Reproductive Life Cycle of Mussels

The cycle begins when a male mussel releases sperm into the aquatic habitat.
The sperm enter the female mussle as she siphons water over her gills. 
The female's eggs are fertilized and develop within her gills for several weeks.
They will emerge as larvae know as glochidia.

The glochida develop as parasites on the gill tissues of certain fishes.
The female of some mussel species will attempt to attract preferred fish with a specialized lure.
This improves the chances that the larvae will successfully latch onto a suitable host and continuue development.

Upon finding a suitable host, a glochidium will clamp down on the fish's gill tissue which then provides food for the developing parasite.
using a highly mobile host species such as fish enables the relatively sedentary mussels to populate habitats they otherwise could not reach.

Within a few weeks the larval glochidia will transform into a juvenile mussel and drop from the host fish.
If the tiny mussel falls onto an appropriate substrate it will continue to develop toward adulthood.