Cuttlefishes (Sepia officinalis) are classified as Cephalopoda which also includes squid, octopuses and nautiluses but despite their name, they are not fish but molluscs. They have an internal shell – the cuttlebone, large W-shaped pupils, eight arms, and two tentacles furnished with denticulated suckers, with which they secure their prey. They generally range in size from 15 cm to 25 cm and can be seen in the Lagoon between beach and Reef Crest. Cuttlefishes eat small molluscs, crabs, shrimp, fish, octopuses, worms, and other cuttlefishes. Their predators include dolphins, sharks, fish, seals, seabirds and other cuttlefishes. Their life expectancy is about one to two years. Cuttlefishes are sometimes referred to as the "chameleon of the sea" because of their remarkable ability to rapidly alter their skin color at will. Cuttlefishes change color and light polarisation to communicate to other cuttlefishes and to camouflage themselves from predators.

What will I see at the Reef Drop-off? The Reef Drop-off, for its first 50m, is the habitat for many reef fish who find shelter on the Reef Face and eat the plankton in the nearby sea. The Reef Face is usually the richest habitat. Its complex growths of coral provide cracks and crevices for protection, and the abundant invertebrates and algae offer an ample source of food. Fish that cruise the boundaries of the Reef Drop-off include predators, such as pompanos, groupers, certain types of shark, and barracudas. If you are lucky enough to have seagrass meadows nearby you will see mackerel, snapper, porgies and grunts. Herbivorous and plankton-eating fish include fusilier, ray, chromis, and the nocturnal squirrelfish and cardinalfish.

Thin Time for Shelled Creatures: Ocean acidification caused by climate change is making it harder for creatures from clams to sea urchins to grow their shells scientists said today. A thinning of the protective cases of mussels, oysters, lobsters and crabs is likely to disrupt marine food chains by making the creatures more vulnerable to predators, which could reduce human sources of seafood. Human emissions of greenhouse gases include carbon dioxide from burning fossil fuels, and some of that carbon dioxide ends up in the oceans, where it dissolves to form acid. The ocean acidification makes it harder for creatures to extract calcium carbonate - vital to grow skeletons and shells according to the study in the journal Global Change Biology.

What will I see in the Reef Lagoon? The Reef Lagoon or reef flat can be entirely enclosed creating a stretch of quiet water or it can be a protective area bordering a beach, or a flat, rocky area between the reef and the shore. Whatever its character the lagoon offers protection to juvenile and small fishes. Consequently, the number of species living there is often the highest of any reef zone. Stretching from the beach to reef crest, the lagoon's sandy bottom is littered with chunks of coral. As one swims towards the crest the flat area becomes more rocky and the number of browsing fishes higher. You will spot juvenile sharks and rays, Black-spotted Pompanos, juvenile Triggerfishes, a large variety of Goatfishes, Sandperches, Monacle Breams and the occasional school of Mullets.

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.