There are about 60 species of seagrass or eel-grass, the only known flowering plant to live permanently submerged in coastal waters in the world, that grow from the mid-intertidal to 50-60m depth. They inhabit all types of substrate but the most extensive sea-grass beds are on soft sediments like sand and mud. The roots and rhizomes of seagrass stabilise large areas of shallow water sediment and these meadows are highly productive and important nursery grounds for many species including commercially exploited fish and crustaceans. These grasses are the main food of manatees, dugongs and green sea turtles, all threatened species of great public interest.

The extent of meadows has been greatly reduced due to such human activities as pollution, damage by fishing equipment, leisure activities and introduced grazers and pathogens. Sea-grasses receive little protection despite the myriad of threats to this habitat. Climate change is likely to have significant consequences, not only due to changing temperature but also to changing sea levels with effects on coastal hydrography and increasing frequency of damaging storms. While the changes may make new areas available for colonisation by seagrasses and improve conditions for them in others, in the short-term, the overall effects of climate change are likely to be negative.

Most of our knowledge of seagrass derives from developed nations but little is known of the seagrass ecosystem in less developed areas of the Caribbean, Indian Ocean and Pacific.