The compressed Anodontus, the thick mussel, the river mussel. These are three species of freshwater mussels in danger of extinction which have been identified in the Seine. Good news for the ecosystem and water quality.
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“We were super surprised to find her in Paris. We thought she had disappeared! “Last summer, while taking water samples for environmental DNA research, technicians from a laboratory specializing in monitoring aquatic and terrestrial biodiversity detected the unexpected presence of a species of mussels. fresh water in the arms of the Seine in the heart of Paris, especially around the Île de la Cité and ÎIe Saint-Louis.
On the bed of the river, not one, but three species classified as endangered by the International Union for Conservation of Nature have been identified: the compressed Anodontus, the thick mussel relatively common in the watershed of the river and above all, the river mullet.
“All organisms lose skin cells constantly and we recover the DNA from these cells in the environment. We filter water and sequence it. This potentially gives us the list of everything that lives. And That's what's interesting, is that we didn't expect to find them in Paris at all because they are threatened.” welcomes Vincent Prié, hydrobiologist specializing in freshwater mussels from the Sygen laboratory.
The river mussel, for example, was detected in 2017 above Troyes in Aube but “was considered to have disappeared from the Seine basin around ten years ago”. “We had lots of data from shells of an ancient presence, but not on a living population” (…) We have a real population, several dozen on the bed of the Seine”, explains the hydrobiologist.
The size of the yellow dots where the sampling took place indicates the proportion of mussels identified.
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© Vincent Prié
Freshwater mussels act as “true bio-indicators” of water quality. But to assert that the “reappearance” of the river mussel is due to the efforts undertaken to improve the quality of the Seine is not scientifically proven, specifies Vincent Prié. “It's a bit of a shortcut. Honestly, scientifically, we don't know anything about it. We can't say that it reappeared or if it was always there! It is also entirely possible that it is reappeared in Paris from populations that are not known elsewhere in the Seine basin.”
In any case, the improvement of the aquatic environment contributes to the development of these species “extremely sensitive to water quality”. “To the extent that it reproduces in an environment that is restored, in doing so, it will play its ecological role, a functional role. By filtering water up to 40 liters per day, it contributes to the natural purification of the river. underlines Vincent Prié.
The thick mussel, protected species
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© Vincent Prié
Hydrobiologist Vincent Prié does not have data to precisely assess the population and distribution of these three species in the Seine. Their development depends on the presence and proliferation of fish.
“The mussels emit a small larva. Released into the water, this larva parasitizes a fish. It will cling to its gills. It will form a small cyst and throughout the metamorphosis, that is say a few weeks, the fish will move almost everywhere in the Seine watershed. The baby mussel, which will be less than a millimeter, falls where the fish is and this is how the mussel will disperse. And colonize new environments“, he explains.
The freshwater mussel did not make its sudden “reappearance” in Paris this summer. One of her cousins has been colonizing the capital's canals for many years: that of the Ourq, Saint-Denis, and the Saint-Martin canal. Thus we can see in their waters the Unio Pictorum, nicknamed the painters' mussel, so called because the artists used their shells to mix colors.
According to the Interdepartmental Federation for Fishing and the Protection of the Aquatic Environment of Paris, Hauts-de-Seine, Seine-Saint-Denis, Val-de-Marne, the rediscovery of three other species that were thought to have disappeared in the capital is rather encouraging news for the ecosystem.
“There are species like the Bouvière, a small 6 cm fish that lays eggs exclusively in mussels and when there are no more mussels, there are no more Bouvieres.”explains Adrien Aries, project manager at the Federation. “There are also fish that consume molluscs such as carp or small catfish. It can benefit certain species, whether for feeding or reproduction. he adds.
