Reproduction In Unionids:
Unlike many bivalve taxa which simply release sperm and eggs into the water column, which will develop into trochophore larvae and eventually into veliger larvae which will settle in substrate. Unionids are unique in that they use a different life cycle to reproduce.
A Female Unionid will drawn in sperm from males into her gills, where fertilization will take place internally. The eggs produced from this will develop into Glochidium larvae. Later, when the female is ready she will use special lures known as superconglutinates, which are small-fish shaped extensions of her mantle, to draw fish in close. When said fish is close she will release her larvae which will then attach themselves to the fish's gills or digestive tract. From there they will feed on their host's bodily fluids for several weeks. They will then dislodge themselves and drop off their host. Next, they will develop into juveniles in the substrate and the process repeats itself.
The strategy is advantageous to the Unionids because of their chosen habitat. Most unionids occur in freshwater rivers where water moves downstream. Since bivalves are sessile they have no means to disperse upstream rivers, a problem marine bivalves don't face. By parasitizing fish glochidia can disperse against the current and access new feeding grounds.
Read More about Unionid reproduction [1] [2]
Watch a Video of a Glochidium in action!