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Freshwater mussels (Unionoida) have a unique lifecycle in which their larvae (called glochidia) must attach to a host (usually a fish) where they undergo metamorphosis.
Male Mussel siphoning Males release sperm into the water that are taken in by females downstream.  Despite having no eyes, mussels have shown the ability to congregate during spawning periods.
Mantle Lure of Lampsilis fasciola

Because mussels can't track down their more mobile hosts to infect them, they have to attract fish to come to them.  Mussels have a variety of ways to attract fish or snare them with their larvae.  This mussel mimics a small fish, like a darter, to attract the attention of its predatory host, the Smallmouth Bass.  

See video and pictures of these amazing adaptions in North Carolina Species here.

glochidia of Alasmidonta varicosa Glochidia average 0.2 - 0.3 mm in size making them barely visible to the naked eye.  Most come out of the adult female open and ready to attach to a fish.  Those species that attach to fish fins (rather than fish gills) have hooks on the end of the shell to aid in holding on.
Tangerine Darter To complete their life cycle, glochidia must attach to a host (usually a fish), where they undergo metamorphosis to the juvenile stage.
glochidia attached to fish gills Here, glochidia are attached to the gills of a small fish.  When glochidia come in contact with their host, their shells close on the tissue, and they become encysted by a single layer of gill cell tissue.  Here they remain while they undergo metamorphosis.
Juvenile Mussels During metamorphosis, most species don't grow, but they develop their internal organs and their foot that they use for burrowing and for obtaining food.  Newly transformed juveniles are quite mobile obtaining their food with their foot.

Aquatic Epidemiology Conservation Laboratory