Freshwater mussels are one of the most threatened animal groups in the world. In the European Union,
threatened and protected mussel species are not adequately monitored, while species considered to be common
and widespread receive even less attention. This is particularly worrying in the Mediterranean region, where
species endemism is high and freshwater habitats are severely affected by water scarcity. In the absence of hard
data on population trends, we report here a long-term comparison of freshwater mussel assemblages at 132 sites
covering 15 different hydrological basins in Portugal. This study reveals a widespread decline of 60 % in the
number of sites and 67 % in the overall abundance of freshwater mussels across Portugal over the last 20 years,
indicating that all species are rapidly declining and threatened with extinction. These results show that current
legislation and conservation measures are largely ineffective and highlight the importance of updating the
Habitats Directive to enforce standard monitoring protocols for threatened species in the European Union and to
extend monitoring to other freshwater species thought to be common and widespread. Efficient water management,
restrictions on irrigation expansion in important biodiversity areas, mitigation of hydrological changes
and loss of aquatic habitat connectivity caused by physical alterations are urgently needed to reverse these
declining population trends. For the severely endangered species Margaritifera margaritifera, Potomida littoralis,
and Unio tumidiformis, where populations are now critically low, more urgent action is needed, such as ex-situ
conservation, protection of remaining populations and large-scale habitat restoration.
We would like to thank Jake Dimon, José Tourais, Filipe Rolo, and
Elza Fonseca for their help in the surveys. This research was developed
under the project EdgeOmics - Freshwater Bivalves at the edge: Adaptation
genomics under climate-change scenarios (PTDC/CTA-AMB/
3065/2020) funded by the Portuguese Foundation for Science and
Technology (FCT) through national funds. FCT also funded MLL under
contract 2020.03608.CEECIND, EF under contract CEECINST/00027/
2021/CP2789/CT0003, AGS under the grants SFRH/BD/137935/2018
and COVID/DB/152933/2022, and JGN under the grant 2020.04637.
BD. The baseline survey was funded by the project “Documentos
Estruturantes” (POA 1.100021) of the Instituto da Conservação da
Natureza.
