Francis Markus BBC Shanghai correspondent

They say if it enters the country's rivers and lakes it could have an unforeseeable effect on the country's aquatic environment.

Piranhas

There are 17 species of the freshwater fish, varying in size, colouring and ferocity - though all share the same razor-sharp bite In Tupi Indian language "pira" means fish and "ranha" tooth Piranha soup - thought to be an aphrodisiac - is an Amazonian delicacy Piranhas keep river waters fresh and prevent disease by eating animal carcasses

The authorities in Beijing have ordered piranha fish at aquaria and amusement parks in the capital to be destroyed.

The piranha fish may be an unfriendly creature, but the sharp-toothed native of South America has been gaining popularity as a pet in China.

The fish, which can strip the flesh off an unwitting victim within minutes, have been appearing in ocean parks and on sale in pet markets, in various parts of the country.

But their days may now be numbered.

The authorities in Beijing have ordered all piranhas being kept at aquaria and amusement parks in the capital to be destroyed, and have threatened heavy fines against traders who continue to sell them.

The [piranha's] head with its staring malignant eyes, and gaping, cruelly armed jaws, is the embodiment of evil ferocity Theodore Roosevelt, Through the Brazilian Wilderness (1914)

In Shanghai so far some traders say they have been banned from restocking with piranhas, but one shop promised fresh stocks by next week.

The measures come amid growing concern over the proliferation of the fish, with reports that they are being bred along northern China's Yellow River and may already have entered its river system, even though some scientists say that the winter temperatures there are too cold for them to survive.

Piranhas are not the danger to people as portrayed in popular [culture]... Many piranha species feed principally on seeds or the fins and scales of other fishes Amazon fish specialist Michael Goulding

Ecologists have been warning that if the fish do find their way into China's rivers and lakes, aquatic life could be seriously jeopardised.

One commentator called for the government to strengthen the laws on importing such foreign species into China.

But it may already be too late to prevent the fish from having an, as yet, unforeseeable impact on China's aquatic environment.