 Freshwater bivalves in the order <unk> have a different lifecycle . Sperm is drawn into a female 's gills with the inhalant water and internal fertilization takes place . The eggs hatch into glochidia larvae that develop within the female 's shell . Later they are released and attach themselves parasitically to the gills or fins of a fish host . After several weeks they drop off their host , undergo metamorphosis and develop into juveniles on the substrate . An advantage of this to the molluscs is that they can disperse upstream along with their temporary hosts , rather than being constantly swept downstream by the water flow . 
