BBC News Online, London
While a piranha was being fished out of the Thames in east London last month, another non-native was basking on the river's shores.
Staff at Fothergill & Co captured the Richmond seal on film
Looking out of the office window, staff at Fothergill & Co were not shocked to see a seal's head bobbing in the water, oblivious to the passing traffic near Richmond Bridge.
Structural engineer Richard Philips, who caught the Richmond seal on film, said it indicated the change he has seen in the Thames over the past 30 years.
"Occasionally after you have heavy rain, some less than pleasant stuff gets washed in," he said.
"There's a bit of flotsam and jetsam on the surface, but apart from that it's remarkable.
"I have been rowing since the mid-70s and back then there were times when you
would go and be ploughing through oil slicks - none of that is there now."
But he appears to be in the minority. Those monitoring the river find that while it is well-regarded internationally - its success comes as a surprise closer to home.
"[The Thames] is actually an environmental success story but it's not well known in this country," said Steve Colclough, a fisheries specialist at the Environment Agency
"People see a few bits of floating plastic which are unsightly and they assume it's pollution.
Among world's cleanest
"We have had a recovery but the public does not realise it."
To the untrained eye it might appear to be a moving mud bath, but the water has improved enough to sustain 121 species of fish.
In 1960, a 20-year project began to extend London's major sewage treatment works and improve river quality. Now the Thames is one of the cleanest metropolitan estuaries in the world.
It is a sea change from 1957, when the Natural History Museum declared it "biologically dead" or incapable of supporting life.
Now teeming with life - including large nurseries of Dover Sole, Sea Bass, oysters, cockle fisheries and some salmon - it reportedly supports the most diverse ecosystem of all Europe's metropolitan estuaries.
The red-bellied piranha which landed on a "Thames Bubbler"
From the estuarine prawn at Battersea, the perch in Putney eastwards to Woolwich's thick-lipped mullet and the five-bearded rockling at Erith, the river is crawling with creatures.
Seals have been spotted as far up as Waterloo Bridge while dolphins and porpoises surface in the late summer and autumn.
And the burrowing Chinese Mitten Crab, which runs "like a spider" across the river bed, has been wreaking havoc on the banks of the Thames for some time.
"We often get calls from people in Lewisham or somewhere else saying: 'Am I seeing things? There's a dead crab in the river'," said Mr Colclough.
Last year the Drinking Water Inspectorate informed us London had among the best quality tap water, largely drawn from the Thames, in the country.
And Coca Cola obviously agrees - it purifies tap water from Sidcup, just down the road from Peckham, for its new Dasani drink.
But for those who just cannot shake the image of a murky Thames seeping through Westminster, Mr Philips suggests walking along just that bit further.
"People look at the river and what they see is mud.
"But the colour is dictated by the silt suspended in the water.
"At the top and bottom of the tide the silt does settle for a short period and then it's quite clear. The river is remarkably clean."