The word oyster is used as a common name for a number of distinct groups of bivalve molluscs which live in marine or brackish habitats. A new, wide-ranging survey that compares the past and present condition of oyster reefs around the world finds that more than 90 percent of former reefs have been lost in most of the bays and ecoregions where the prized molluscs were formerly abundant. In many places, such as the Wadden Sea in Europe and Narragansett Bay, oysters are rated “functionally extinct,” with fewer than 1 percent of their former reefs persisting. The declines are in most cases a result of over harvesting of wild populations and disease, often exacerbated by the introduction of non-native species. Oysters have fueled coastal economies for centuries, and were once astoundingly abundant in favored areas.