Mussel imitates fish

[Al Agnew]

And yes, mussel larvae are parasitic on fish.  The larvae, called glochidia, have to attach to the gills of the host fish, and different mussel species need different species of fish for the glochidia to take hold of the gills and survive.  Some species use the lure structures to attract the right kind of fish, and then spew their glochidia when the fish gets close, but others release groups of glochidia in packages that resemble the fish's food, and the fish eats the package.  If the fish is the correct species, the glochidia feed on the blood from the gills for a few days or weeks, grown into juveniles, and drop off.