Anyone who finds a mussel on a lakeshore usually sees only its unassuming shell, but beneath it lies an astonishing world of genetic diversity. Freshwater mussels (Unionida) live largely hidden in the sediment, where they filter water day in, day out, thereby helping to purify and stabilise our water bodies. Furthermore, freshwater mussels act as ecosystem engineers: where they occur in higher densities, they form mussel beds that alter the bottom structure and create microhabitats, such as hiding places and settlement areas for invertebrates.

Hardly any other group of animals is so closely linked to the quality of our waters and, at the same time, so severely threatened. Habitat loss, pollution, the decline of their host fish and invasive species are taking their toll on them worldwide. In Switzerland, too, their populations are steadily declining. “Despite their ecological importance, surprisingly little is known about most native mussel species,” says Julie Conrads, co-author of the study and a researcher in the Department of Aquatic Ecology at Eawag. The results of her research were recently published in an article in Aqua & Gas.

Almost impossible to tell apart by sight

“Many mussel species look so similar that they are sometimes difficult even for experts to tell apart,” she explains. Using modern DNA analysis, the genetic profile of each mussel can be determined. This data reveals not only which species an individual belongs to, but also how closely related different individuals and populations are to one another. This reveals the extent to which mussels from different lakes or rivers differ genetically from one another, providing an indication of how isolated populations are or how much contact they have had with one another over longer periods of time.

Particularly isolated or small populations often exhibit lower genetic diversity and can be more sensitive to environmental changes. This is exemplified by two native species, the duck mussel and the swan mussel: the duck mussel, which has separate sexes, is genetically far more diverse than the swan mussel, which is both female and male in one individual; the swan mussel's populations are also more isolated from one another. This reduced diversity can limit their ability to adapt to new environmental conditions and increase the risk of local population declines, particularly in the face of growing pressures from rising water temperatures, invasive species or altered flow regimes.

Genetic monitoring makes it possible to track such structures and changes over time and to identify early warning signs. It thus provides an important basis for assessing the resilience of species and populations, as well as for informed conservation and management decisions in aquatic environments.