The rainbow shell is a small mussel that is widely distributed in the Mississippi basin.  It occurs in many tributaries of the White River system in the Missouri Ozarks, where it is usually found in cool, clear streams.  Rainbow shell and its relatives use mantle flap lures to attract their host fish, which include basses and other sunfish.  They have also evolved the ability to "dance" by anchoring their foot in the substrate and rocking back and forth.  

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VILLOSA_IRIS-SHELL.jpg (45685 bytes) The female rainbow mussel uses both morphology and behavior to convincingly mimic a small crayfish.  Elongate papillae on the mantle margin resemble antennae and legs.  The mantle margins are folded to one side to present a lateral view, and the mussel usually positions the lure with papillae toward the substrate (putting its "feet" on the ground).
VILLOSA_IRIS_6C.jpg (80082 bytes) The "eyespot" of the lure is uniformly dark, like that of a crayfish. It lacks the light peripheral ring seen in Lampsilis, which mimic fish.  The coloration and the shape of the mantle flaps suggest the thorax and abdomen of a crayfish.  Most remarkably, the female's movements also mimic crayfish.  The papillae move independently like legs, and periodic swinging motions are accompanied by "tail tucking" movements like a crayfish.   See the movie!
VILLOSA_IRIS_2A.jpg (71967 bytes) In the adjacent photo, the marsupial gills of the female are visible between the two mantle flaps that form the lure.