I just came across this video through some guys on Reddit, and have to share it. This mussel (Lampsilis Mussel ) can only make the transition from young to adult by living inside a preferred host. All mussels can use a variety of fish as hosts, but the Lampsilis prefers the largemouth bass!
So, natural selection has helped these mussel develop into a "lure" of sorts that looks like a bass' natural prey (darter, silver striker). If they get a bite, they squirt their young into the bass, where the young hang on the gills until they drop off as fully formed.
Strange, huh? Guess we're not the only ones trying to attract the bass after all! Nature is really something beautiful. These mussels can't even see the fish that they're imitating.