Shellfish reefs are used in living shoreline projects in combination with other practices to increase habitat diversity, or as a substitute for temporary growing materials and stone sills. Living shoreline shellfish reefs are most successful at locations with evidence of a healthy, natural shellfish population already present such as native oysters and ribbed mussels. Shell-based reefs are constructed with native shell either loose or contained in mesh bags or cages. Other reef-building alternatives are pre-cast structures that support the settlement and growth of shellfish. Pre-cast reef structures are typically placed like a marsh sill and can be combined with other practices such as sand fill and planted tidal marsh. Living shoreline shellfish reefs are presently considered to be experimental approaches under investigation to determine how well they reduce shoreline erosion.