Overfishing and unchecked coastal development have resulted in the disappearance of 85 percent of all oyster reefs, making the ecosystem one of the most severely affected marine habitats in the world, according to a study released Thursday.

The Nature Conservancy study found that several reefs in China have seen drastic declines over the past 30 years, while those in Europe have almost entirely disappeared. Half of the shellfish populations in South America are under threat, while flat oysters have been virtually wiped out in Australia.

Native oyster reefs _ essentially mountains of the bivalves cemented together _ were once dominant features of many temperate estuaries around the world. Much as coral reefs are critical to marine habitats, the bivalve shellfish are vital to bays and estuaries, creating habitats for a variety of plants and animals, the study said.