Giant Clams: Snorkelling over the Reef Drop-off one is immediately struck by the beauty of the 'underwater garden' comprising table corals – often hundreds of years old - waving soft corals with brightly coloured fish darting in and out and above all the varied colours of the giant clams. Clams are bivalves which have turned themselves over so that the hinge that joins the two shells is pointed downwards. The mantle between the two shells is upwards and spills out of the shell. This mantle can vary in colour from bright blue, through green and yellow to brown depending on the influence of microscopic algae called zooxantellae cultivated by the clam which then feeds on part of them. Its upward position enables the clam to expose its body to maximum sunlight in order to grow the zooxantellae as abundantly as possible.

Starry Pufferfish (Arothron stellatus): Up to a metre long the Starry Pufferfish is an impressive sight. The fish seems almost unseaworthy as it swims very slowly rocking from side to side with the currents on the outer edge of the Drop-offs where it is often found. It particularly likes coral rich areas. The predominant colour of the Starry Pufferfish is grey, light for the body and darker for the hundreds of dot and dash markings that cover it from snout to tail. Also distinctive is the black dot dash markings at the base of the pectoral fins. It's diet comprises a wide range of bottom-dwelling animals and some algae.

Snorkelling while the time is right. If you want to snorkell on some decent coral reefs do it now because in 20 years there won't be any left according to Roger Bradbury, a resource management ecologist at the Australian National University. Within a human generation they will collapse, he says. There will be remnants here and there but the global coral reef ecosystem will cease to be.

Over fishing, ocean acidification and pollution are pushing coral reefs into oblivion. Each of these forces alone is capable of causing the global collapse of coral reefs; together they assure it. Each is growing broadly in line with world economic growth.

Global fishing pressure is still accelerating; it is set to double in a decade even as the global fish catch is declining each year.

Ocean acidification is yearly more extreme because of increased carbon dioxide absorption from the atmosphere. Coral can make their calcareous skeletons only within a special range of temperature and acidity. That range will be exceeded in the next 20 to 30 years.

As for pollution, coral reefs can't survive in nutrient-rich waters. The only things that are encouraged are microbes soaking up the sun's energy by photosynthesis and lots of jellyfish feeding on them. What will be left is a slimy algal-dominated hard ocean bottom.

So remember: the underwater photographs that you take today could be of historic interest tomorrow.

Did You Know? Over 4,000 species of fish inhabit coral reefs. Healthy reefs can produce up to 35 tons of fish per square kilometer each year, but damaged reefs produce much less.

Humpback Red Snapper (Lutjanus Gibbus): The Humpback Red Snapper prefers waters that are not too deep and are rich in currents so they can be seen often both on the Reef Crest and on the slope of the Drop-off. A very distinctive fish growing to 25 cm its silver grey body is outlined by black dorsal and anal fins and a black forked tail with rounded ends and a broad upper lobe giving it the alternative name of Paddletail. In bright contrast are the orange pectoral fins and the yellow rimmed eyes. The Humpback is often seen in red livery also with yellowish pectoral fins. The head in both versions features a slightly concave profile. Diet comprises invertebrates and small fishes and an occasional feed on zooplankton.

Black Pyramid Butterflyfish (Hemitaurichthys zoster): A typical plankton feeder, the Black Pyramid Butterflyfish, which is very common in the Maldives, can be found in schools feeding near the surface of the Reef Drop-off. The 18 cm Butterflyfish is identified by the broad black, white, black body stripes with a yellow flash on the dorsal fin.