The Chinese mitten
crab (Eriocheir sinensis) - an Ecological
The purpose of this paper is to develop an understanding of the
ecological principles that are relevant for the management of the problem
that the introduction (or the threat thereof) of the Chinese mitten crab
(Eriocheir sinensis H. Milne-Edwards, 1854)
poses. One part of this undertaking is to describe the lifecycle
of the species and the impacts from an ecological perspective; another
part is to determine the areas of
uncertainty; and the last part is to evaluate the Management Model in the
of ecological considerations.
Non-indigenous species (NIS) pose a variey of challenges to the manager.
threats to the native ecosystem can occur on different levels. NIS can
outcompete native species,
they can destroy infrastructure, they can even impact human health. The
potential of a given NIS to do so depends on its own adaptability which
is mainly characterized - apart from physiological prerequisites like
adaptability - by a high
reproduction rate, which facilitates the
establishment in the new environment, and an
opportunistic feeding strategy (Moyle/Light, 1996; Groscholtz 1996).
The Ecological Model aims to give a better understanding of the ecology of
the NIS in order to achieve more appropriate management responses.
Model combines the lifecycle of the non-indigenous species with the
various impacts at each developmental stage. It cites the applicable
regulatory measures where they actually address impacts or vectors.
The Chinese mitten crab
is a native of the South China sea where it
inhabits as a catadromous species the
coasts and rivers. Since its first
occurrence outside its native habitat, the mitten crab has become firmly
in Europe and - important for the Washington State situation - in the San
Francisco Bay Area [details]. The crab
the grapsoid family with
the characteristic that "larvae escape from their eggs to drift free and
pass through various stages before settling on the bottom", differing thus
fundamentally from all pure fresh-water crabs which do not go through
these stages when the larvae drift (Panning 1938).
The fact that the larvae drift freely through the water is probably what
made the transfer from point of origin possible in the first place
(Panning 1938, 1952): Reproduction obviously requires a female and male
individual. A single ballast tank can contain enough larvae to
make reproduction in non-native waters more likely since female and male
individuals are "relocated" in sufficient numbers to make encounters
The Ecological Model identifies three likely secondary introductions in
addition to the initial introduction described in the
Model [figure 1a
the transport by ballast water in the intercoastal traffic; the transport
through recreational boating (as by-catch) in bait traps or in
water buckets; and the transport through shipping and packaging. All
these vectors add an "internal" or national component to the dispersal of
The mitten crab is practically a fresh-water animal as it spends most of
its life in
In spring the larvae escape from their eggs and spread in the water
develop immediately into a 1.7 mm zoeae, three
additional stages follow subsequently before the larvae reach the last
larval stage of a megalopa, which is 3-4 mm long. As a megalopa
the larvae change their shape to that of a crab; instead of
about, they take up a life at the bottom. During this development the
animals move gradually from the salt to the fresh water, i.e. the river
mouths. The crabs are too small at first to be able to make their way
the strong currents.
During their first summer the small crabs are brought with the
tidal current into fresh water, during their second summer the crabs
remain in the coastal regions where the water recedes at ebb tide until
they have grown sufficiently large to migrate upstream.
It is in the larval stages that a successful introduction into
non-invaded systems most likely happens, since large numbers of the larvae
can be introduced at once.
The Ecological Model marks for this stage, the juvenile stage, the most
impacts (see also table), many of
which are however also applicable to the adult stage.
Direct and indirect parasite transfer to humans can happen at all places
crab comes into contact with humans, either directly as food (raw crab
meat) or indirectly through predators of the crab (again raw meat).
The impact on fisheries may consist of direct consumption by the crab or
damage caused to the catch and the nets or traps (when the crabs cut the
threads with their jaws).
It is mostly in the tidal areas that the crabs burrow their
descending holes in the embankments which retain water when the tide
recedes; these burrows can also erode levees and dams.
The crab can pose a threat to agriculture by eating the shoots of plants
which has been described for rice agriculture (Vogel, 1994; Hieb, 1997).
Additionally, diked fields may be endangered by the crab's burrows.
The sheer number of crabs can have the potential of clogging intakes of
power plants and irrigation systems (Halat/Resh, 1996; Hieb, 1998;
Ecological interactions are the least known but encompass the
the mitten crab on the native ecological system. It can be reasonably
the basis of experience with other non-indigenous species that there
are impacts on the food cascade by competition and predation. Possible
impacts are depicted in [figure 2c]
and [figure 2d].
Two major reasons seem to drive the crabs on their migrations:
In late fall the crabs generally return to the middle of the rivers
for deeper water, causing such crowding in the narrow river-channels that
the younger and weaker crabs are forced to migrate further upstream for a
winter rest place.
The distance of their migration, however, is predominantly determined
by the salinity gradient, only subsequently by population
density (Panning 1938).
The second reason for the migration seems to be that converted rivers do
not hold sufficient nutrition for the crabs. Panning describes that in the
20s, when mitten crabs were first discovered in Germany, the migration
in the rich tidal feeding grounds, "it was th[e] enormous increase
[in numbers] that forced them to move on farther upstream in their search
for food" (Panning 1938).
If crabs encounter an obstacle on their migration on the bottom of
the river they try to get by it in many ways eventually, for exmple,
to pass a dam by land. The ability of the crab to stay ashore
certain amount of time - up to 38 days if it is emersed as by dew, for
example (Peters/Panning 1933) - enables the crab also to enter another
riversystem over land.
This leads within the Ecological Model to an additional path for secondary
invasion. Natural invasion may not only occur along the coast and the
riversystems but also across the land.
With development of their sex instinct the crabs leave their upstream
feeding grounds to begin their migration toward the estuaries.
The sex organs develop during this migration, and crabs reach puberty on
the last leg through brackish waters in the tidal regions.
In fall the crabs gather in large swarms to breed in the brackish waters.
Females fasten their eggs 24 hrs after the mating [pic]
to small hairs on their
pleopods on the underside [pic]
with a cementlike substance which only hardens
in brackish water with a salinity of more than 25; also, the
need salt water to mature (Panning 1938).
In the summer months after the mating (and brooding) males and females
set out for the banks in the estuaries where they gradually perish.
Mitten crabs never repeat their journey up the rivers; among the reasons
may be biological adaptation, general exhaustion, or the
great distance (Panning 1938).
The observed coverage by barnacles of older adults has been suggested as a
of their fading strength
after migration and breeding period: they are too slow and weak to fight
off encrusting organisms (Panning 1938).
Under normal circumstance the single
breeding period (semelparity) is compensated by an enormous egg
production (250,000 to one million eggs per female (Cohen and Carlton 1997)),a
characteristic which renders the mitten crab a successful invasive
The time schedule in the lifecycle (like hatching and migration) seems to
mainly influenced by temperature, thus hatching may be delayed in colder
springs and migration times may vary, the migration and development of the
individual is apparently determined (or "guided") by the salinity gradient
(Panning 1938; Panning/Peters 1933).
In general little is known about the
actual population sizes and the behavior of the crabs. Assessing and
by the fact that the crabs migrate in deep water and tend to be
Scientific uncertainties [figure 2b] exist in
particular in relation to the impacts and in
relation to the vectors of introduction.
An example for the former is the impact of parasites. The Chinese mitten
crab is secondary host for
the Oriental lung fluke
(Peragonimus westermannii [details]).
While the direct transfer to humans by
consumption is easily conceivable (if there is a market), the indirect
through mammals -
in Germany the crabs were ground and fed to lifestock at some stage
(Peters/Panning 1933) - or possibly fish bears more uncertainties and
What impact the crab might have on fisheries is yet unknown. Panning describes it
that the crab would even be able to catch healthy fish. Yet, when fishing
reach the bottom the crabs might crawl up on the net and
caught and thus defenseless fish [
pic] (Panning 1938). Another impact on
fisheries is the possibility that they destroy the nets with their jaws
and/or wear them out sooner with their rough carapaces, equally fish might
The impacts on agriculture are uncertain as to their probability and extent. While
in places with rice agriculture can cause damage by eating the rice
shoots (Vogel 1994; Hieb 1997) the consequences of an invasion for
agriculture are not predictable, though levees are certainly at risk as
they are in
Generally, experiences in other
geographic settings have to be considered with some caution as one
characteristic of successful invasive species is their adaptability
Thus the observations from Germany about the lifecycle seem to be not
directly transferable as crabs do not wander as far, return to the
brackish waters earlier, and start their temperature dependent
behavior earlier in the year (compare Panning, 1938 and Hieb, 1997).
The largest uncertainty is the one about the ecological interactions. The
impact on the food web is not described. As an opportunistic, omnivorous
animal the crab has the potential of disturbing the food web on more than
level, i.e. not only as a predator of other herbivores (or even
carnivores) but also as a competitor of herbivores (the crab eats plants,
Panning 1938). In Washington State's waters it may compete with crayfish,
compete with other predators, it may be an additional predator on native
With respect to the uncertainties about vectors, it is not yet
established whether and to what extent the crab "naturally" would
invade Washington State's waters. San Francisco Bay - so far apparently
area with an established population on the West coast (Veldhuizen/Hieb,
1998)- has a huge
watershed, which makes it unlikely for the crab to enter another
riversystem via land (though the dispersal within the watershed might very
well happen by land). Another possibility is the dispersion along the
coast through drift of the larvae into other estuaries.
Evaluation of the Management Model
The Management Model does not account for the gaps in the knowledge about
the mitten crab. Regulations in place address some of the vectors but do
them. Thus federal
regulations prohibit the illegal harvest and the
transport of the crab but they do not prevent the transfer of parasites,
directly or indirectly. Equally, the
and the Coast Guard regulations based on it (or rather the federal
Act implementing the IMO resolution) address the vector of ballast
water but they do not interrupt the introduction as the measures, i.e.
ballast water exchange, are only voluntary.
None of the impacts are addressed directly, the regulations only try to
interrupt or prevent the introduction of injurious species in general. No
measures are taken to prevent the spread of the mitten crab along the US
voluntary ballast water exchange refers only to international vessel
traffic; intercoastal traffic, i.e. US intern from California to
Washington, is not covered. Dispersal over land is not addressed at all,
neither the natural nor the one by shipping and packing material.
No measures are taken to limit the spread within the freshwater system,
pic] at dams. In short there is a blind spot in the Management
it comes to "internal" introductions.
Panning suggests as a possible consequence of river degradation that "many
native animals [are deprived of their natural
habitat], as for instance ... the
predatory fishes which would have been of the greatest importance in
fighting and checking these mitten crabs" (Panning 1938, also
Moyle/Light 1996 who point at the importance of habitat instability for
the establishment of non-indigenous species in general). The
Management Model does not emphasize the significance of an intact habitat
for the natural defense of non-indigenous species.
The Management Model does not provide for mitigating or abatement actions
where the damage already occurred. The burrows of the crab pose a severe
threat to levees, yet the Management Model does not address this
Finally, there are no monitoring or evaluation measures which would be
important to find out about the actual population, its behavior and
development. No educational actions are taken which would be important to
utilize any "voluntary potential" in the public for repelling mitten