Traces of rare, pollution-sensitive mussels once thought to be nearly extinct in France have been discovered in the Seine River in Paris, raising hopes that efforts to clean up the waterway are making progress.

The discovery follows last year’s Olympic swimming events, marking the first time in a century that swimming was deemed safe in the Seine.

Scientists analysing large water samples from eight points in central Paris identified DNA from 23 species of mussels—including three classified as near-extinct—as well as 36 species of fish, a dramatic increase from the 1960s when only a fraction of that biodiversity was recorded.

“All organisms lose skin cells all the time and we recover the DNA of these cells from the environment,” said Vincent Prié, a hydrobiologist specialising in freshwater mussels at the Sygen laboratory that carried out the research.

“We filter the water and sequence it. This potentially gives us a list of everything that lives. And that’s what’s so interesting, because we didn’t expect to find them in Paris at all, because they’re under threat.”

The researchers, originally studying the effects of urban lighting on biodiversity, used a groundbreaking method called environmental DNA (eDNA) analysis, which detects species based on the genetic traces they leave behind.

Among their findings were traces of the thick-shelled river mussel, black river mussel, and depressed river mussel—species nearly extinct in France. The depressed river mussel, also known as the compressed anodont, can grow up to 8cm and had nearly disappeared from the country, except in the northeast.

These mussels play a crucial role in water quality, with each mollusk capable of filtering up to 40 liters of water per day.

“It contributes to the natural purification of the river,” Prié said.

However, Prié cautioned that it is too soon to attribute their presence to specific clean-up efforts by city authorities. Other factors, such as rising water temperatures or artificial lighting, may also play a role in their return.

“It’s a bit of a shortcut. Honestly scientifically we don’t know. It is quite possible that it has ‘reappeared’ in Paris from populations we don’t know about elsewhere in the Seine basin.”

Vincent Vignon, an ecologist who took part in the study, said the rare mussels were “very demanding and only settle in water that is not too polluted”. He added: “There’s clearly something special going on in Paris that we don’t yet fully understand.”