The RSPCA say they had "no choice" but to put down a family of yellow-bellied terrapins dumped by a canal near Northampton at the risk of "untold damage" to wildlife.
The nine fresh-water turtles were spotted by two joggers on the banks of a canal in Blisworth this week (May 22).
Each reptile was the size of a dinner plate and some could have been up to 30 years old.
But under UK law, the yellow-bellied terrapin is listed as an "invasive, non-native species" and releasing them into the wild can be punished with a fine of up to £5,000 or two years in prison
Local resident Andrew Carter, who collected the turtles and cared for them while waiting for the RSPCA, said: "They're voracious eaters. They will eat anything they can get their beaks on. They'll take a duckling, frogs, weeds, tadpoles, even a pigeon.
"If they had got into the water they would have caused untold damage.
"Many people bought them as pets during the Teenage Mutant Ninja Turtle craze. But they don't know they live decades and have enormous appetites. They are often dumped when they become inconvenient."
The yellow-bellied terrapin is native to Florida and thrives in swampy conditions.
A RSPCA officer collected the Terrapins - but all nine had to be euthanised the same day.
The RSPCA officer who collected them said: "Terrapins can literally decimate local wildlife in the UK. It's not rare to see a duckling snapped up by one.
"They are endemic. You'll be hard-pressed not to go to one pond or river in England and not find a terrapin.
"Even if they could be rehomed, they could not be sold as pets and we could not expect any centres to accept nine fully-grown terrapins."
The RSPCA say they are forced to destroy hundreds of illegal species every year in the UK - many of which were likely once pets that became unwanted.