The Thames is home to over a hundred species of fish but never, until now, has a piranha been found in its murky waters.
One of the deadly Amazonian fish, shoals of which can devour big prey in seconds, mysteriously landed on a moored vessel in Dagenham, east London, having apparently fallen out of the sky.
The three crew of the Thames Bubbler environmental boat, which pumps oxygen into the river, identified the razor-toothed fish as a piranha and called in marine experts to investigate.
Tom Cousins, a fisheries officer at the Environmental Agency, immediately recognised the specimen as a red-bellied piranha. He surmised that the fish had been accidentally dropped by a passing seagull which had plucked it out of the water.
Mr Cousins said: "I have to admit that I was pretty sceptical when I got a call from them telling me that a piranha had fallen out of the sky on to the vessel. But when I saw it, I recognised it by its shape, and it had a really good pair of teeth on it. It was very fresh and had obviously only just died. You could see the marks made by the seagull's beak on its back."
Mr Cousins thought that the fish, normally found in the warmer fresh waters of the Amazon, had probably been released by its former owner and swum along the Thames before it was picked up on Wednesday afternoon by a seagull.
If the fish had managed to survive - almost impossible for a species that is used to hunting in a large shoal - it might have infected some of the 119 native species of Thames fish, including eels, carp, mullet, Dover sole and bass, with new bacteria.
"Introducing a new species to the river can upset the ecological system as foreign species transmit parasites and diseases to native species that have no immunity to them.
"This was a reckless and illegal act by whoever set it free. If, heaven forbid, it had survived, it would have been a pretty dangerous predator," said Mr Cousins.
Paul Hale, curator at the London Aquarium, said piranhas were acclimatised to swimming in at least 15C waters and the fish would not have survived for longer than an hour in the Thames' current 10C temperatures.
"It could not have survived in the low temperatures of the Thames. And piranhas are generally nervous and not the ferocious killers people think they are. They prey on weak and injured animals, including fish, birds and mammals, as well as carrion," he said.
Piranhas are often kept as pets in Britain, despite the fact they need to be kept in large tanks heated to within a narrow temperature range.
Paul Griffin, manager of Aqualand Pet Centre in Birmingham, sells red-bellied piranhas for between £5 and £11 depending on size.
"We sell between six to eight every month and I would say that the piranha is mainly a boy's fish," he said.
"I've been raising piranhas since the Seventies and Eighties and I have always found them to be very interesting pets."
Although there are vegetarian species of piranha, which eat fruit and nuts, their carnivorous cousins are vicious. Piranha attacks on bathers have been increasing in Brazil following the damming of rivers.
SHORT AND SNAPPY
Name: Pygocentrus nattereri
Age: Can live for 10 years
Home: Large, slow rivers in the Amazon
Appearance: Up to 30cm long, silvery-grey fish with red bellies
Mouth: Powerful jaws and razor-sharp teeth. Typical bite is 12.5cm across
Favourite food: Live fish or dead meat
Likes: Hunting in shoals and devouring prey in seconds
Dislikes: The cold. Ideal water temperature is 24-27C
Loves: Eggs laid among plants in a pit. Defended by parents
Most frightening fact: Believed to have eaten 300 people when a boat capsized near Obidos, Brazil, in September 1981.Reuse content