Mollusk means soft body in Latin and octopus, squid, and snails are included in this classification. Oysters need hard bottom habitat shell or rock and cannot survive if buried in mud. They are sessile, spending most of their entire life attached to something and cannot move from that location.
Oysters prefer oyster shells as substrate, although they can live on any hard surface.
This surface material or substrate is called cultch. Cultch that is clean and free of mud and debris is preferred, since some pollutants, including oil can inhibit the setting of oyster larva. That is why it is important to return old shell to the oyster beds to provide cultch for future oysters. The egg and sperm fuse in the water and become a fertilized egg or zygote and begins cell division. After about four hours, the zygote has developed tissues and organs and is called a trochophore, then continues to develop and is called a veliger.