Background information; by edgardowelelo@yahoo.com, Master of the Game.
The warm waters of the TROPICS are relatively poor in nutrients and oxygen, making substantial stretches of the shallows somewhat barren. However, in certain areas, where conditions are favourable, SEAGRASS BEDS, MANGROVES and the most diverse marine communities of all – CORAL REEFS – can be found. The efficient recycling of nutrients in these habitats allows them to flourish.
CORAL REEFS
Built by many millions of tiny, colonial animals, coral reefs are cities of limestone that are simultaneously growing and being destroyed. Warm seas stretch around the Planet Earth in a wide belt either side of the equator: where land interrupts the open ocean and where the sea floor rises to the surface, a number of different habitats – from apparently lifeless sandy flats to flourishing CORAL REEFS – are found. Seen from the air, isolated coral reefs appear as turquoise jewels in an otherwise empty ocean. Seen underwater; when snorkelling or diving, the profusion of life and the riot of colour and activity are a cause for constant wonder: the eye barely has time to rest on one curiosity or delight before its attention is demanded by another. But CORAL REEFS are far more than visual masterpieces: without the intricate framework that they provide, marine plant and animal diversity in the TROPICS would be a fraction of what it is. REEFS are places of infinite interest, their richness, beauty and complexity rivalling that of any other habitat on Planet Earth.
BUILDING A REEF
Scattered throughout Planet Earth’s shallow, sunlit, tropical oceans, CORAL REEFS provide shelter and food for thousands of different fish and invertebrates. Essentially, REEFS are like cities, corals themselves being the bricks, and coralline algae the cement. The result is a rampart of consolidated life which can grow so large that it can become a geological feature in its own right. However, REEFS do differ in their origin and composition, and consequently in the life that they support.
How do REEFS form?
There are several different types of REEF, each of which is formed in a different way. How a REEF grows and its overall structure depends on the geology of the underlying SEABED, the water temperature and the impact of waves. It was in 1842 that CHARLES DARWIN devised a simple way to classify REEFS, which is still broadly used today. He (Charles Darwin) recognized three main types – FRINGING REEFS, BARRIER REEFS and ATOLLS and noted that one sometimes develops into another. FRINGING REEFS develop along the coast in the shallow waters lining a TROPICAL ISLAND or CONTINENT.
The CORALS grow upwards to sea level, or just below, and outwards towards the OPEN OCEAN. If a lagoon or channel develops between the FRINGING REEF and the land, and the REEF continues to grow outwards, the REEF becomes further and further offshore – often reaching the edge of the continental shelf – and is called a BARRIER REEF. BARRIER REEFS can also originate offshore if the depth of the SEABED further out is shallow enough to allow CORALS to grow. CORAL ATOLLS are rings of REEF, often encircling an island of sand which is colonized by plants, animals and sometimes by man. They typically have a shallow, sandy, sheltered lagoon in the middle, which has access to the open, deep blue sea beyond through a number of channels. CORAL ATOLLS are the tops of submarine mountains. When a volcano erupts and emerges from the sea, its edge becomes fringed, in time, by coral. This fringing reef may gradually turn into a BARRIER REEF as the CORAL grows out towards the sea. Over time, as the volcano subsides, the coral continues to grow up, striving to maintain its position in shallow, sun – lit water. When the VOLCANO itself eventually becomes invisible, what remains is a RING OF CORAL REEF surrounded by Deep Ocean.
The most well-known of all REEFS is probably the GREAT BARRIER REEF off the north eastern coast of Australia. It is actually a combination of many different types of REEF stretched out along 2300 km (1429 miles) of coastline. It includes 2100 REEFS, which make up the BARRIER, along with 540 continental islands which have extensive FRINGING REEFS. Freshwater run-off limits the extent to which CORALS can grow near the mainland itself, which is one of the principal reasons for CORAL GROWTH being comparatively luxuriant offshore.
EARLY SEAS
Before the continents (landmasses) moved to the position we know today, there was an expanse of tropical water called the TETHYS SEA. Bordered by northern Europe and Asia on one side and southern Europe and the Indian continent on the other; it gradually closed up due to CONTINENTAL DRIFT. The animals and plants evolving in the TETHYS SEA were squeezed eastwards into what is now the WESTERN PACIFIC. The shallow warm waters and multitudinous islands of that area made it a perfect crucible for marine life, and the greatest diversity of inshore and reef life is still found here. The Indo Pacific boasts 730 species of coral compared to the Atlantic’s 65. The northward drift of the AFRICAN CONTINENT also meant that the Atlantic Ocean became separated from the Indian Ocean, so Atlantic animals and plants had no opportunity to cross – fertilize with the Indo – Pacific stock. The rise of Isthmus of Panama about 2 million years ago ensured that no further contact was possible with the eastern Pacific, so the Atlantic became completely isolated. Its animals and plants began to evolve separately and today there are no species common to both regions.
DISTRIBUTION OF REEFS
Perhaps surprisingly, CORAL REEFS are not strictly limited to the TROPICS and they do not necessarily thrive here. Certain physical factors limit their distribution. CORALS need warm water, ideally between 180 and 300C (640 and 860F). When a cold current runs through an area, as in the Galapagos Islands, the development of REEFS is poor to non – existent. Equally, if a warm current persists to the north or south of the TROPICS, as around Bermuda, which is bathed in the Gulf Stream, reefs can develop. CORALS obviously need a good solid substrate to grow and build a REEF upon. They need access to light (for the plants in their tissues), so the water needs to be shallow and clear. Too much sediment in the water both smothers the CORAL POLYPS and cuts down the amount of light that penetrates. Also, too much fresh water kills them. These demands mean that CORAL REEFS do not grow near the MOUTHS OF RIVERS, even where the water temperature and the substrate is suitable. For example, in Brazil, the freshwater and sediment carried down from the forest by the Amazon and other large rivers such as the SAO FRANCISCO prevent substantial CORAL GROWTH offshore. As a result, Brazil’s coast, much of which is tropical, has few well developed REEFS. Each CORAL has its own food requirements, its own way of growing, its own way of coping with storms, diseases and predators. Each competes differently for light and space. This gamut of physical and biological factors controls both where REEFS grow and the distribution of different types of CORALS over REEF.
What is coral?
CORALS belong to a group of animals called CNIDARIANS. This group encompasses hard and soft corals, sea fans, gorgonians, hydroids, jellyfish and sea anemones. Although the group is remarkably diverse, there are a few features shared by all. They have a free – swimming larval stage and a simple body plan; a central mouth, through which material passes in and out of the body, and a ring of tentacles. The other distinguishing feature of the CNIDARIANS is the presence of nematocysts (stinging cells) which are used to catch prey.
COLONIAL LIFE
A CORAL POLYP is a single animal. In isolation it looks very similar to a SEA ANEMONE, but unlike SEA ANEMONES, many CORALS have made a huge evolutionary leap – they have evolved to form colonies. A few species remain solitary, but in most cases, new POLYPS bud off the initial founding POLYP and gradually colonies of thousands or even millions of POLYPS will grow, each connected to its neighbors by living tissue. Freed from the limitations of living alone, colonies can grow to immense sizes and live a very long time. CORALS come in all shapes and sizes, but the basic plan is the same: Polyps live as a surface layer on some sort of structure, be it hard and inflexible or rubbery. The hard or true corals, for which reefs are most famous, build limestone skeletons beneath the living tissue. SOFT CORALS, as their name implies, do not form solid skeletons; instead they secrete limestone crystal structures called sclerites, which are embedded within a jelly – like matrix beneath the POLYPS. When they die, little is left of most SOFT CORALS as the limestone element of their make-up is so small and easily breaks up. By contrast, some sea whips and fans and, notably, the BLACK CORALS have such dense skeletons of limestone, protein and minerals that they are highly durable and are collected and polished for jewellery. The skeleton of BLACK CORALS is more dense the slower it grows, and it grows more slowly at depth. So it is the very deep, older colonies that are most highly prized.
CORAL BLEACHING
Much of the colour we see in CORALS comes from the pigments of microscopic algae living within CORAL TISSUE. Sometimes all the algal cells are expelled from a CORAL, which then becomes totally white, this is called CORAL BLEACHING. In large – scale bleaching events, an entire reef can become ghostly white, as if made of ice. Environmental conditions, such as disease, too much shade and a change in salinity, can induce bleaching, but it seems that the most prevalent cause is a sustained increase in water temperature. If CORAL regains some algae, it will probably survive in the long term. However, bleaching can be irreversible, and then the CORAL will die. When this happens on a large scale a significant change in the REEF COMMUNITY follows. Algal turf grows over the DEAD CORAL, allowing herbivorous fish and invertebrates, particularly SEA URCHINS, to flourish. SEA URCHINS graze algae, but in doing so they scrape away the surface layer of rock. By grazing the surface so effectively, they interfere with the recruitment of new CORAL colonies and perhaps delay the recovery of the REEF as a whole.
BUILDING WITH LIMESTONE
CORAL REEFS would not exist if it were not for the ability of CORAL POLYPS to secrete limestone or calcium carbonate. SEA WATER surrounding a CORAL is very rich in dissolved calcium carbonate, but the fluid inside the POLYP cannot retain a large quantity of calcium carbonate, so it is laid down as microscopic needle – shaped crystals beneath and around the POLYP. This process occurs in two stages. As the POLYP expands to feed with its TENTACLES at night, it lifts off the skeleton, rather like a glove coming part – way off a hand. At this stage, the calcium carbonate crystals form ridges. During the following day (when the CORAL POLYP is retracted and lying on its new structure), the valleys between the ridges fill in with more calcium carbonate and skeleton takes on a smoother appearance. Because the skeleton of hard corals is made of limestone or calcium carbonate, it is pure WHITE. Each CORAL SPECIES lays down its skeleton in a different way. This gives rise to the extraordinary range of shapes and forms that HARD CORALS take. Some form large BOULDERS, where the POLYPS live in small, isolated depressions or grooves in the skeleton. Some grow in branches, which can be small and stubby, while others are spreading tree – like structures. Still others grow delicate, leaf – like plates or flat tables. The range is huge, and to complicate the matter further, the same species will grow in a different way depending on the physical characteristics of the place in which it finds itself. Although a coral’s genetic blueprint is fundamental in determining what it looks like, its appearance is also affected by waves, currents, light and competition for space on the REEF. Such variations in CORAL FORM have complicated the matter of identification over the years.
A COSY RELATIONSHIP
None of the hard reef – building corals could lay down their stony skeletons at the rate that they do without help. They simply could not acquire enough food to grow that fast. Help comes from millions of single – celled algae, called ZOOXANTHELLAE, which live within CORAL TISSUES. It is the relationship between these microscopic cells and their hosts that is the key to REEF FORMATION. ZOOXANTHELLAE, like all plants, photosynthesize to produce simple sugars and oxygen, so the CORAL benefits by having its own in house supply of food and oxygen. The relationship is symbiotic because the algal cells benefit from the coral’s carbon dioxide and nutrients essential for growth. They also have a safe, stable home in which to live. The importance of ZOOXANTHELLAE in the building of REEFS cannot be overstated. CORALS grow up to three times faster because of them, and thus out – compete the slow deterioration of the REEF from erosion. However, not all CORALS house these cells. Most of those living below 40 m (130 feet) do not have ZOOXANTHELLAE due to the lack of light for photosynthesis. Consequently, such CORALS grow slowly and are not major contributors to the REEF STRUCTURE.
Being only about 0.01mm (0.0004in) across, there may be as many as two million single – celled algae living in each square centimeter of coral tissue, making them the most extensive plant species on CORAL REEFS.
SHAPING REEFS
CORAL REEFS the world over conform to the same basic profile in cross – section. Wherever the REEF is, be it the CARIBBEAN or INDO – PACIFIC, and, therefore, whatever species are responsible for building it, the REEF by and large takes on a similar shape. This is because the factors affecting CORAL GROWTH, namely wave action, currents, sunlight and heat, are the same.
ZONATION
The shallowest part of a REEF is called the REEF FLAT. CORALS here cannot grow above the water level as they would dry out ad die, so they grow horizontally, extending the REEF FLAT out towards the SEA. Even in shallow water, the heat can be too much for most species of coral, so the variety found on the REEF FLAT is usually quite limited. On the very outer edge of the FLAT there is often a narrow crest running along the REEF. This is formed partly by BOULDERS or CORALS that have been thrown up by wave action and then cemented to the REEF by a calcareous red alga. The growth of this alga, which does particularly well in this exposed position, adds considerably to the height and breadth of the crest. On the landward side of the REEF FLAT, there is a lagoon or area of shallow water which typically has a sandy bottom. Depending on the depth of the lagoon, some CORALS may be able to live here and form PATCH REEFS (literally, a small patch of reef) or the even smaller “bommies”. The sand may also be colonized by SEAGRASSES, which develop a habitat in their own right. On the seaward side of the REEF is the REEF SLOPE. This is where CORALS REEFS are at their most diverse. The majority of CORALS prefer the slightly less turbulent waters of the REEF SLOPE, which receives less battering from waves than the crest. CORALS near the top of the slope grow faster as they receive more SUNLIGHT than those at depth. This could lead to the slope becoming steeper and steeper, even overhanging, as the shallower CORALS grow further and further outwards, but corals and chunks of REEF break off and tumble down, ensuring that much of the material formed in the shallows lands up in the deeper areas.
CREATING SAND
REEFS are dynamic structures that change their shape continually, but the change is usually so gradual as to be unnoticeable. None the less, day by day a REEF is both growing and eroding in a variety of ways. The two key factors responsible for the erosion of REEFS are waves and boring (tunneling) animals. Both can cause even the largest CORAL to break up, turn into rubble and eventually become sand. Clearly, the constant pounding of waves will erode the structure, and a severe storm can wreak serious damage to an entire REEF over a very short space of time. More subtle, but no less damaging, is the impact of animals, particularly SPONGES and BIVALVE MOLLUSCS that come to seek protection. Sponges, which have no means of mechanically drilling into the CORAL, rely on acids to dissolve away the limestone skeletons. BIVALVES, on the other hand, rely mainly on their scraping powers. CORALS are also eaten by PARROTFISH, particularly the large humphead parrotfish. Whole schools can inflict serious damage on CORALS as they move across a reef. They bite off sizeable chunks of CORAL and swallow the skeleton as well as the living tissue. The limestone they consume grinds up both the algae and the organic tissue they have eaten, making it easier to digest. It is then excreted as a curtain of sand as they swim along the REEF. Sand is also produced from other sources. Halimeda, a green alga that grows in chains of flat discs, has a large limestone content. When the discs break off or die, they fall to the seabed and in time are eroded into sand. Mollusc shells take longer to break up, once the owner has died, and add larger pieces to the coral sand accumulating on the REEF.
REEF PLANTS
Algae or seaweeds are non-flowering plants, which are extremely abundant on CORAL REEFS, such as the GREAT BARRIER REEF (In Australia), and are an enormously important part of the ecosystem. The four (4) categories of algae are divided by colour: brown, green, red and the primitive blue-green algae. The colours come from pigments inside the cells, which are responsible for trapping different wavelengths of light for photosynthesis. Algae take on a wide range of forms, from stony encrusting growths to branching plants and solitary bubble-like structures.
Brown algae are present on CORAL REEFS, but they do better in colder water, where they tend to dominate. On CORAL REEFS the growth rate of green algae is particularly fast- an estimated 1-5 kg per square metre per year – but there are so many herbivorous fish and invertebrates living on the REEF that these plants are cropped back to mere stubble. Red algae are most obvious on the REEF CREST – sometimes called the ALGAL RIDGE – where they thrive in the area of greatest wave action. Here they consist of 95 per cent limestone rock and only 5 per cent living tissue. Unlike CORALS, they can withstand the force of big waves because they grow in low – lying encrusting sheets.
THE FATE OF SAND
If the waves and currents of a REEF are favourable, sand gradually piles up and a CORAL CAY (bank) develops. It may only be temporary and get washed away again, or it may continue growing until the area is well established and ripe for colonization. Initially birds will use it as a resting place, their guano adding nutrients to the sand, so in time plants may colonize it. This encourages more animal life, and gradually the cay will develop into a flourishing island. Alternatively, as sand settles and becomes compacted in nooks and crannies on the REEF, red encrusting algae grow over it, sealing it in. A combination of physical and chemical processes then takes place, which binds the sand particles together to form limestone again. In this way, little by little, perhaps by only 1mm (0.04in) per year, the REEF grows. While this scale of growth may seem tiny compared to that of CORALS, the limestone is very dense and hard and does not suffer the same degree of erosion that CORAL heads experience, making it important in the long – term construction and durability of a REEF. Some sand grains become finer and finer until they dissolve into the sea water again. This calcium carbonate, now in solution, is recycled and deposited by the living CORALS as delicate limestone skeletons once more. Although the recycling of sand on a REEF is complex, it is an indication of how the reef is constantly developing and changing.
MOUTHS OF THE REEF
Diversity is a defining characteristic of the REEF and with it comes a variety of feeding techniques and defence strategies unrivalled in the marine world. Diversity is the hallmark of life on the REEF, especially when it comes to feeding habits. From animals that unassumingly eat debris off the sea floor; to immobile sponges, to lightning – quick barracuda and sharks, the variety of feeding strategies used is astonishing. It is the revolutionary arms race between predator and prey and the high level of competition of food and space on CORAL REEFS that has led to such specialization. The predators may be equipped with snaring traps, sharp teeth, sucking mouths or harpoons, and they may seek their prey by using sight, smell or electrical senses. They may then use sheer speed or stealth to catch them. Prey, in their turn, have defences, such as camouflage, mimicry, poison, armor and spines. The more vulnerable may simply hide in cracks and crevices on the REEF and eke out a living there, but those that venture out must have additional means of escaping the many mouths ready to make a meal of them.
PLANKTON EATERS
One of the best ways in which to understand the complex organization of communities on a CORAL REEF is to look at the feeding structure. Only if you take into account all the primary production of ALGAE on the REEF (including the ZOOXANTHELLAE), the work of billions of bacteria, which break down organic matter, the filter feeders, the detritus feeders, all the animals that come out at night, as well as those seen during the day, does a reef’s blood web begin to take shape. What emerges is a tightly organized ecosystem where the recycling of nutrients is highly efficient. It has to be because the warm tropical waters that bathe CORAL REEFS are very poor in nutrients. Only by efficiently using and recycling what is already present on the REEF and what little comes to it in the way of drifting PLANKTON can the reef ecosystem maintain itself.
Exposed on the REEF SLOPE
If you swim along a REEF SLOPE when the current is running, the water is filled with little fish lined up, facing the current and busy feeding. Indeed the majority of fish we see on CORAL REEFS are specialized zooplankton feeders. Even those that do not feed on ZOOPLANKTON as adults usually eat PLANKTON as larvae and young juveniles. Plankton – eating reef fish rely largely on sight to pick out their food. Their snouts are short, which allows them to focus both eyes on small targets directly ahead. Many species have extendible mouths that are designed to catch prey that is floating by. This is an energy – efficient feeding strategy for fish because they do not have to move to swim towards their prey, instead a quick gulp does the trick. In some species the protrusible mouth creates significant suction which draws the prey into the mouth. Once caught, the PLANKTON cannot escape because the gills have rakers, which are especially long and closely spaced. To avoid being eaten, daytime plankton living above the REEF tend to be transparent – except for pigmented parts, such as the gut and very small. However, plankton carried on to the REEF from the open ocean tend to be more visible and fall victim to the numerous fish which station themselves above the reef slope. These thousands of little fish are vulnerable to predation themselves, from fish such as jacks, so they too have adaptations which reduce their chances of being eaten. Their best form of defence is to close ranks and dash for the REEF, which they do at the slightest provocation. They are also stream – lined and have deeply forked tails for fast swimming. These physical adaptations are more pronounced the further out from the REEF the fish go. For instance, damselfishes, which roam only a metre or so from the cover of CORAL, are less streamlined and have stubbier tails than anthias, which venture further out. Two closely related species that feed in different ways can have very different shapes. Thus herbivorous surgeonfish, which stay on the REEF to graze algae, are notably rounder in shape than Thompson’s surgeon – fish, which is a PLANKTIVORE. There are exceptions to the rule, of course. The pyramid butterflyfish is a PLANKTON FEEDER, and although it feeds some distance from the REEF, it has neither a streamlined body nor a forked tail. Its defence lies in discouraging attack, which its deep body and long spines effectively achieve. The main disadvantage of this body shape for fish is that they might have some difficulty swimming in stronger currents.
CLEANING STATIONS
Like other animals, fishes need to be cleaned, but as they do not have the wherewithal to clean themselves, they get others to do it for them. The appropriately named cleaner wrasse has a distinctive way of swimming, lives in one place, “a cleaning station” and advertises its services by almost dancing in the water; its clients, ranging from small fishes to large groupers, mantra rays and even sharks, adopt a passive posture to assure the cleaner wrasse that it is safe. This little fish will then eat parasites, dead scales, food scraps and fungus from the skin, mouth and gills of its clients and both parties benefit. Numerous other creatures on the REEF also act as cleaners. Herbivorous blennies and tangs will clean a turtle’s back of fouling algae, butterflyfishes will clean ocean – going sharks that visit the REEF, and various shrimps are cleaners too, working on the teeth of moray eels, among other creatures.
VEGETARIANS
Although there are many different plants associated with the REEF, algal turf, which predominates on the REEF FLAT, is by far the most productive. Indeed, it is one of the most productive plant communities in the world. Little wonder, then, that an army of fish and invertebrates feed on this bounty.
Who eats plants?
HERBIVORES come in all shapes and sizes. On Caribbean reefs, invertebrate herbivores, such as the SEA URCHIN DIADEMA, are important, but on Indo – Pacific reefs only fish have a significant role. The dominant plant – eating fishes on all reefs include SURGEONFISH, DAMSELFISH, PARROTFISH and RABBITFISH. There are also smaller fish, such as BLENNIES and GOBIES, some of which are herbivores, but they play a minor role in harvesting plants. Surprisingly, given the poor nutritional content of reef algae in general, most of these fishes have large populations and fast growth rates. They do well, it seems, on diets that are low in protein. However, they do not eat small crustaceans, detritus and bacteria as they graze within the turf. Also, many herbivorous fishes eat planktivore faeces, which are extremely rich in nutrients such as nitrogen. Herbivorous reef fish have their own set of characteristics, including a small mouth which scrapes the substrate quickly and often, and a body designed more for fine control while grazing than for fast swimming. As they eat they probably take in quite a lot of calcareous material, which is of little nutritional benefit to them, so they must eat a lot to ensure they get enough organic material.
Territory defence
Virtually all areas of the REEF are fiercely defended by their owners. Boundaries are patrolled, and sometimes the mere presence of a territory owner will deter an intruder. Some herbivores tolerate different species residing within their territory either because they pose little threat to the amount of algae available, or their presence assists in territorial defence. Some herbivores, such as convict tangs, do not hold territories, but roam over the REEF to graze. The only way they can get access to the closely guarded algal turf is to invade a territory in mass numbers. The resident fish, perhaps a pair of powder blue surgeonfish, have little chance against the invading hordes. After a minute or so of frenetic grazing, the swarm of convict tangs moves on, leaving the surgeonfish alone on their territory with a vastly diminished crop of algae.
MEAT EATERS
With so many planktivores and herbivores living on the REEF, there are inevitably many carnivores, including numerous invertebrate predators such as CRABS, ANEMONES, CONE SHELLS, MANTIS SCHRIMPS, and even brittlestars. BRITTLESTARS stand up on their arms and entice small fish into the shelter underneath their central disc. Once a fish has entered, the brittlestar twists its arms and imprisons the fish. The REEF is also known for its predatory fish, which range from the tiny fish, such as gobies to the largest fish, such as SHARKS.
Strategies
Predators, particularly PISCIVORES (fish-eaters), use a number of different tactics to catch their mobile and vigilant prey. The open – water species, such as JACKS, simply pursue their prey, sometimes hunting in packs if the prey is a schooling species. By simultaneously attacking a mass of small fish, JACKS have a better chance of separating individuals from the school. When attacked, small fish, such as SILVERSIDES, try to stay together to gain some safety in numbers. Also, the visual effect of millions of tiny silver fish acting as one, flashing and shattering like breaking glass, prevents the JACKS from being able to home in on one victim. Lizardfish use a very different hunting strategy; they are ambush predators. Their camouflage is excellent, so they simply lie on the seabed or the REEF and keep very still. When an unwary grunt swims by on its way to feed in SEAGRASS BEDS after dusk, the lizard – fish launches upwards and grabs it. GROUPERS are among the largest fish on the REEF. They appear docile and hardly move at all, but that is part of their strategy. They place themselves in caves, under overhangs and in holes on the REEF, remaining still for so long that their prey becomes used to their presence and forgets the danger. Once the victim is within the strike zone, the lunge comes with extraordinary speed. The trumpet fish has yet another strategy, using non – predatory fish for cover. It swims closely aligned with its innocent partner so that the small fish it hunts cannot see it. When the pair pass close enough to a potential victim, the trumpet fish streaks forward to suck in its prey. Jacks sometimes use the same technique, hiding behind humphead parrotfish.
Defences
The presence of predators has led prey species to develop an extraordinary variety of avoidance mechanisms. Hard exteriors, such as the shells of crustaceans, are commonplace in the invertebrate world, but fishes also use armour of sorts: boxfishes have tough skin and bony plates, scorpionfishes have fin spines they can erect when threatened, and surgeonfishes have scalpel – like spines near their tails. The highly territorial blue – lined surgeonfish goes one step further and has venom within its spines. Other fishes are shaped so that they are difficult to attack: angelfishes, for example, have deep bodies that are difficult for predators to get their mouths around, and pufferfishes swell up with water at the slightest provocation. The fantastic array of colours and design seen on a REEF is due, in part, to the constant threat of predation. Some colours, such as those seen on NUDIBRANCHS (SEA SLUGS), the blue – ringed octopus and certain pufferfishes, indicate that the carrier is poisonous. But colour is more often used as a means of disguise. The harlequin ghost pipefish, for example, is almost indistinguishable from its chosen host, a featherstar. The skin extensions of the pipefish and its coloration match the feeding arms of the featherstar perfectly. Some prey fish have evolved to resemble predators. For example, the comet hides in a hole head first when threatened, leaving its tails exposed. What remains visible of this small fish – a rounded spotty tail with a large eyespot – resembles a moray eel peering out of a rock. Eyespots are commonly used to trick predators into misdirecting an attack. Many butterflyfishes have eyespots near their tails so that a predator will go for the less vulnerable part of the fish, the tail, rather than its head. Also, stripes often cover the eyes of butterflyfishes and others so that the precise outline of the body is less discernible. Defensive behaviour is equally varied. Cowries and hermit crabs withdraw into their shells, while other animals escape into holes on the seabed or on the REEF. Some fish school together for protection, whereas others make a quick dash for cover. Despite such strategies predators still win sometimes, not least because they are on the reef in abundance.
CORAL AS FOOD
CORALS cannot move, of course, so most CORAL POLYPS spend the daylight hours withdrawn into their protective limestone skeletons where, for the most part, they are safe. As a second line of defence, they have nematocysts (stinging cells) like barbed harpoons, which contain poison. Primarily, nematocysts are used for catching ZOOPLANKTON, but they are almost certainly discharged when a fish touches the CORAL. The venom in stringing cells varies in strength, and some species, such as FIRE CORAL and many of the HYDROIDS (small, soft tree – like Cnidarians), can leave a painful weal on your hand. Despite these deterrents, certain fish remain specialized CORAL – EATERS. Butterflyfishes have delicate snouts, which are perfectly adapted to pick at individual POLYPS, and some species, such as the CHEVRON BUTTERFLY FISH, depend entirely on CORAL for their food. They also fiercely defend their territory, usually a single table coral, from intruders. PARROT FISH are in advertent coral eaters; they use their beak – like mouths to scrape algae off the REEF and ingest tiny coral colonies while doing so. The HUMPHEAD PARROTFISH actually bites large chunks off coral heads, but could well be targeting the ZOOXANTHELLAE CELLS within the CORAL rather than the CORAL TISSUE itself, as it is primarily a herbivore. There is a delicate balance between the herbivores, the CORAL and the ALGAE. If grazing is too heavy, the newly settled coral larvae, developing POLYPS and early coral colonies get scraped off before they have become properly established, so no new coral heads will develop. Conversely, if there is too little grazing, the new recruits can become smothered in the faster – growing algal turf and cannot survive.
STAR FISH PLAGUES
Attractive but deadly, the Crown – of – thorns starfish is notorious for its devastating effect on CORAL REEFS. When these creatures occur in sufficiently large numbers as they did on the GREAT BARRIER REEF in the late 1960’s and again in the early 1980s, it was feared that the entire REEF would eventually disappear. However, although crown – of – thorns starfish do feed on and kill CORAL, the plagues always seem to disappear; leaving the REEF to recover.
Killing Coral
Crown – of – thorns starfish usually measure 25-35cm (10-14in) across, but individuals of up to 80cm (32in) have been found. As adults they prefer to feed on branching CORALS, such as STAGHORN, and PLATE CORALS of the genus Acropora, but if corals become scarce they will also eat SOFT CORALS and ALGAE. Crown – of – thorns pushes its membranous stomach through its central mouth and smothers the CORAL. Enzymes are secreted which break down the living coral tissue and this is then transported by tiny hairs, called CILIA, to the digestive organs. The whole process can take 4-6 hours, and the starfish leaves behind nothing but the white skeleton of the coral. If the coral is large, a starfish might return night after night until the entire colony is dead. In severe crown – of – thorns outbreaks, 90 per cent of the CORALS can be killed.
Natural or Man – Made?
Although one starfish eats only about 5 sq m (54 sq feet) of CORAL per year; the impact of hundreds of STARFISH is considerable, but an aggregation of starfish is not considered an “outbreak” unless there are thousands of them. The worst outbreaks recorded have involved several million individuals. Scientists do not yet agree on whether or not outbreaks are natural. The most likely explanation is that they do occur naturally, but that human activities exacerbate the problem. One possible cause of outbreaks is that starfish predators have been reduced because of the activities of shell – collectors and spear – fishermen. The giant triton in its highly prized shell, is an effective crown – of – thorns predator. This mollusc holds the starfish down with its muscular foot while it cuts through the flesh with its radula (a saw – like cutting organ). It then inserts its long proboscis into the starfish and eats the soft tissues. However, triton numbers have never been very high and it is unlikely ever to have exerted much control over crown – of – thorns populations. Blue – finned and yellow margin triggerfish also prey on crown – of – thorns. These fishes have hard, plate – like scales ad strong, sharp teeth, so they can handle the venomous starfish with impunity. They pick up the starfish, flip it over and attack it from beneath, where its spines are shorter and less sharp.
Baby boom
Another outbreak theory involves the survival of the starfish larvae. Female crown – of – thorns attract males chemically, then both simultaneously release millions of eggs and sperm into the sea. One female alone can produce about 60 million eggs in a breeding season. Fertilized eggs spend 10-28 days in the PLANKTON as larvae. They feed on PHYTOPLANKTON, which are very abundant when there are lots of nutrients in the water. Typically, water in the open ocean and on CORAL REEFS is poor in nutrients, so PHYTOPLANKTON numbers are limited. .But when the nutrient content increases, as a result of high agricultural run – off or sewage and other pollutants entering the system, PHYTOPLANKTON will bloom and many more crown – of –thorns larvae will survive. Plankton eaters will consume some of the larvae, but they are present in such huge numbers that many will still manage to settle on the REEF. Physical forces, such as favorable currents and winds, will play their part too, and when combined with unlimited food and low predation, the results can be dramatic. Because so many millions of eggs are fertilized, even the slightest percentage increase in larval survival will result in a huge increase in the adult starfish population.
NIGHT FEEDERS
At night everything changes and the adaptations and rules that govern the daytime feeders do not necessarily apply. As the sun goes down, smaller fish seek shelter on the REEF first, and gradually, in order of size it seems, all the daytime fish retreat to the shelter of the REEF. For about 15-20 minutes there is little visible activity on the reef; the diurnal fish are all hidden and the nocturnal fish have not yet emerged. This is referred to as the “quiet time”. In fact, certain nocturnal species, such as juvenile cardinal fish, emerge before the quiet time, but as these fish are virtually transparent and less than 3 cm (1.2 in) long, they go unnoticed by the visual hunters patrolling the REEF.
CATCHING PLANKTON IN THE DARK
After dark, the nocturnal fish swarm out from their daytime resting places – caves and overhangs – where they sought refuge in large groups, and disperse over the REEF. Many of them, such as soldierfishes and bigeyes, are planktivores who have evolved exceptionally large eyes in order to find their prey at night. Small ZOOPLANKTON tend to be overlooked, probably because they cannot be seen. Larger ZOOPLANKTON, mostly minute crustaceans, who cannot afford to be out on the REEF during the daytime, risk emerging at night when the armies of daytime planktivorous fish have retired. However, it is not just fish that eat PLANKTON at night: all manner of invertebrates emerge to feed under cover of darkness. Brittle stars crawl out from SPONGES, featherstars make their way to elevated vantage points and basket stars spread their intricate branching arms up into the current to catch PLANKTON. SOFT CORALS, HARD CARALS and SEA FANS extend their POLYPS, and sieve the water indiscriminately for PLANKTON of all sizes. At night then, as the ZOOPLANKTON rise from the REEF to feed, they run the gauntlet of millions of CORAL POLYPS and all the other invertebrate traps that are waiting to catch them. Even though the water is full of danger, it seems that the risks of getting caught at night are less than during the day when there are so many fish about.
NOCTURNAL HUNTERS
Some daytime fish, such as SURGEONFISHES, FUSILIERS and MOORISH IDOLS; adopt a darker coloration at night, to reduce their chances of being spotted by a nocturnal predator. Their dark hues blend in well with the gloom and help to break up their outline. This adaptation cannot be essential, as many do not appear to change colour at all. Some fish spend the night drifting under a CORAL, while others tuck themselves right away into holes. Certain PARROT FISHES have an unusual form of protection: they hide in a hole and secrete a cocoon of mucus around themselves each night. This is thought to disguise them chemically from predators such as MORAY EELS, which hunt by smell. But such a cocoon is no defence against vampire snails. These have an extendible mouthpart which acts like a proboscis. When they encounter a sleeping fish, they extend the proboscis into the soft tissue of its mouth and draw blood. Blood is pumped along the tube of the proboscis into the snail. At night, when there is little light to see by, whitetip reef sharks hunt by homing in on tiny electrical impulses emitted by moving fish. But even inactive fish are not safe because sharks have an acute sense of smell and can detect a fish hidden under a CORAL. Consequently, many fish tuck themselves away when a shark’s ferociously rummaging head cannot reach them.
SEX ON THE REEF
Reproduction in a competitive world is complicated. Most REEF ANIMALS either change sex or are both male and female at the same time. Reproduction on CORAL REEFS, particularly among fish populations, is a complicated business. They do not limit themselves to one male mating with one female, who nurses the developing young. Instead a range of tactics is used, each of which, for the species concerned, enhances the individual’s chances of success. While some, such as butterflyfishes, do form male – female pairs and remain together until either one falls prey to a predator; it is much more common for one male to have a HAREM of females. To complicate reproductive matters further; many reef fishes change sex, and some are even HERMAPHRODITE; both male and female at the same time. In the invertebrate world where many species, including CORALS, are immobile, being HERMAPHRODITE is an obvious solution to the problem of not being able to search for mates.
SEX CHANGE
Within the animal kingdom there are some species that can either change sex or act as both male and female at the same time. A significant percentage of the animals that can do this are FISHES, and many of them live on CORAL REEFS.
Transsexuals
One of the most problematic aspects of identifying fish is that many of them change sex and colour during their lives. Some PARROTFISH species, for example, have brightly coloured males and drab females, but to confuse the matter, the same species also have drab juvenile males which look very much like the females. HERMAPHRODITISM, where a fish has both female and male reproductive tissues, is common among REEF FISHES. Usually, however, they can reproduce only as one sex at a time. Some fish are PROTOGYNOUS, meaning that they are born as females and develop into males later. Others are PROTANDROUS – born males, later changing to females. An exception to this is a group of small fish in the CARIBBEAN whose members are both male and female at the same time (SIMULTANEOUS HERMAPHRODITES). Among the wrasses, PARROTFISHES, GROUPERS, ANGELFISHES and some DAMSELFISH and GOBIES it is usual for the females to change sex. Anthias or fairly basslets, relatives of GROUPERS, are also born as females and later turn into males. ANTHIAS are common throughout the INDO -PACIFIC, but are nowhere more apparent than in the RED SEA. Indeed, the CORAL REEFS of the RED SEA are renowned for their clouds of orange – coloured scalefin anthias which hover just above the REEF, feeding on PLANKTON. The purple males have harems of orange females which remain close to them. These fish swarm above the REEF and it is difficult to tell which females belong to which male, except when the fish seek shelter. Then the males and their harems are more clearly defined. A male anthias stops the females in his HAREM changing into males by being aggressive to them. This ensures that he is the only male in his group and therefore that his sperm fertilizes any eggs produced. He directs most of his aggression to the largest females, which are next in line for becoming males. It seems that his aggressive behaviour has an impact on the females’ hormones and they remain female. However, should he die or be eaten, the largest female in his HAREM will develop into a male, acquiring longer fins and new colours within a few days. The black hamlet, a small Caribbean fish, is both male and female at the same time. At spawning time, pairs take it in turns to release their eggs and sperm, swapping roles after each bout.
Born Male;
Fish that start life male and later become female include SCORPIONFISH, BREAM, SNAPPER, BASS and some ANEMONE fishes. These last – named fish leave the PLANKTON as tiny males and seek out an ANEMONE. (They are protected from the anemone’s sting by a coating of mucus). If accepted by the resident fish, they join the perking order at the bottom. The largest fish in an anemone is female, and she prevents the males changing sex by harassing them. Only if she is removed from the anemone will the largest male change sex and assume the dominant role. Among fishes where the females turn into males later in life, there can also be individuals that are born male and remain so (known as PRIMARY MALES). For example, the PRIMARY MALES amongst BLUE – BARRED PARROTFISH start life being drab, but can later turn out to be the most gaudily coloured fish of all, even more so than the SECONDARY MALES (males that developed from females).The primary males among green wrasse retain their dull colours and look like females. This gives them an advantage at SPAWNING TIME, when they otherwise could not get into the secondary males’ spawning territories without being detected. When the females release their eggs into the water, these clandestine primary males swim up and release their SPERM at the same time as the territorial male in the hopes that some of their sperm will be successful.
Why change sex?
All FISH want to maximize the number of surviving offspring they have, so here lies the main benefit in changing sex – the ability to reproduce throughout life. Take the PROTOGYNOUS FISH which start life as females. They can produce many thousands of small eggs as they are growing up and have them fertilized by a successful male who has proved his worth. When they themselves have grown up and proved that they can survive, they rank high in the pecking order. At this stage they would do better to fertilize the reproductive cells of as many other fish as possible, and to achieve that it is best to be male. PROTOGYNY, therefore, allows fish to maximize their reproductive output throughout their lives, rather than being born male, waiting to become DOMINANT, and reproducing only later in life. If there are not very many fish of one kind on a REEF (as is often true of CORAL REEF FISH) and predation is high, then being HERMAPHRODITIC makes sense. If individuals of one or other sex are taken by a predator, the chances of reproducing successfully are not diminished at all; if a male is taken, the largest female can turn into a male and all the other females can carry on reproducing. If a female is taken, the male still has other females to mate with, and the other females still have the male. But, if too many of a single sex were to be taken by predators and there was no HERMAPHRODITISM, the remaining fish of the opposite sex would be stuck for partners.
SPAWNING;
All CORAL REEF FISH try to ensure that their genes continue into the next generation, and they have evolved to do this in a number of ways. Some simply release their eggs and sperm into the open water and hope that at least a few will be fertilized and survive. Others put a little more effort into it and build a nest of sorts. The truly dedicated, however, carry the developing eggs around with them.
SEX IN THE OPEN
If you dive on a REEF in the evening, you can sometimes see thousands of SURGEON FISH streaming across it. Follow this River of fish and you will probably end up at a prominent CORAL OUTCROP on the REEF EDGE. Without warning, a small group of the fish will rise above the REEF in perfect synchrony and release a cloud of eggs and sperm into the water. More and more groups will follow suit, returning to the safety of the REEF between each bout. This method of reproduction is called BROADCAST SPAWNING. Fish that broadcast their SPAWN produce a great number of small eggs, because there is a high chance that many of them will either remain unfertilized or be eaten by PLANKTON FEEDERS at some stage. Producing lots and lots of eggs improves the chances of a few surviving. Fish enhance their offspring’s chances of making it through the drifting PLANKTON STAGE and then finding a REEF in a number of ways. They spawn at dusk, when most PLANKTIVORES are retiring to the safety of the CORAL COVER; they spawn at places where the currents take the fertilized eggs away from the hundreds of PLANKTIVORES that inhabit the REEF; they also time their activities to the moon’s cycle so that the tides and currents are favorable for taking the developing larvae away from the REEF and later delivering them back so they can find a home. Some methods of BROADCAST SPAWNING are more haphazard than others. While SURGEONFISH and GROUPERS may spawn en masse in their thousands, GREEN WRASSE MALES establish individual temporary spawning territories into which females are attracted. ANGELFISHES, BUTTERFLYFISHES, BOXFISHES, LIONFISHES and some WRASSES and PARROTFISHES have one – to – one sex and broadcast their spawn in pairs. Pair spawning usually involves a degree of COURTSHIP. The male may court the female by displaying to her, showing off his colours in some way, and then nuzzling her belly. When she is ready, they will rise high off the REEF together, positioning themselves so that the released eggs are immediately covered by sperm. The chances of successful fertilization are higher with this careful approach. Once finished, the pair dive back to the safety of the REEF.
SHARK BABIES
Male sharks have “claspers,” organs which, in some species, function much like a human penis. Before mating, sea water is taken into a pair of muscular sacs which lie along the male’s belly, and, when ready, the sacs contract, flushing the sperm into the female. Mating can be a vicious affair: the male bites the female on her flank, back and pectoral fins to maintain his position and possibly, make her receptive. With a grip on her from behind, the male twists around her and inserts one of his claspers. The fertilized egg then develops in different ways, depending on the species. Some, such as bullhead and horn sharks, produce eggs encased in leathery a capsule which is left among weeds or rock. Others give birth to live young, which develop as isolated eggs inside the female. But in some sharks, such as Hammerheads, the yolk sac feeds the embryo early on, then develops a placenta that receives nutrients and oxygen from the mother via an umbilical stalk in much the same way as human embryos.
UNDERWATER NESTS
Many fishes prefer not to leave the REEF at all to reproduce. Some create a nest of sorts on the sand or on the REEF and lay their sticky eggs there. Known as DEMERSAL SPAWNERS, these fish invest much more in their offspring by producing larger eggs with a larger yolk, and by tending the eggs once they have been fertilized. The survival rate at this stage is much higher than with BROADCAST SPAWNING, but fewer eggs can be produced at any one time. Nesting may occur en masse, with hundreds of fish clearing small patches of sand cheek by jowl with others. When green chromis spawn, the sandy areas in between shallow REEFS begin to look like patchwork quilts from above. The process begins when a male excavates an area, smaller than a saucer, by flicking his tail in the sand. Now in his breeding colour of lemon yellow, he attracts females to his patch by swimming up and down above his nest, flashing his colours. He also “chirps”, a sound that advertises his readiness to spawn. A blue – green female, attracted by this display, will dive down to his nest, release her eggs on to the sand and swim off again. The male swims alongside her as she deposits her eggs, covering them with sperm. There is no long – term pair bonding, and the female will go and spawn with another male elsewhere on the sand. This frenetic activity lasts throughout the afternoon until dusk, when the fish have to return to the CORAL for the night. Large TRIGGERFISH also excavate nests in the sand, but these are bigger and require a great deal more work. Large lumps of CORAL and STONE are carried away, and the sand is blown out using their mouths. Usually the female lays her eggs at dawn, leaving the male to tend the nest and defend the developing eggs. Many fish like to feed on the large and nutritious eggs of TRIGGERFISH, so the nests are often surrounded by BUTTERFLYFISHES, WRASSES and others waiting for their chance. Any diver who has ventured near a male titan triggerfish on guard duty will testify to its aggression. It will not only scare away other fish, but will also chase and bite divers too.
Broody Males
The more time and energy invested in offspring, the better their chances of survival. Bigger eggs have a better start in life, but they need a lot of tending. Among the fish prepared to do this are PIPEFISH, where the male actually broods the eggs. Banded pipefish are common on shallow REEFS of the INDO – PACIFIC, particularly in more sheltered waters. They form long – term pairs and although they do not necessarily spend the day feeding together, they do greet each other every morning. At dawn the female swims to the male’s home patch and the two swim for a while, gently twisting their bodies together in a simplified version of their courtship dance. On the morning that the female’s eggs are ready, the pair dance together, entwining their bodies as they rise up off the CORAL. In a sudden rush, the female transfers her eggs to the male’s sticky pouch. He then carries the developing eggs for about 10 days before they hatch out into the PLANKTON as independent miniature pipefish. During the breeding season, banded pipefish mate every 10 days. It is only because the male takes full responsibility for the fertilized eggs that the female can spend enough time feeding and regaining energy to produce another batch of eggs so soon. Similar dedication to safeguarding eggs is seen in male cardinal fish. When the female extrudes her eggs in a sticky ball, the male fertilizes them immediately and then keeps them safe in his mouth. He ensues that all the eggs have a good supply of oxygen by turning them around in his mouth at regular intervals, but for the week or so that he carries them, the male appears not to eat at all.
CORAL SEX
Anyone who has dived on the GREAT BARRIER REEF at night a few days after the full moon in November will have witnessed one of the great spectacles of nature: the spawning of CORALS along the entire 2300 km (1429 miles) of REEF. Soon after dark, tiny bundles of eggs and sperm (which look like the small POLYSTYRENE BALLS used in beanbags) float up from the REEF to the surface: the water is thick with them.
A question of Timing;
Being immobile, CORALS cannot go in search of a mate, so to achieve cross – fertilization between one colony and another of the same species, which may be some distance away, CORALS have to synchronize their spawning very precisely. Eggs, which take longer to develop than sperm, begin to grow in the CORAL POLYPS as early as April. At first they are white, but as winter passes and the spring sun begins to warm the water, development speeds up and eggs become either pink, orange, red, purple, green or even blue. In some CORALS one POLYP will be female and another male, but in most cases the POLYPS are HERMAPHRODITE, producing both eggs and sperm. The cue to spawn seems to come from the season, the water temperature and the moon. All three (3) factors must be right for spawning to occur. Along the entire REEF, spawning takes place between the second and seventh nights after the full moon, but each species will only spawn on one night during that period. Shortly after dusk the egg – sperm bundles of some species can be seen just under the POLYP mouth, ready for release. On cue, each species will release its eggs and sperm at the same time, some early in the evening, some later. The accuracy with which CORALS spawn is an extraordinary feat, but one that makes absolute sense. By spawning at night they avoid the daytime PLANKTIVORES, and by spawning all together, they overwhelm the nocturnal PLANKTIVORES. They also spawn at a time in the month when tidal movement is low. This increases the chances of eggs and sperm meeting rather than being swept away from each other.
Variety of Sex Cells
HERMAPHRODITE CORALS COAT their bundles of eggs with sperm only 30 minutes or so before spawning. On reaching the surface, the bundles break up, the sperm swimming off in search of eggs from another coral head of the same species, and the eggs drifting, waiting to be fertilized by sperm from another colony. Some CORALS extrude their bundles all at once, almost explosively. For others it happens over a longer period of time, sometimes as long as 30 minutes. Most massive Porites coral heads are not HERMAPHRODITE: the females release millions of tiny eggs and the males produce sperm that hangs like a mist, creating an effect very like theatrical dry ice. The sperm of some Porites corals actually swim to find another Porites coral head with ripe eggs which are then fertilized and brooded internally. Only later are these larvae released into the water. On a coral spawning night a scum of eggs and sperm forms at the surface. In this soup, eggs are being fertilized and the next generation of CORALS is developing. Though they can survive adrift in the PLANKTON for longer, most coral larvae settle out on the REEF after only a few days.
