Some mussels may have bissel threads at this stage of development, which help them to anchor themselves to the river bottom. Development with a host fish is incredibly important to mussel propagation. Without these fish, mussel populations would never be able to spread upstream. Sounds pretty chancy. Well, mussels have developed very sophisticated ways of attracting their host fish.
All mussels possess an organ called the mantle, which is a soft lining of the mussel's shell.
Mussels often use this mantle to lure their host fish with incredible displays. In some species, for example, the mantle has evolved to look like a minnow, a great attractant to both minnows and larger fish that feed on minnows. In the dwarf wedgemussel, the mantle swells, changes shape and color, and moves, perhaps suggesting something crawling along the river floor. Some scientists believe that fish release some chemical which the mussel can sense, so that it knows when an appropriate host fish is in the neighborhood kind of like smelling underwater. Sometimes, the glochidia can themselves be an attractant.