 Some of the species in the freshwater mussel family , Unionidae , commonly known as pocketbook mussels , have evolved an unusual reproductive strategy . The female 's mantle protrudes from the shell and develops into an imitation small fish , complete with fish @-@ like markings and false eyes . This decoy moves in the current and attracts the attention of real fish . Some fish see the decoy as prey , while others see a conspecific . They approach for a closer look and the mussel releases huge numbers of larvae from its gills , dousing the inquisitive fish with its tiny , parasitic young . These glochidia larvae are drawn into the fish 's gills , where they attach and trigger a tissue response that forms a small cyst around each larva . The larvae then feed by breaking down and digesting the tissue of the fish within the cysts . After a few weeks they release themselves from the cysts and fall to the stream bed as juvenile molluscs . The fish are relatively unharmed . 
