The river mussel is gonochoristic which means that it has separated sexes. It reaches reproductive maturity at an age of 4 to 5 years.
During spring, male mussels release their sperm into the water where it is taken in through the female’s inhaling siphon and fertilises the eggs. The larvae grow inside the brood pouch for about 4 to 6 weeks (depending on the water temperature). Healthy females in good condition may produce between 50,000 and 100,000 larvae, also called glochidia. Sometime between the month of May and June, the glochidia are expelled through the female’s aperture. In the open water, the 0.2mm glochidia will be able to survive for a few days only. Therefore they need to find a host fish as soon as possible.
As the host fish breathes larvae-infested water, the glochidia attach themselves to the host’s gills. The fish’s immune system induces the formation of cells that cover the attached glochidia and form a cyst within 2 days. The larvae mature into young mussels inside the cyst within 10 to 35 days (depending on the water temperature). During their parasitic phase, the larvae develop into young mussels that have a ciliated foot, a digestive and nervous system and gills, however, they do not grow perceivably. After metamorphosis, the young mussels detach from the host and develop into mature river mussels inside the gravel bed over the next years.
Host species include a variety of native freshwater fishes. In the Eifel and Ardennes, the most common host fishes are the minnow, chub and bullhead. Since the hosts develop an immunity to the infections caused by the river mussel, young fish are essential to the reproductive success of the river mussel.
The young mussels move through the gravel bed using their ciliated foot, at the same time they swirl food particles to their mouth opening in the mantle cavity. When the gills are fully developed, the beating of the gill’s cilia create the water current that swirls the food to their mouth opening. The diet of the river mussel includes both biotic an abiotic organic particles such as bacteria, unicellular organisms, algae and fine dead plant material (detritus). Adult mussels filter 3 to 4 litres of water per hour. A healthy population of mussels in a river or stream filters a large proportion of the water and assists the self purification process of the water body.