bays (Chin Mitt Crab Control Comm 2002, Ruiz et al
2006). The ports of Baltimore and Norfolk are two of the most vulnerable to future introductions.
Due to the large number of crabs reported in Baltimore Harbor since 2005 there is potentially a
small population of crabs already in the Chesapeake, however the crabs could have originated
from ballast transfer (Ruiz et al 2006).

On the Pacific coast of the North America the Colombia, Willamette, and Tillamook river
systems are at risk due high levels of shipping, and the movement of recreational boats between
these systems and those of Northern California (Veilleux and Lafontaine 2007). Using these river
systems the crab could extend its range as far upstream as northeastern Washington and Idaho.
The Snake and Willamette Rivers have natural barriers that will prevent its spread farther. In
these river-dominated estuaries crab populations may be limited by the small amount of estuarine
habitat (Hanson and Systma 2005). However, nearby estuaries including Coos Bay, Yaquina
Bay, Tillamook Bay, Puget Sound, Grays Coast, and Willapa Bay have sufficient estuarine
habitat. Coos Bay is less susceptible then the rest because of its, “lower flushing rates” (Hanson
and Systma 2005). The northern extent of the mitten crab invasion on the Pacific coast of North
America will be limited by water temperatures below the crab’s mortality threshold; this
threshold will probably be reached in Southern Alaska (Hanson and Systma 2005).

Almost the entirety of Europe, excluding Scandinavia, the Alps and other mountainous regions,
is vulnerable to invasion as is most of the Mediterranean, and coastal portions of the Middle East
(Herborg et al 2007). In the UK the mitten crab has the potential to spread throughout the entire
Thames catchment and potentially the entire island due to the extensive network of rivers and
canals (Clark et al 1998, Ruiz et al 2006). The same is true for Ireland (Clark et al 1998).

The Chinese mitten crab has never been reported in any area within the Southern Hemisphere.
Further research into why this might be is needed to determine (1) whether it can invade
estuarine systems within the Southern Hemisphere, which seems likely based on its invasion
history (2) whether global dispersal methods for the species makes introduction to the Southern
Hemisphere likely.

Global climate change has the potential to increase the Chinese mitten crab’s potential for range
expansion. According to Dukes and Mooney (1999) short-term increases in water allowed for
long-term range expansion in Artemisia tridentata. Dukes and Mooney (1999) proposed that
alteration fop weather patterns due to global climate change could increase this type of
expansion. Since short-term decreases in precipitation have already been shown to facilitate the
expansion of established population of mitten crabs into new areas, it is likely that decreased
rainfall in certain areas could facilitate its spread (Rudnick 2000). Due to its short generation
time, and rapid dispersal abilities the mitten crab may be able to take advantage of changes
created by global climate change to a greater extent than other species (Dukes and Mooney
1999). Since there is a marked increase in larvae survival and salinity tolerance with increases in
temperature the increase in temperature associate dwiht global climate change could facilitate the

invasion of crab as well. On the other hand increasing precipitation due to global climate change
could decrease salinity in certain estuarine systems it may make certain areas less hospitable to
the Chinese mitten crab reducing its potential to invade.

The highly oscillatory population levels, wide physiological tolerances, migratory nature, ability
to rapidly spread and high propagule pressure associated with established populations of mitten
crabs makes eradication improbable and management difficult. Differences between different
populations within the same area, such as the North and South Bay populations in San Francisco
Bay that result from its high phenotypic plasticity and genetic diversity create further problems
when trying to control the species (Rudnick 2002).

The challenges associated with control means the best option is to not let the mitten crab get
established. Since the primary method of introduction is through ballast water, the use of ballast
water exchange has been proposed as a control mechanism (Lafontine 2008). Unfortunately,
ballast water exchange has had low mortality rates for catadramous species similar to the mitten
crabs suggesting it would not be an effective method of introduction prevention (Lafonainte

Once the mitten crab is established eradication is improbable and probably impossible. The
methods that have been suggested are the use of biological control and establishment of a mitten
crab fishery. Although biocontrol has been suggest it is at a low priority as little is known about
pathogens and predators of the mitten crab and there is a general fear over maintaining species
specificity of the biocontrol agent (Chin Mitt Crab Control Comm 2002).

Creation of a mitten crab fishery seems like the best management option. Harvest of the mitten
crab may reduce local demand for imported mitten crabs and support ailing fisheries. The mitten
crab is a traditional food in China where it supports a fishery worth $1.25 billion and in Portugal
a fishery has already been created that gets higher market returns than other decapods (Herborg
et al 2005, Hymanson et al. 1999, Cabral and Costa 1999). There is a known demand in Asia,
North America and Portugal for mitten crab, as illegal, live shipments of mitten crabs have been
intercepted (Chin Mitt Crab Control Comm 2002). Demand in the rest of Europe is unknown or
low (Chin Mitt Crab Control Comm 2002). In Europe where there is a low demand for Chinese
mitten crabs however, the mitten crab could be used for fertilizer and as feed for livestock (Chin
Mitt Crab Control Comm 2002). The combined food, fertilizer and feed markets could create
enough demand to reduce mitten crab populations. However, in Germany where there is an
annual catch of 242 metric tons the establishment of a mitten crab fishery has failed to
effectively reduce mitten crab range expansion or stop population increase. (Herborg et al 2005).

Future Research:

The widespread lack of information available on the Chinese mitten crab means there are many
areas where research needs to be done. This means that research must be prioritized. The mitten
crab’s ability to rapidly spread, and the significant impact it can have on local ecosystems once it
is established means a focus needs to be placed on (1) preventing its spread (2) determining its
impact once it has spread and (3) decreasing the mitten crab’s impact within the introduced

Currently the main dispersal method for the mitten crab appears to via ballast water. Therefore,
one of the primary areas of research should be into potential methods of killing mitten crab
larvae within the ballast tanks. To prevent further mitten crab spread areas that are highly
sensitive to invasion by the mitten crab need to be mapped. To do this more research needs to be
done on the crab’s tolerances and habitat requirements within its native range. Further research,
on the crab’s ability to invade areas with and tolerate poor water quality, lack of water and the
compounding effects of multiple stressors should be done. No information exists on the crab’s
chemical, mineral or day-length requirements, or mortality rates with regard to freshwater
discharge or dissolved oxygen. Further research should be done to determine this as a greater
knowledge of the crab’s tolerances will allow for greater prediction of its potential range.

 The impacts of the mitten crab on local ecosystems is vaguely understood, in order to minimize
these impacts more research needs to be done on them. In general, more information on crab
predation and feeding habits is needed to efficiently determine the threat Chinese mitten crab
invasion may pose to aquatic invertebrates and fish. Some of these studies should look at feeding
behavior within the crab’s native habitat. Further research should also look at historical
extinctions caused by the mitten crab as they allow for prediction of future extinctions.

The significant impact the mitten crab has once it has been introduced also means that research
efforts should focus on reducing mitten crab populations once the species has become
established. Since there is limited information on pathogens and parasites affecting Chinese
mitten crabs in their native range, further research into these interactions would very useful as
these pathogens maybe potential biocontrol agents.

The Chinese mitten crab is well-adapted to invasion. It is a generalist species with a high
reproductive rate, rapid dispersal and range expansion and wide range of physiological
tolerances; this allows it to establish populations in almost any environment. It has already
established populations in North America and Europe, and will likely spread throughout the
Northern Hemisphere as there appear to be no real limitations to its spread, making every
estuarine and riparian ecosystem susceptible. The behavior of the mitten crab makes its inherent
invasability of increased concern as it appears to have deleterious impacts on the ecology,
economy and human health of areas to which it is introduced. Due to the significant challenges

associated with controlling and managing aquatic invaders, especially rapidly reproducing ones
such as the mitten crab, there are no established management strategies. This set of traits makes
the Chinese mitten crab the ideal aquatic invader. As the ideal aquatic invader studies on the
mitten crab could reveal unique and novel patterns in aquatic invasion. Due to this potential, the
deleterious impact of the Chinese mitten crab on local ecosystems and economies and the
relative dearth of knowledge we have on Chinese mitten crab evolution, taxonomy, physiological
tolerances, and interspecific interactions especially within its native range more research needs to
be done.


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