M. trossulus reaches sexual maturity when the shell reaches approximately 15 – 20 mm in length. Gonad development occurs in the cooler months. Depending on location, the gonad development period varies. For instance, gonad development occurs throughout the winter in Prince William Sound, but is delayed until early spring on Vancouver Island, B.C. Spawning of gametes is thus also dependent on the local conditions. In Prince William Sound, spawning occurs in the summer while spawning is delayed until late summer and fall on Vancouver Island. Gametes are released through the gonoducts into the mantle cavity near the excretory structures called nephridia and finally into the surrounding water. Within two days of being fertilized, the tiny eggs (60-65 µm) form bilaterally symmetrical veliger larvae. Veliger larvae exist as plankton for four to seven weeks feeding via a characteristic velum. During this time, the larvae develop two valves. At the end of the larval period the larvae seek a place to settle on such as hydroid colonies, filamentous algae, other mussels’ byssal threads, barnacle shells, wharf pilings or bare rock. As the larvae settle, they shed the velum and metamorphose into juvenile M. trossulus. In California, M. trossulus lives to be approximately three years old. In cooler, northern water they are believed to live longer. During their lifetime they may reach approximately 10 cm in length.
