The body parts of the fish to which the glochidia attach themselves vary between the different mussel genera. There are gills and fin parasites, as well as transitional forms. It is also possible that glochidia of some mussel species attach themselves to the skin of the fish. The freshwater pearl mussel is a pure gill parasite. After a period of time that varies between mussel species, the very simple glochidia have transformed into juvenile mussels. In the freshwater pearl mussel, development can take from 1 to 10 months. The glochidia of the freshwater pearl mussel are 0.04 - 0.07 mm long when they are expelled from the mother. They grow on the fish and fall off as juvenile mussels with a shell length of 0.4 - 0.7 mm.
Instead of the two shell valves and one sphincter muscle previously present, the juvenile mussels now have all the organs of the adult mussel, including two sphincter muscles, a heart, two pairs of gills, an intestinal tract and a movable foot. After they have fallen off the fish, the juvenile mussels bury themselves in the stream bed for about 5 years, where they are dependent on sufficient flow through the sediment spaces.
If the glochidia does not find a host within a few days, it dies. The glochidia are immobile and therefore defenceless against the environment. As a young mussel about 2 cm in size, it returns to the surface of the stream bed and reacts extremely sensitively to the water quality.
