By Alex Kirby
BBC News Online environment
Many marine creatures, from seahorses to turtles, are at risk from the rapid destruction of the Earth's seagrasses, according to the United Nations.
Seagrasses: Home to the manatee
It has released the first map of their global distribution, and says 15% of them have gone in the last 10 years.
Seagrasses are flowering plants - not seaweed - that flourish in some of the shallow waters that line our coasts.
They provide an important habitat for a range of other sea life and benefit people by helping to combat erosion.
The World Atlas Of Seagrasses is the work of the UN Environment Programme's World Conservation Monitoring Centre (Unep-WCMC), based in Cambridge, UK.
It estimates the area of seagrasses in the world at 177,000 square kilometres, an area about two-thirds the size of the UK.
But this is likely to be an underestimate, because there have been no surveys of seagrasses off the western coasts of Africa and Latin America.
BRITISH ISLES DISTRIBUTION
Flowering plants in seawater
Have ribbon-like, grassy leaves
Live in coastal bays, estuaries
Food and shelter for animals
The 60 or so species of seagrass, which grow in large meadows in both tropical and temperate seas, are extremely varied.
They range in length from the 2-3-centimetre leaves of sea vines in deep water off Brazil to the strands of eelgrass that grow to more than four metres in the Sea of Japan.
The meadows are home to fish, manatees, dugongs and green turtles, and provide a good habitat for many other plants.
They also protect coral reefs by binding sediments, and help to clean the water and protect the coasts from storms.
Yet the authors say the seagrasses are being steadily destroyed by the run-off of nutrients and sediments from human activities on land, and by boating, land reclamation, dredging, and some fishing methods.
Protection for seahorses will also benefit the grasses
Dr Klaus Toepfer, the executive director of Unep, said seagrasses were "a vital marine ecosystem whose importance has largely been overlooked until now".
He said: "The scientists have presented us with a worrying story. In many cases, these vitally important undersea meadows are being needlessly destroyed for short-term gain without a true understanding of their significance."
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Ed Green, one of the co-editors of the atlas, said: "There are few places where seagrass meadows are protected. We now know that vast numbers of fish use seagrass for a short but critical part of their lifecycle.
"We are also becoming aware of the role that seagrass plays in the climatic and oceanic carbon cycles and in coastal protection. The true economic value is difficult to measure, but this work suggests it is immense."
Dr Mark Collins, director of Unep-WCMC, said the seagrass beds had been overlooked by conservationists and coastal development planners throughout their range.
There are about 60 species of seagrasses
He said: "The public can play an important role. By insisting on protection for seahorses, turtles and dugongs they will also safeguard the ecosystem that supports them and has intrinsic benefits that are less obvious."
The UK has seagrass meadows in south-west England, sheltering two rare species of seahorse as well as cuttlefish, crabs and sea bream.