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Page last updated at 16:29 GMT, Monday, 27 July 2009 17:29 UK
Freshwater crabs 'feel the pinch'
Matt Walker
Editor, Earth News

The freshwater crab Geothelphusa ancyclophallus
Geothelphusa ancyclophallus has little defence against habitat destruction

Two thirds of all species of freshwater crab maybe at risk of going extinct, with one in six species particularly vulnerable, according to a new survey.

That makes freshwater crabs among the most threatened of all groups of animals assessed so far.

The study is the first global assessment of the extinction risk for any group of freshwater invertebrates.

Crab species in southeast Asia are the most at risk, from habitat destruction, pollution and drainage.

Scientists from the Zoological Society of London and Northern Michigan University led the survey, which produced the first International Union for Conservation of Nature (IUCN) Red List of Threatened Species for the 1280 known species of freshwater crab.

Of those, the survey found that 227 species should be considered as near threatened, vulnerable, endangered or critically endangered.

For another 628 species, not enough data exists to adequately assess their future, says the survey published in the journal Biological Conservation.

However, while the most optimistic scenario is that 16% of all species are at risk, the worst case scenario suggests the figure could be as high as 65%, or two-thirds of all species.

Keystone species

Freshwater crabs are essential to many freshwater ecosystems. Some feed on fallen leaves and algae, while other species help cycle nutrients by eating vast quantities of detritus.

The crabs themselves are an important source of food for a range of birds such as herons and kingfishers, reptiles such as monitor lizards and crocodiles and amphibians such as frogs and toads. Mammals that like to dine on freshwater crabs include otters, mongooses, civets as well as wild boar and even macaque monkeys.

Because most species require pristine water to survive, they are also excellent indicators of good water quality.

But species are increasingly being impacted by habitat destruction and pollution.

Singapore freshwater crab (Johora singaporensis )
The Singapore freshwater crab Johora singaporensis lives on an island, making it particularly vulnerable to disturbance and pollution

Most vulnerable are crabs living in southeast Asia, which is also home to the greatest diversity of species.

For example, 40 of 50 species living in Sri Lanka are threatened.

Those species that live a semi-terrestrial life, breathing air, living in burrows and dividing their time between water and land, appear most at risk, possibly because their habitats are most easily disturbed by human activities.

No species are yet known to have gone extinct, but some species such as the terrestrial crab Thaipotamon siamese and the waterfall crab Demanietta manii from Thailand have not been seen alive for over a century, and their original habitats have since been built over by urban developments.

The loss of natural forest to land development and agriculture has also impacted almost every habitat in which freshwater crabs live, the report notes.

The proportion of freshwater crabs threatened with extinction is equal to that of reef-building corals, and exceeds that of all other groups that have been assessed except for amphibians.

"We must set clear goals to reverse these trends and ensure that our enduring legacy is not to wipe out the small things that provide us with great benefits, such as nutrient cycling," says Ben Collen, one the survey scientists from the Zoological Society of London.

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