Researchers recently discovered three endangered species of freshwater mussels in the Seine River, indicating an improvement in the ecological health of Paris's waterway. The thick-shelled river mussel Unio crassus, the river mussel Pseudanodonta complanata, and the compressed pond mussel Potomida littoralis were found during a study on environmental DNA (eDNA) analysis, according to Le Monde.
These freshwater mussels are highly sensitive to pollution and play a crucial role in their ecosystem by filtering large quantities of water. "They can filter up to 40 liters per day. It's a real purification service!" said Vincent Vignon, an ecologist and project director at Alkïos, as reported by La Croix. Their presence suggests that measures to improve water quality in the Seine are yielding positive results.
The eDNA study involved collecting and filtering 20 liters of water at eight different points in the river during the summer of 2024. The samples were analyzed by the laboratory Spygen. "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," explained Vincent Prié, a mollusk specialist at Spygen, according to The Guardian.
In total, scientists uncovered the DNA of 23 different types of mussels in the Seine, including five invasive species and three endangered ones. Among these, the thick-shelled river mussel, the river mussel, and the compressed pond mussel are particularly noteworthy due to their status on the Red List of Threatened Species of the International Union for Conservation of Nature. These species were once believed to have disappeared from the Seine basin. "Finding the river mussel is really spectacular; we thought there were only two populations left, near Troyes and in the Oise," said Prié, as reported by Le Monde.
"The re-emergence of these species, which are particularly sensitive to water quality, is a strong signal. It shows that our joint efforts to rehabilitate the Seine are bearing fruit," stated Prié, according to Die Zeit. Freshwater mussels contribute to their environment by filtering water, which makes the river more transparent and benefits the growth of plants and the presence of other insects and invertebrates. These organisms become food for fish, creating a vital link in the river ecosystem, according to Avvenire.
Over the past few years, approximately 1.4 billion euros were invested in sewage treatment plants and the sewage system in the Paris metropolitan area, including the modernization of two wastewater treatment plants and the construction of a stormwater basin at Austerlitz. These efforts were partly in preparation for the Olympics and aimed at improving the river's water quality.
The Seine River, once synonymous with pollution, saw a remarkable transformation. The number of fish species increased from three to over thirty, with about fifty species now recorded, including 36 identified using environmental DNA techniques. This resurgence is encouraging news for the ecosystem and the city's residents.
Despite these improvements, challenges remain. Due to fluctuating water quality, some swimming competitions during the Olympics had to be postponed, prompting debates among athletes regarding its cleanliness. The decisive factor for swimming safety in the Seine is the concentration of certain bacteria that do not affect the mussels.
Nevertheless, plans are underway to enhance recreational use of the river. Three outdoor swimming pools are planned to be created in the Seine starting next summer, one of them not far from the Eiffel Tower. Swimming in the Seine River was prohibited for more than a century, but recent improvements have made it possible again. "Being able to swim again in the Seine River was a long-held wish in Paris, especially since swimming was prohibited since 1923," reported The Guardian.
“There is clearly something particular happening in Paris that we don't fully understand yet; it opens a field of exploration," said Vignon, according to Le Monde. The abundant artificial light in Paris may favor the presence of phytoplankton, which the mussels feed on, as suggested by Ouest-France.
The article was written with the assistance of a news analysis system.