An animated fish decoy, complete with rhythmically undulating flaps and an "eye" spot, rides atop the rear of the freshwater mussel Lampsilis. The female mussels dump their larvae when real fish approach, attracted by the decoy. Some of the larvae find their way to the gills of the real fish where they grow. This is how Lampsilis reproduces.
Such perfect designs fuel "Intelligent Design" debates. Is this "intelligent design"? Not if there can be an adaptive advantage to the incipient stages of this useful structure.
Preadaptation - functional change with structural continuity - the idea that a feature initially developed for a specific purpose, finds itself "preadapted" and useful for a completely different purpose further in the evolutionary path of the organism.
For example, the undulating flap of Lampsilis might have provided other advantages earlier - like aeration of the larvae.
Ever Since Darwin: Reflections in Natural History by Stephen Jay Gould
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