The freshwater pearl mussel, Margaritifera margaritifera, is our longest lived freshwater species, often exceeding 100 years, and sadly among the rarest. The species was almost hunted to extinction for the pearls they occasionally contained, and is now threatened because the clean, pebbly rivers they live in are disappearing.
Swedish research into the much larger populations of mussels in Scandinavia show they can live to 280 years and play a vital role in moderating river flow. They stick out of the riverbed, enabling small trout to thrive by reducing river flow, and they clean the water by filtering out filth.
The mussels need a good population of brown trout and salmon to survive. When they breed they release clouds of tiny larvae which are breathed in by the fish and clamp themselves on to their gills, remaining there for months without the fish noticing. When the fish swim upstream the larvae drop off hoping to find a suitable stony bed to start a new colony. The chances of this breeding strategy working are slim, so the mussels live a long time and produce more than a million larvae in a lifetime to ensure survival. Most known mussels colonies are already mostly old specimens, so a breeding programme is under way to try to revive lost populations.
