Asian Green Mussel Perna veridis
The larval stage of the green mussels is spent drifting on the currents. As the mussel develops, it undergoes metamorphosis and begins producing the shell. When this happens the scallop settles out of the water column onto a surface, preferably hard substrate, and attaches itself using structures known as byssal threads. During this phase bivalves are referred to as "spat". Unlike bay scallops which detach after attaining sufficient size, green mussels will remain in one place unless forcefully removed.
Image Credit: FWC