The 25cm long oyster was discovered at Talmont-Saint-Hilaire in the Vendee department last Friday.
“I was working in the channel and walking along I felt something with my foot,” said Mathieu Naslin, an employee of the Viviers de la Guittière oyster farm.
“I picked it up and it was a huge oyster,” he told AFP.
Another mammoth oyster was found at a farm close by in the Sables d'Olonne area in April, weighing in at 1.3kg.
“It's extremely rare,” said Naslin, adding that he had never seen anything like it during eight years in the job.
“No doubt there are more oysters this big out at sea but in the farms you do not expect this.
“The salinity level occasionally drops a lot because we are next to a lake that puts water into our channel. That restricts the growth of the oysters.”
The oyster, which has since been returned to the Atlantic waters, was nicknamed “Georgette” after one of the workers who recently retired.
“She was a sort of matriarch at the company,” Naslin explained.
The team has estimated Georgette was 13-15 years old taking into account that a commercial oyster normally grows about three centimetres a year and stays in the water for three years.
“Nonetheless it has been shown from the growth of the shells that an oyster can live for between 35 and 65 years,” he said.
Georgette is edible but the farm has no intention of selling it.
“There are people who go for very big oysters, but we want to keep it alive and why not keep it growing? It's like a trophy,” Naslin said.
“Quite a few people have offered to buy it.
“As a joke, the boss told us he would not let it go for under €2,000.
“But we would not sell it, even for €30,000.”