They may look harmless, but one bit a chunk off the finger of its Watford owner.
The menace of terrapins breeding in the sewers of the South of France was reported last week. But Britain is not immune from the threat of 2ft reptiles grown from tiny novelty pets bought for children and then disastrously freed into the wild, where they eat the chicks of water birds. According to experts, some US species weigh up to 75lb, and consume 10lb of beef a day.
'We have been warning the governments of Europe for years that it would come to this,' said Andrew Highfield, 38, director of the Bath-based Tortoise Trust.
'We are on record back in 1986 predicting that these things would naturalise in Europe and pose a serious threat throughout the Continent. It is too late now.' Mr Highfield, a zoologist, wants the Government to impose tough restrictions on the importation of terrapins. Only Sweden currently operates a ban.
Terrapins are bred in their millions on farms in the southern United States and exported mainly to Europe as pets, and to Asia as food.
In France, Spain, Italy and Greece they have begun to multiply, threatening local wildlife and its habitat and unsuspecting humans.
According to Mr Highfield, and zoologists at the London Zoo and the British Association of Tortoise Keepers in Birmingham, there is no evidence that terrapins have begun to reproduce in Britain - the summers are not long and hot enough for their eggs to hatch. But the volume of imports more than makes up for that.
The terrapins reaching Britain from the United States come in two varieties: the red- eared terrapin (Chrysemys Scripta Elegans) and the common snapper (Chleldrya Serpentina), the aggressive carnivore which, by the time it is three years old and nine inches long, can bite off part of a human finger.
It is the common snapper - living in the muddy bottom of ponds and streams - which is playing havoc among the bathers and waders of France.
'It is highly aggressive,' a keeper in the Reptile House at London Zoo said. 'It is an efficient eating machine that will certainly take a bite if two pink toes pass before it.'
I went searching for a snapper last week and found one in a terrace house at Watford, Hertfordshire, eating a piece of bacon from the local butcher. The house is an animal sanctuary, Reptile Rescue, run by Rob Hawkins and his partner, Madeleine Humphries. It was packed to the ceiling with lizards, snakes, birds, turtles, tortoises and terrapins, many of them abandoned by their owners or passed on from London's petshops and zoos.
The two-year-old snapper was dumped with Reptile Rescue after it bit a chunk of flesh from its owner's finger last Christmas. It had cost pounds 20 and the owner paid no attention to danger warnings from the petshop, Mr Hawkins said.
'People buy these things for the bullshit factor,' he added. 'It's very macho keeping a reptile, especially if there is an element of danger. The problem is, they are sold too cheaply.'
The British firms that import terrapins pay dollars 1 (65p) for a baby red-ear and dollars 10 for a snapper. Petshops sell them on average for pounds 8 and pounds 10 respectively. Snappers cost more because they are more expensive to farm, needing a much greater area of water, importers said.
Travel last week through Regent's Park in central London or Clissold Park to the north and you could have seen shoals of terrapins basking in the sun on rocks, duck ramps and water-logged branches.
At Clissold Park, a former Hackney council gardener said that 20 years ago, when the first terrapins were abandoned in the park's ponds, they were a curiosity. Now, with a population of about 50, they are a menace, eating the chicks of water birds. London Zoo officials report sightings of terrapins in the Thames.
Few statistics exist in Britain on the importation of terrapins. None have ever been kept for red-ears. On 1 January, when the European single market came into existence, all documentation ceased for the common snapper.
Statistics available from the City of London Corporation's quarantine centre at Heathrow show that 4,000 entered Britain through the airport between 1986 and 1990.
Only in the United States, where the terrapin is farmed, are statistics available. In 1990 three million were exported. Japan was the biggest market (600,000), followed by France (500,000), Hong Kong (300,000), Spain (300,000), Britain (200,000), and West Germany (100,000).
The most curious fact is that the sale of small terrapins, or turtles as they are known in the US, is proscribed there on health grounds - the terrapin carries salmonella.
Michael David, a veterinarian with the US Department of Agriculture, said the sale of small turtles was banned by state governments at various times from the end of the Sixties to the end of the Seventies.
They 'were associated with a high incidence of kids getting sick', he said. 'The stores just stopped buying them and you don't see them being sold anymore. The sale of turtles above four inches was not prohibited, but the restriction just made it difficult to move them around so the stores gave up.'