Assignment #2

The Chinese mitten crab (Eriocheir sinensis) - an Ecological Analysis



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) [ pic] 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 scientific uncertainty; and the last part is to evaluate the Management Model in the light of ecological considerations.

Non-indigenous species (NIS) pose a variey of challenges to the manager. The 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 climate 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.

Ecological Model

The Ecological 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 established in Europe and - important for the Washington State situation - in the San Francisco Bay Area [details]. The crab belongs to 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 happen.

The Ecological Model identifies three likely secondary introductions in addition to the initial introduction described in the Management 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 crab.

The mitten crab is practically a fresh-water animal as it spends most of its life in freshwater. In spring the larvae escape from their eggs and spread in the water column. Prezoeae 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 drifting freely 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 upstream against 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 where the 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; Veldhuizen, 1998).
Ecological interactions are the least known but encompass the effects of the mitten crab on the native ecological system. It can be reasonably assumed on 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 ended 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, climbing onshore to pass a dam by land. The ability of the crab to stay ashore for a 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 eggs 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 sign 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 species.

The time schedule in the lifecycle (like hatching and migration) seems to be 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).

Scientific Uncertainty

In general little is known about the actual population sizes and the behavior of the crabs. Assessing and monitoring is complicated by the fact that the crabs migrate in deep water and tend to be nocturnal.
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 transfer 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 intraceabilities.
What impact the crab might have on fisheries is yet unknown. Panning describes it as unlikely that the crab would even be able to catch healthy fish. Yet, when fishing nets reach the bottom the crabs might crawl up on the net and eat the 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 be damaged.
The impacts on agriculture are uncertain as to their probability and extent. While the crabs in places with rice agriculture can cause damage by eating the rice shoots (Vogel 1994; Hieb 1997) the consequences of an invasion for Washington's agriculture are not predictable, though levees are certainly at risk as much as they are in other regions.
Generally, experiences in other geographic settings have to be considered with some caution as one characteristic of successful invasive species is their adaptability (Groscholtz 1996). 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 one 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, it may compete with other predators, it may be an additional predator on native species.
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 the only 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 not close 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 IMO Resolution 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 coast. The 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, such as traps [ pic] at dams. In short there is a blind spot in the Management Model when 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 problem.
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 crabs.