WHAT'S KILLING THE MERMAID?

They may not be the world's most beautiful creatures. In fact, some would consider them the Ugly Ducklings of the sea world. Yet their innocent eyes and whiskered snouts give manatees, or sea cows, an appeal for humans comparable to that of dolphins. Believed by some to be the prototype of the mermaid myth, these gentle creatures have become one of southern Florida's major tourist attractions. But maybe not for much longer: a mystery disease is threatening to kill off Florida's entire manatee population.

Their numbers have been dwindling for years: from eating polluted sea grass, from getting tangled in fishing lines or from being slashed by pleasure-boat propellors. But this year something different - and much more sinister - has been happening.

Around 250 of the Florida subspecies of the West Indian manatee (Trichechus manatus latirostris) have died off Florida in the first four months of this year. That compares with 201 deaths during the whole 1995. Of this year's fatalities, 150 have been since early March, when scientists, alarmed by the escalating number of deaths, first identified the presence of the deadly illness. At the current rate, unless the cause of the disease is discovered and eliminated, the Florida manatee could be extinct within two years.

The cause and exact nature of the disease is still a mystery. "This animal is in otherwise perfect health. It's just dead," marine research scientist Dr Scott Wright said as he cut open a manatee recently at a makeshift morgue on Sanibel Island, off Fort Myers on Florida's south-eastern coast. Most of the deaths have occurred in that area. Autopsies on the dead animals have revealed discoloured lungs, filled with fluid - symptoms of pneumonia. But, so far, laboratory tests on the diseased tissues have offered no further clues about the cause - or possible cure.

A number of possible causes have been eliminated. A recent cold spell in Florida has been discounted, because only adults have so far died and the cold normally affects calves first. Nor does the so-called Red Tide, a toxic micro-organism that has affected algae and shellfish off Florida, appear to be to blame; that, the scientists say, would lead to manatees becoming sick or disoriented, not to their dying suddenly.

In the past, the greatest threat to manatees has been the 800,000 pleasure boats in the waters around southern Florida. Manatees swim close to the surface, very slowly - usually around 4mph - and have to come up for air every few minutes; marine scientists often identify individual manatees by their propellor scars. The boats also contribute to the pollution of the creatures' main food source, sea grass.

The manatee (the name is thought to come from the pre-Columbian Carib Indian word for "woman's breast") is a vegetarian mammal believed to have evolved from a wading, plant-eating animal related to the elephant. Fossils found in Florida suggest that the beast - which averages 10ft in length and weighs half a ton - is at least 45 million years old. Manatees winter off Florida, but venture in summer as far north as New England. One was named Chessie after a transmitting device attached to his tail tracked him to Chesapeake Bay, near Washington, DC.

Manatees exist elsewhere in the tropical Atlantic and in the Caribbean; their close relative, the dugong, is found in the Pacific north of Australia and in the Indian Ocean. But Florida has made something of a pet of the manatee, which has been protected there since 1893. The state government helps raise funds for its protection by selling customised "Save The Manatee" car number plates. The state-owned power company, Florida Power and Light, keeps at least one steam turbine turned on at each coastal plant at all times, warming the waters for the manatees who gather there. The company is also behind a public awareness campaign which issues "I slow for manatees" stickers to boaters.

Concern about the disease that threatens to wipe out these well-loved mammals runs deep. "The trouble is," said state governor Lawton Chiles, "even if scientists manage to pin down the cause, they may not be able to do anything about it."