Seagrasses around the world are disappearing, with some species now threatened with extinction. The first global survey of individual seagrass species has found that 14% are at risk of going extinct. More common species are also in decline, meaning both seagrass habitat and diversity is being lost. Seagrasses provide food and habitat for a variety of ocean species including manatees, sea turtles and fish such as sea horses. Seagrasses are flowering plants that grow on the ocean floor. They form vast meadows that flower and seed underwater, having evolved from land-based plants that entered the water millions of years ago. Seagrasses alone form important marine habitats. They act as nurseries for young fish and shellfish, and are the primary food for large marine mammals such as manatees and dugongs, as well as reptiles such as some sea turtles. They also contribute to the health of coral reefs and mangroves, salt marshes and oyster reefs. It has been known for a while that seagrasses are declining in many parts of the world. The reasons are many, seagrass expert Frederick Short told the BBC. Professor Short, of the University of Hampshire in Durham, US is the director of SeagrassNet, an international seagrass monitoring program with 114 sites around the world. For example, seagrasses are gone from the most developed coastlines due to pollution, said Professor Short. Seagrasses are in decline in the developing world due to sedimentation, caused by run off from impacted watersheds and deforestation, and being overloaded with nutrients flowing into the sea from sewage and agricultural runoff. Seagrasses are also being directly damaged by the dredging of seafloors. But there has never been a review of individual species status, said Professor Short. Many widespread, common seagrass species which are not presently threatened are nonetheless in decline, so we have both an overall loss of habitat and a loss of biodiversity, said Professor Short. BBC