Image caption Turtles are among the animals making use of the gulf's plentiful food and shelter

Two small islands in South Asia's first marine biosphere reserve have sunk into the sea primarily as a result of coral reef mining, experts say.

The islets were in a group in the Gulf of Mannar, between India and Sri Lanka.

The Indo-Pacific region is considered to contain some of the world's richest marine biological resources.

The group's 21 islands and islets are protected as part of the Gulf of Mannar Marine National Park, covering an area of nearly 560 sq km (216 sq miles).

Fishermen had indiscriminately and illegally mined invaluable coral reefs around the islets of Poomarichan and Villanguchalli for many decades, said S Balaji, chief conservator of forests and wildlife for that region of Tamil Nadu state.

"The absence of any regulations prior to 2002 led to illegal mining of the coral reefs, which came to an end when environmental protection laws were enacted," he told the BBC Tamil Service.

Mr Balaji said rising sea level as a result of global warming was also a factor behind the islands' submergence.

But this was questioned by Simon Holgate from the Proudman Oceanographic Laboratory in Liverpool, UK, who said observations showed that the sea level in the region had been rising slower than the global average.

Lost islets are indicators, and can even be considered as a warning S Balaji, GOMBRT

"I think that global sea level rise had little impact on the disappearance of these islands and it must be due to other reasons, possibly the mining of coral reefs," Dr Holgate told BBC News.

Though these islets were only 3-5m (10-15 ft) above sea level, their submergence sounded an alarm bell about the danger many more small islands faced in the long run, according to Mr Balaji, who is also director of the Gulf of Mannar Biosphere Reserve Trust (GOMBRT).

The Gulf of Mannar was chosen as a biosphere reserve by the Indian government in 1989 because of its biological and ecological uniqueness, and the distinctive socio-economic and cultural profile shaped by its geography.

Most of the 21 islands are uninhabited, and the corals were mined for use as a binding material in the construction industry, as they were rich in calcium carbonate.

Rich biodiversity

The biosphere reserve is a storehouse of about 3,600 species of marine flora and fauna.

Many more wait to be studied, said Deepak Samuel, marine biologist and project associate with the Energy and Environment Unit of the UN Development Programme (UNDP).

"The Gulf of Mannar is a unique reserve with ecosystems like coral reefs, mangroves and seagrass," Mr Samuel said.

"It is a nursery for shell and fin fishes, which means the entire breeding and juvenile raising takes places in these three ecosystems."

More than 300,000 fishermen depend on the Gulf of Mannar for their livelihood. It is also the dwelling place for many endemic species, notably the dugong or "sea cow".

Studies have proved that this gulf is home to 117 species of corals belonging to 37 genera, and 13 out of the 14 species of seagrasses in Indian seas.

The area has also been famous for pearl harvesting for over 2,000 years.

According to marine biologists, a quarter of the 2,000-plus fin fish species in Indian waters are in this gulf, making it one of the region's most diverse fish habitats.

Image caption Most of the islands are uninhabited, and rise just a few metres above sea level

The loss of these two islands should be a "wake-up call" for all those in the entire Asia-Pacific region, said Mr Balaji.

Though the lost islets were small, he cautioned that a similar fate may happen to larger islands in the long run as a result of global warming coupled with large scale mining.

Losing the reefs may result in migration of fish populations to other regions, which would result in loss of the gulf's biodiversity, according to Dr Samuel.

"Lost islets are indicators, and can even be considered as a warning," he said.

With the threat of climate change in years to come, factors such as coral mining will have an accelerating effects on the submergence of many island, he warned.

People in the area have gone on record many times as saying that the coral reefs in the Gulf of Mannar saved them from destruction when the devastating tsunami struck in December 2004.

Experts also point out the need to keep the remaining 19 islands and islets "pristine" in order to offer them some protection them from processes such as climate change.

The Indian National Oceanographic Institute point out that very few of the islands and islets in the gulf are in good shape.

Collection of coral by students for research over many decades, and heavy industrial pollution caused by onshore industries, have inflicted an irreversible damage to the coral reefs in this unique marine biosphere.