Miles of unblemished coral reefs have been turned to slime-covered rubble because of rising sea temperatures caused by global warming.
A study into the extensive bleaching of the Seychelles corals in 1998 has found that these Indian Ocean reefs failed to recover, with many of them crumbling to broken fragments.
Scientists said the findings showed that rising global sea temperatures could have a more devastating impact on the world's tropical corals than previously thought.
"Some of the reefs have collapsed to almost mobile beds of rubble. They are no longer solid structures and some have been overgrown with fleshy green mats of algae," said Nicholas Graham, a coral ecologist at the University of Newcastle.
"They have basically turned into reefs of rubble and algae, with very little fish life. It's a depressing story and it's very sad to see what's happened to these reefs," said Mr Graham, a member of the survey team.
The Seychelles once boasted mile upon mile of luxuriant coral reefs but in 1998 the local sea temperatures rose dramatically because of the general rise in global temperatures combined with the effects of a strong El Nino - an occasional reversal of the warm ocean currents in the Pacific Ocean.
Reefs around the world were badly affected by the temperature rise. The Great Barrier Reef off Australia suffered its worst bout of bleaching in 700 years - only to be surpassed by even worse bleaching in 2002.
Temperatures in the Indian Ocean in 1998 rose to unprecedented and sustained levels, causing the stressed coral to eject the tiny single-celled algae that feed and clean the coral's animal polyps as well as giving the reefs their vibrant colours.
Without its beneficial algae, the bleached coral animals die within several weeks, leaving behind their empty "skeleton" - the chalky reef which is built by generations of corals over many centuries.
Scientists from Britain, the Seychelles and Australia carried out extensive surveys of 21 coral sites in 1994. They did the same in 2005 to assess the extent of the damage caused by the 1998 bleaching event.
"The coral reef system of the inner Seychelles has undergone a widespread phase shift from a coral-dominated state to a rubble and algal-dominated state," the scientists report in the journal Proceedings of the National Academy of Sciences.
Before 1998, about 50 per cent of the area covered by the survey was covered in growing coral. Today, just 7.5 per cent of the same area is living coral and less than 1 per cent is now covered in vital fast-growing species, Mr Graham said.
The variety and number of fish living in the area have also suffered. Out of 134 fish species known to be living in the region, about half have disappeared from the most heavily-affected areas.
A total of four fish species - a type of butterfly fish, two species of wrasse and a damsel fish - have probably gone extinct locally and six other species have reached critically low numbers.
"We have shown that there has been very little recovery in the reef system of the inner Seychelles islands for seven years after the 1998 coral bleaching event," Mr Graham said.
He added: "Reefs can sometimes recover after disturbances, but we have shown that after severe bleaching events, collapse in the physical structure of the reef results in profound impacts on other organisms in the ecosystem and greatly impedes the likelihood of recovery."
Dead and bleached coral is soon attacked by other marine organisms such as sea snails, worms and clams which bore into the calcium carbonate structure of the reef, causing it to weaken and collapse.
Bleached reefs can sometimes recover with the help of floating coral larvae arriving from distant colonies, but the Seychelles are relatively remote, making this scenario less likely.
Mr Graham said that many coral reefs around the around the world had been damaged by rising sea temperatures caused by global warming.
"Unfortunately it may be too late to save many of these reefs but this research shows the importance of countries tackling greenhouse gas emissions and trying to reduce global warming and its effect on some of the world's finest and most diverse wildlife," he said.
Corals grow around the shallow waters of tropical islands and submerged volcanoes and can form extensive lagoons and atolls if the land subsequently subsides.Reuse content