(Including the Malay Archipelago)



Southeast Asian Reefs And Reef Life

Most of the reefs around Southeast Asia are - at least in geological terms - quite young. Towards the end of the last ice age, sea levels were as much as 100 metres lower than they are today, and much of the area between the large islands of Borneo, Sumatra and Java was dry land. Since then the sea has reasserted itself, so that now this area is flooded with warm, shallow water dotted with islands and reefs.

The Nature Of Corals And Reefs

Tropical reefs are built mainly from corals, primitive animals closely related to sea anemones. Most of the coral types that contribute to reef construction are colonial; that, numerous individuals - polyps - come together to create what is essentially a single compound organism. The polyps produce calcareous skeletons; when thousands of millions of them are present in a single colony they form large, stony (in fact, limestone) structures which build up as reefs. What happen s is that, when corals die, some of the skeleton remains intact, thus adding to the reef. Cracks and holes then fill with sand and the calcareous remains of other reef plants and animals, and gradually the whole becomes consoloidated, with new corals growing on the surface of the mass. Thus only the outermost layer of the growing reef is actually alive.

There are about 450 species of reef-building coral in the seas around Southeast Asia and the Pacific. Corals grow slowly, adding about 1-10cm ().4-4in) growth in a year. Once over a certain age, they start being able to reproduce, releasing tiny forms that float freely among the plankton for a few weeks and settling to continue the growth of the reef. The forms corals create as they grow vary enormously according to the species and to the place on the reef where it is growing. Colonies range in size from a few centimetres in diameter to giants several metres across and many hundreds of years old. Some are branched or bushy, others tree-like, others in the form of plates, tables or delicate leafy fronds, and yet others are encrusting, lobed, rounded or massive.

Microscopic plants called zooxanthellae are of great important to the growth and health of many corals. These are packed in their millions into the living tissues of most reef-building corals (and of various other reef animals, such as Giant Clams). Although reef corals capture planktonic organisms from the water, a significant amount of their food comes directly from the zooxanthellae. It is for this reason that the most prolific coral growths are in the shallow, well lit waters where the zooxanthellae thrive. The presence of coral communities does not, necessarily lead to the development of thick deposits of reef limestone, for example, the Karakatoa Islands off the southern tip of Sumatra consist mainly of slab of volcanic rock with a patchy veneer of corals.

Types of Reef

In most regions with plentiful coral communities, the Calcareous skeletons have built up to form a variety of different types of reef:

.   fringing reefs
.   patch reefs, banks and shoals
.   barrier reefs
.   atolls

Fringing Reefs

Fringing reefs occur in shallow water near to land. Typically they extend to depths of 15m-45m (50-150m) depending on factors such as the profile and depth of the seabed and the clarity of the water. Islands that stand in deep water, like Palau Sipadan, have precipitous fringing reefs that descend hundreds of metres, but these are exceptions rather than the rule. Many mainland coastlines in Southeast Asia are too close to the river estuaries for reefs to develop, and instead support stands of mangroves - another marine ecosystem of enormous importance in the region. But the offshore islands, away from the influence of freshwater run-off, are often surrounded by reefs.

Palau Reefs

Patch Reefs, Banks and Shoals

In theory, reefs can develop anywhere that the underlying rock has at some time been close enough to the surface for corals to become established and grow. Sea levels may have risen considerably since then, or other geological changes may have returned to lower the depth of bed beneath the surface, either way, there are many places where reefs exist as isolated mounds or hillocks on the seabed. Such patch reefs are widespread throughout the Southeast Asian region in relatively shallow waters surrounding the islands and on the continental shores. They vary in size from tens to thousands of metres in diameter, usually with their tops coming to within a few metres of the surface - indeed some emerge about the surface and are topped by sand cays. Patch reefs further offshore, lying in waters hundreds of metres deep and with even their tops 20m (66ft) or more below the surface, are usually referred to as banks or shoals. Some of the most extensive lie in the South China Sea.

Barrier Reefs

Barrier reefs occur along the edges of island or continental shelves, and are substantial structures. The major difference, apart from size, between them and fringing reefs is that they are separated from the sore by a wide deep lagoon. The outer edge of the barrier drops away steeply to the ocean floor, beyond. Initially these reefs formed in shallow waters then, as sea levels increase, they built progressively upwards so that their living topmost parts were still near the surface of the water. There are a few barrier reefs in the Philippines and Indonesia, but the best-developed are found around Papua New Guinea - for example the 180km (110-mile) barrier running along the outside of the Louisiade Archipelago of the southern tip of the mainland.


These are formations of ancient origin - millions of years old - and take the form of ring-shaped reefs enclosing a shallow lagoon and dropping away to deep water on the outsides. Atolls begin life as fringing reefs around volcanic islands and kept growing as the underlying base gradually subsided beneath the water level. Most of the world's atolls are in the Indian and Pacific Oceans, but there are a number to explore in Southeast Asian water, particularly around Papua New Guinea and the eastern provinces of Indonesia. Taka Bone Rate Atoll, reputedly the third largest atoll in the world, is off the southern coastline of Sulawesi.


The reef ecologies of Southeast Asia - and those of northern Australia - harbour a greater range of species than anywhere else in the Indo-Pacific; they are packed with all manner of bizarre and beautiful plants and exotic animals. It is likely the region became established in a centre of evolutionary diversification millions of years ago, it has remained so, despite changes in sea levels and in the fortunes of individual reefs, right up until the present day. On most reefs your attention is likely to be held initially by the fish life in a single dive's casual observation you might see well over 50 different species, while a concentrated effort would reveal hundreds. Even that is only part of the story.

The reef and associated marine habitats of most Southeast Asian and Pacific countries support well over one thousand species, but many are hidden from view within the complex framework of the reef - gobies, for example, usually in fact the most numerous of all the fish species on a reef, are seldom noticed.   

Reef Zones And Habitats

Reefs can be divided into a number of zones reflecting differences in such features as depths, profiles, distance from the shore, amount of wave action, and type of seabed. Associated with each zone are characteristic types of marine life.

The Back Reef And Lagoon

The back reef and lagoon fill the area between the shore and the seaward reef. Near the sea-beds is usually a mixture of sand, coral rubble, limestone slabs and living coral colony. The water depths vary from a few meters to fifty meters (165 ft) or more, and the size of the lagoon can be anywhere from a few hundreds to thousands of square meters. The largest and deepest lagoon are those associated with barrier reefs and atolls, and may be dotted with islands and smaller reefs.

Sites within lagoons are more obviously more sheltered than those on the seaward reefs, and are also affected by sedimentation. Here you will find many attractive seaweeds; most of the corals are delicate, branching types. Large sand-dwelling anemones are often found, and in places soft corals and "false corals" are likely to form mats over the seabed. Especially where there is a current you may encounter extensive beds of sea-grasses, the only flowering plants to occur in the sea. Among the many species of animals that make these pastures their home are the longest sea cucumbers you will find anywhere around the reef.

Although some typical reef fishes are absent from this environment, there is no shortage of interesting species. On the one hand, there are rowing predators - snappers, wrasse, trigger fish, emperors and others - on the lookout for worms, crustaceans, gastropods, sea urchins and small fish. Then there are the bottom-dwelling fish that burrow into the sand until completely hidden, emerging only when they need to feed.

Most entertaining to watch - if you spot them - are the small gobies that live in association with Pistol Shrimps. In this partnership the shrimp is a digger and the goby, stationed at the entrance to the burrow, is the sentry. The small fish remains ever on the alert, ready to retreat hurriedly into the burrow at the first sign of disturbance. The shrimp has very poor eyesight; it keeps its antenna in close touch with the goby so that it can pick up the danger signal and, likewise, retire swiftly to the safety of the burrow.

The Reef Flat

Reef flats are formed as their associated reefs push steadily seaward, leaving behind limestone areas that are eroded and planed almost flat by the action of the sea. The reef flat is essentially an inter-tidal area, but at high tide it can provide interesting snorkelling. The inner part of the reef flat is the area most sheltered from the waves, and here you may find beautiful pools full of corals and small fish. Among the common sights are: micro-atolls of the coral genus Porites; their distinctive doughnut shape, with the ring of coral surrounding a small, sandy-bottomed pool, occurred as a result of low water level and hot sun inhibiting the upward growth of the coral. In deeper water, as on the reef rim, the same coral forms huge rounded colony.

Towards the outer edge of the reef flat, where wave action is much more significant, surfaces are often encrusted with calcareous red algae, and elsewhere you will usually find a fine mat of filamentous algae that serves as grazing pasture for fish, sea-urchins, gastropod, molluscs and other animals. Some fish are permanent inhabitants of the reef-flat area, retreating to pools if necessary at low tide; but others, like parrotfish and surgeon fish, spend much of their time in deeper water, and then crowding over onto the reef flat with the rising tide.

The Seaward Reef Front

Most divers ignore the shoreward zones of the reef and head straight for sites on the reef front, on the basis that here they are most likely to see spectacular features and impressive display of marine life. Brightly lit, clean, plankton-rich water provides ideal growing conditions for corals, and the colonies they form help create habitat of considerable complexity. There is infinite variety, from shallow gardens of delicate branching corals to walls festooned with soft corals and sea fans. The top 20 meters (66 ft) or so of the seaward reef is especially full of life. Here small, brilliantly coloured damsel fish and anthies swarm around the coral, darting into open water to feed on plankton. Butterfly fish show their dazzling array of spots, stripe and intricate patterns as they probe into the crevices or pick at coral polyps - many have elongated snout especially adapted for this delicate task. By contrast, you can see parrot fish biting and scraping at the coral, over time leaving characteristic white scars.

Open-water species like fusiliers snappers and sharks cover quite large areas when feeding, and wrasse often forage far and wide over the reef. But many species are more localised and can be highly territorial, on occasion even being prepared to take on a trespassing diver. Fish-watching can give you endless pleasure but there is much else to see. Any bare spaces created on the reef are soon colonized, and in some places the surface is covered with large organisms that may be tens or even hundreds of years old. These sedentary reef-dwellers primarily rely on, aside from the omnipresent present waterborne food. Corals and their close relatives - anemones, sea fans and black corals - capture planktonic organisms using their tiny stinging cells. Sea squirts and sponges strain the plankton as seawater passes through special canals in their body-walls. Other organisms have rather different techniques: the Christmas-tree worm, for example, filtered out food with the aid of its beautiful feathery "crown" of tentacles.   

Apart from the fish and the sedentary organism, there is a huge array of other life forms on the reef. Tiny crabs live among the coral branches and larger ones wet themselves into appropriate nooks and crannies, often emerging to feed at night. Spiny lobsters hide in caverns coming out to hunt under cover of darkness. Gastropod molluscs are another type of marine creatures seldom seen darting the day, but they are in fact present in very large numbers, especially on the shallower part of the reef; many of them are small but on occasion you might come across one of the larger species, like the Giant Triton (Charonia tritonis). Some of the most easily spotted of the mobile invertebrates are the echinoderms. Most primitive of these are the feather stars, sporting long delicate arms in all colours from bright yellow to green, red and black. The best known of their relative, the sea urchin, is the black spiny variety that live in shallow reef areas and is a potential hazard to anyone waling on to the reef.

Many of the small, brightly coloured starfish that wander over the reef face feed on the surface film of detritus and micro-organisms. Others are carnivorous browsing on sponges and sea mats, and a few feed on living coral polyps. The damage they cause depends on their size, their appetite and, collectively, their population density. Potentially the most damaging of all is the large predator the Crown-of-Thorns Starfish; fortunately population of this creature has so far reached plaque proportion on relatively few of the Southeast Asian and Pacific reefs apart from the Australian Great Barrier Reef.   


Reefs in the Southeast Asian region are among the most biologically diverse in the world, they are also valuable to the local people as fishing grounds and as sources of other important natural products including shells. Unfortunately, in the past the few decades they have come under increasing pressure from human activities, and as a result they are, in places showing signs of wear and tear. Corals are slow-growing, if damaged or removed they may require years to recover or be replaced in the natural course of events, storm-driven waves from time to time create havoc, on coral reefs, especially in the typhoon belt. But some human activities are similarly destructive, especially blast fishing and the indiscriminate collection of corals to sell as marine curios.

Overfishing is a further deadly hazard to reef environments, and has already led to perilously declining population of target sources in some areas. Another way overfishing can cause great damage is through altering the balance of local eco-system; for example, decreasing the population of herbivorous fish can lead to an explosive increase in the algae on which those species feed, so the corals of the reef may be overgrown and suffer. Some areas are being damaged by pollution, especially where reefs occur close to large centres of human population. Corals and other reef creatures are sensitive to dirty, sediment-laden water, and are at risk of being smothered when silt settles on the bottom. Sewage, nutrients from agricultural fertilizers and other organic materials washed into the sea encourage the growth of algae, sometimes to the extent that corals become overgrown.

Growing awareness of environmental issues has given rise to "ecotourism". The main underline principle is often summarized as "take nothing but photographs, leave nothing but footprints", but even footprints - and indeed any form of touching - can be a problem in fragile environments, particularly among corals. A better way to think of eco-tourism is in terms of managing tourism and the tourists themselves in such a way as to make the industry ecologically sustainable. The necessary capital investments is minimal, and therefore much-needed employment becomes available for the local population. In the long term, the profits would exceed those from logging or overfishing.



These beautiful fish, with their minute, brushlike teeth, browse on sponges, algae and corals. Their vibrant colouring varies according to the species, like those of the butterfly fish and were once thought part of the same family. They are, however, distinguished by a short spike extending from the gill colour. They are territorial in habit and tend to occupy the same caves or ledges for a period of time.


With their elongated, streamlined silvery body and sinister-looking jaws, barracudas tend to appear rather fearsome and they should be approached with caution. Barracudas are effective reef predators who tend to school in large numbers when young, however by the time they mature to a length of two meters or over, they prefer to hunt singly or in pairs. 


As the name suggests, these small, nocturnal fish have extremely large eyes. They are effective predators which hide in protective holes in the coral by day and venture out at night to feed on other small fish, crabs, larvae and the larger planktonic animals.


These are often extremely hard to spot. Since they are usually well camouflaged and blend into the reef bottom where they live. The carnivorous blennies are ferocious hunters, whipping out so quickly from their hiding place to snatch small prey that the entire split-second action can go completely unnoticed.


Among the most of reef inhabitants, butterflyfish have flat, thin bodies, usually with a stripe through the eye and sometimes with a dark blotch near the tail; this serves as a camouflage and confused predators, who lunged for the wrong end of the fish. Butterflyfish can also, unusually, swim backwards to escape danger. Many species live as mated pairs and have territories while others school in large numbers.

Damselfish And Clownfish

These pugnacious little fish often farm their own patch of algae. Found almost everywhere on the reef, they also sometimes form large groups to feed on plankton. Clown fish which live among the stinging tentacles of the sea anemone, are also members of this family. Of the twenty-seven clownfish species known from the Indo-Pacific, 15 are found on the reefs of Southeast Asia.

Cardinal Fish

These tiny fish live in a wide range of depth down the reef. Their colours vary widely, but most have large eyes which help their night vision as they come out of hiding to feed on the plankton that rises up through the water as night falls. Cardinal fish also have large mouths and, in some species, the male incubates the eggs inside his mouth. During this period, the male juggles the egg mass from time to time and refrains from feeding. This process is known as mouth brooding and is a reproduction strategy that is used by certain species of marine fish.


Easily recognized by their chin whiskers, a pair of long barbels which they use to hunt for food, goatfish are often seen moving along sandflats, stirring up small clouds of sand as they feel beneath the surface for prey. They sometimes forage in small groups or large school. Goatfish are benthic, or "bottom dwellers", which is the name for fish that either feed or lie camouflage on the ocean floor.


The goby is another "bottom dweller" which can remain undetected on the seabed for a long period of time. They have large protruding eyes which are raised above the level of the head and powerful jaws which enable them to snatch prey and dart back to safety. Gobies are among the most successful reef families, with literary hundreds of species. Their colouring varies from brightly coloured to quite drab.


Groupers range from just a few centimetres long to the massive giant grouper which can grow to 12 feet long. They can  vary enormously in colour; grey with darker spot is the most common. Movement is slow except when attacking prey with remarkable speed. All groupers are carnivorous, feeding on invertebrates and other fish. Like wrasse and parrot fish, some start off as females and become males later.

Jack And Trevally

Jacks and trevallies are fast predators which range in size from small to very large. They can be silver, black, green or yellow and are usually found in open waters but are occasionally visitors to the reef since they follow the current as they feed. Cruising the outer slopes, they dash in with lightning speed to snatch unwary reef fish. They can be seen singly, schooling, or in small group.

Moray Eel

This ancient species of fish have gained their undeserved reputation for ferocity largely because, as they breathe, they open and close their mouths to reveal their numerous sharp teeth. They do not have fins or scales. Moray eels anchor the rare portion of their bodies in a selected coral crevice and stay hidden during the day. They emerge at night to feed on shrimp, octopuses, and mussel, and are immediately attracted by the smell of dead or injured fish.

Moorish Idol

This graceful and flamboyant fish reaches a maximum size of 20 centimetres. It is easily distinguished by its long dorsal fin, thick protruding lips and pointed snout. It probes for food in nooks and crannies. Moorish idols are usually seen individually, but may sometimes form large groups prior to spawning. Moorish idols are related to surgeon fish even though their body shape is quite different.


These are distinguished by their sharp, parrot-like beaks and bright colours and are among the most important herbivores on the reef. Many change colour and sex as they grow, with the males developing striking colouration by comparison with the drabness of the female. Many build transparent cocoons of mucus to sleep in at night, the mucus acting as a scent barrier against predators. The beak of the parrot fish enable them to crunch the surface of coral rock to feed on algal double and boring algae within.

Pipefish And Seahorse  

These are both poor swimmers and tend to lurk in seagrass beds or amongst coral away from current. Seahorses use their tail to wrap themselves around corals and seagrasses to stop them from being swept away. Their vulnerability has forced them to become masters of disguise, sometimes mimicking a blade of grass or a gorgonian coral.


These small to medium size, highly poisonous, omnivores feed on algae, worms and molluscs. They are found all the way down the reef to depths of around 30 meters. They are slow moving but when threatened they inflate themselves into big, round balls, by sucking water into the abdomen so that it become almost an impossible task for predators to try and swallow them.


Snappers are important carnivores on the reef, feeding mostly at night. Many are inshore-dwellers, although the yellow tail snapper is a mid water fish and the commercially exploited red snapper dwell at all depths. Snappers are becoming rarer on the reef because they are long-lived and slow-growing which means that once the population are drastically reduced they are unfortunately take a long time to replenish.

Soldierfish And Squirrelfish

Both species are nocturnal fish and are often confused with each other. Soldier fish have a rounder, bulkier body and are more evenly coloured than squirrel fish. The reddish colouration and large eyes are also common among other nocturnal fishes like bigeyes. Dozing under rocks or corals by day, they emerge by night to feed. They have serrated, spiny scales and sharp defensive fins.


Triggerfish are medium to large fish with flattened bodies and often striking markings. They have powerful teeth and feed on crustaceans and echinoderms on the midreef. Large species cruise corals looking for food. When a trigger fish is threatened, it squeezes itself into a crevice and erects its first dorsal spine, locking it into place with a smaller spine: this stays wedged until the "trigger" is released.

Wrasse And Hogfish

Wrasse vary enormously in size from the tiny cleaner wrasse to the giant napoleon wrasse which can easily reach 2 meters in length. Wrasse are usually brightly coloured and go through various colour and sex changes as they mature.

Their distinctive buck teeth are well adapted to pulling molluscs from rocks or picking off crustaceans. Most live in shallow reef areas, although some will frequent greater depth.

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