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According to one dictionary definition, paradise is "heaven; the Garden of Eden; or an ideal place or state". Have you ever described somewhere as 'paradise' and truly meant what you said? Well, after visiting the tiny Pacific nation of Palau, I can indisputably say I've found my underwater Garden of Eden – an almost reality-defying utopia of divine beauty.

With over 500 spectacular, picture-perfect tropical islets making up this archipelago, Palau (known in the native tongue as 'Belau') is one of the world's most magical marine destinations. For snorkelers and scuba divers in particular, Palau offers every reason to part with the extra dollars to visit this remote and exquisite piece of tropical heaven.

NEPTUNE'S GARDEN

Lying in the northwest Pacific Ocean, roughly 800km east of the southern Philippine island of Mindanao, Palau is a sizzling hot-spot of marine biodiversity. The crystal-clear azure waters of the many fringing reefs found around these coral islets boast more than 400 species of hard corals, 300 species of soft corals, over 1400 species of fish and seven of the world's nine species of giant clams. As a marine biologist, I was constantly amazed and intrigued as I took in the profound biological significance of this island system. And my feeling, after working there as a dive guide for five months, was that many visitors overlook just how important Palau is on the world's marine stage – and how important it is that we ensure its pristine environment is protected.

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The basis for this whole story lies within the limestone skeleton matrices of an extraordinary animal group – the corals. These sophisticated invertebrates are related to jellyfish, bluebottles, anemones and hydroids through the presence of specialised venomous stinging cells known as cnidocytes or nematocysts, which they use to catch food and for defence against predators. Corals not only splash the reef with more colours than you'll find in Picasso's paint box, but they're profoundly important ecological players.

There are two types of corals – hard and soft. The hard or stony corals (also known as reef-building or hermatypic corals) secrete a hard internal skeleton made of calcium carbonate or limestone. Soft corals also secrete limestone, however, this is in the form of tiny, thorn-like spicules, which lay embedded in the tissue. Those beautiful gorgonian sea fans we see in many underwater pictures belong to this group.

Most varieties are made up of a whole colony of tiny individuals we know as polyps, and together they make 'heads' of corals, which grow bigger and bigger when each polyp splits in half through a process known as 'asexual budding'. The solitary, single-polyp mushroom corals of the family fungiidae are well known for their ability to clone themselves and sometimes there are entire areas blanketed by members of this successful group.

Colonial corals, such as the branching forms of the acropora species, are capable of relatively fast growth rates, at around 18cm per year, while others, especially those morphological forms that grow into big boulders like some porites species (the 'brain corals') grow very slowly, only around 1cm per year, and can be hundreds of years old.

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Most corals partake in mass spawning events once a year in which, on a few special nights linked in with the phases of the moon, they release thousands of eggs and sperm into the water. The fertilised eggs are then taken away with the currents to find a new home many miles away. It is through the propagation and growth of these corals that reefs are gradually formed over thousands or even millions of years.

Once the coral foundations are in place, the rest of the ecosystem follows, which encompasses a plethora of different animal groups, all fitting perfectly into a complex web of bustling aquatic life, from microscopic bacteria, algae and worms through to the spectacular nudibranchs (often referred to as sea slugs), crustaceans, shellfish and innumerable fish species, all the way to the apex predators, the majestic sharks.

UNDER THE MICROSCOPE

Here's the important part… As I got to know all of the sparkling reefs and coral-rich lagoons surrounding the islands of Palau, I began to explore for myself a very interesting aspect of a coral's life; one that is key to the creation of reefs, which, in fact, support a minimum of 25 per cent of the planet's marine species. I am referring to a microscopic, single-celled, photosynthesising algae known as zooxanthellae (pronounced 'zoo-zan-thel-lee'). Species members of the corals, clams, jellyfish and sponges are well known to house these amazing little organisms, which live in their millions inside the tissues of their hosts. In the case of reef-building corals, these algae 'symbionts' (any organism that lives in a symbiotic relationship with other organisms) can support up to 90 per cent of the coral's nutrient requirements through the process of photosynthesis. It is a beautiful example of hand-in-hand evolution: the coral provides a protected home for the algae and, in return, the algae uses the coral's metabolic waste products (CO2 and ammonia) to assist in the process of photosynthesis.

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Mushrooms coral – seen with their tentacles fully extended for feeding on microscopic organisms
and particulate organic matter.

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The spiny devilfish belongs to the extremely venomous stonefish family. A master of camouflage, contact with its dorsal fin spines can cause incredible pain and can even be fatal.

The nutrient recycling system these two organisms have created as they've evolved together is so remarkably efficient that it remains one of the core reasons that coral reefs have such high rates of growth and energy production, despite the typically low-nutrient water surrounding them.

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A crucial part in the success of this story is the algae's need for light, which is the reason we find most corals growing in the first 50m of water, where light can easily penetrate down to the coral polyps. It is also understood that the zooxanthellae symbionts greatly assist in the creation of a coral's calcium skeleton, which indirectly helps to further expand the reef and thus support even more marine life.

HOT TOPIC

Another specific parameter required for healthy coral growth is temperature. Optimum growth occurs in waters ranging roughly from 20°C to 30°C. It can be generally stated that when a coral is exposed to temperatures of 1°C to 2°C above that of their upper thermal tolerance limit for extended periods, the algal symbionts are made to produce highly corrosive oxygen radicals, which cause the coral to expell them.

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Above left: Spotted garden eels live in large colonies on the seabed. When they rise in numbers
to feed they create a garden of strange swaying 'plant life'.

The photosynthetic pigments in the zooxanthellae contribute towards the colourful nature of the coral, so when large numbers of algae are flushed out, the coral tissues are left largely transparent, revealing the white skeleton underneath, thus giving the coral its classic 'bleached' appearance. This is a 'lose-lose' situation, as the algae loses its home and the coral begins to weaken and starve. If these high sea surface temperatures (SSTs) are relatively short-lived, the few remaining zooxanthellae in the coral tissues may be able to quickly repopulate themselves and the corals are given a chance to recover. However, if high temperatures are maintained for too long, the damage is irrevocable and the whole colony may die.

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Blue Corner – one of the world's top 10 dive sites and Palau's biggest dive drawcard for its reliable sightings of enormous schools of snapper, trevally and barracuda, friendly Napoleon wrasse and the dozens of sharks that cruise by. Divers are hooked in with ropes at the edge of the reef to prevent them from being carried away by strong currents.

Studies revealing crucial information about the resilience of corals and their potential to adapt to warming seas have been gaining pace in recent times. It has been discovered that many corals contain not just one strain of zooxanthellae, but rather several types and, at times of high tissue stress, the expulsion of the dominant strain may allow for the more heat-tolerant variety to take its place. This alludes to a coral's potential to adapt to rising SSTs by choosing to adopt more heat-tolerant algae.

A red lionfish does not use its venomous spines to capture prey, but only for protection from predators.

Another consideration is that the corals already surviving in warmer waters are in a good position to tackle predicted rises in temperatures through being physiologically better prepared. And it may be that these corals, armed with resilient zooxanthellae strains, will be the ones to carry the corals reefs through the next few decades. According to some in the scientific community, coral reefs around the world will come under some pressure. If that turns out to be the case, it is likely to have an effect on the intricate ecosystems they support; an impact yet to be fully understood.

THE ACID TEST

Another issue potentially affecting corals is that studies appear to show that our oceans are becoming more acidic. Oceans naturally absorb CO2 from the atmosphere and have been acting as a major sinkhole for this greenhouse gas over the last two centuries. But many scientists now believe there is a looming problem with acidification.

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The Blue Holes (large coral caverns) are an extremely popular and spectacular dive site in Palau.

Like a piece of chalk in a glass of vinegar, a lowered ocean pH will not only hamper the ability of corals and other key shellfish species to calcify their skeletons, but it may cause their skeletons to weaken and even dissolve altogether. Ocean acidification is a controversial topic amongst scientists, with some making dire predictions for the health of our oceans, while others maintain there is no clear indication of a problem.

However, if you throw in oceanic warming, possible acidification, destructive fishing methods such as explosives and poisoning, over-fishing, pollution, aquarium collecting and coastal development, it appears our planet's reefs could be in serious peril. All we can do is hope that some of the more dramatic predictions fail to eventuate, and that nature finds a way to deal with any looming challenges.

In the meantime, though, as consumers we can all make a difference by not purchasing shells and saying no to jewellery made from corals. We can also become more informed and selective about what seafood we buy and where it comes from. I'd also suggest you make a visit to Palau – it offers one of the most dramatic examples of what we stand to lose if we don't learn to look after our oceans and the life they support.

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The jellyfish in the marine lakes rely partly on nutrition passed on from their zooxanthellae and
they follow the trajectory of the sun across the lake (in their millions) in order to keep their
algae bathed in sunlight.

 

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