"In many respects, freshwater pearly mussels look like any other bivalve mollusc, but what sets some of them apart (notably Lampsilis spp.) is the unusual extension of their fleshy mantle that grows beyond the confines of the protective shell valves to wave around in the water. This fleshy protuberance can look astoundingly like a small fish and this is no coincidence because this fishy appendage is actually a lure to attract fish so they can be press-ganged into the mussel’s reproductive strategy. The lure is very convincing. Not only does it have markings that suggest eyes and skin patterning, but it is even moved by the mussel in a fish-like way. These details are more than enough to grab the attention of a real fish that mistakes the lure for a snack. The fish edges closer and makes a lunge for the fake prey nipping the membrane of a specialised brood gill the lure is concealing. This releases the mollusc’s larvae, nasty-looking miniature versions of the adult, known as glochidia. These larvae are parasitic and they get drawn under the fish’s gill plates where they latch onto the blood-rich tissues of the gills..."Further discussion and more examples here.