Published in Swara, the magazine of the East African Wildlife Society when I was working in Kenya Maya/Jun 1984
The wealth of beautiful and varied corals found along the Kenya coast are each finely adapted to make the most of their different habitats
Coral dominates the Kenya coast: along many parts of the shoreline there is a coral cliff about 4 metres high which is an ancient reef about 100,000 years old; the brilliant, white sand is mainly ground-up coral and the lagoons have an ancient coral floor. Not only are the foundations based on coral but the com-plex web of living organisms above also depends on living coral.
Typically, the coast is lined by a fringing reef between 500 and 800 metres off shore. The lagoons inside the reef may be uniformly shallow, and often uncovered at low tide, as at Tiwi, or may contain pools and channels down to about 10 metres deep, as at Nyali and Diani. Other reef systems can occur out-side the fringing reef, as at Malindi. This situation provides a number of different environmental conditions, from the exposed, seaward side of the reef, which receives a steady battering from the
Indian Ocean waves, to the warm, sheltered, deep rock pools, which have little strong water movement. Each area contains different habitats occupied by different organisms. As one of these organisms corals too have particular preferences. To appreciate the reasons for this it is necessary to have some idea of the structure of the coral organism and how it forms its rock-like skeleton.
There are two main kinds of coral, the soft corals and the hard corals. The former are sometimes known as the Octocorals as in these the tiny coral animals, the polyps, have an eight-rayed symmetry.
Although they do not form reefs they contribute significantly to the ecology of the area as they are very common. Notable types are the large, whitish, rubbery sheets, sometimes a metre across, such as Sinularia, which are always present in the rougher areas outside the reef, for example on the drop-off from North Reef, Malindi. There are also the delicate Xeniid corals which occur in brown and grey clumps,attached to sea grasses, in most of the lagoons. The fragile, pink and jelly-like Nephthyid corals are usually limited to deeper water. Organ-pipe coral, Tubipora musica is an Octocoral which does form a skeleton and is common in rock pools near to the reef.
The hard Hexacorals, on the other hand, have a six-rayed symmetry and are closely related to the sea anemones, although individual polyps rarely reach the size of sea anemones. Their form varies greatly: there are the massive types such as Porites, which make up the huge coral heads, and rounded brain corals like Platygyradelicate, branched type like Acropora,the staghorn coral; and even solitary types like fungus coral, Fungia, which look like large, overturned toadstool caps. All the various forms have three main features in common: they have six-rayed polyps, they have a calcium carbonate skeleton and they all contain living algal cells in their body tissues known as zooxanthellae.
Thus, a hard coral is animal, vegetable and mineral. The relationship between these three parts has been extensively investigated and several important facts are well known. If a coral is kept without light it will die in a few weeks or months, and, in the dark, the rate at which its mineral skeleton grows decreases considerably. This suggests that corals, like plants, need light to manufacture food, although the coral animals, the polyps, are capable of catching and digesting small planktonic creatures. (The polyps have powerful stinging cells which fire tiny barbs into the prey to entangle and poison it; some can inflict a painful sting on a human, like the Hydroid coral, Millepora, which is related to the infamous, floating Portuguese Man o' War, Physalia physalia.) Further investigation has shown that the polyps are indeed partially reliant on the zooxanthellae for food. This association is to their mutual benefit as the coral also provides the zooxanthellae with shelter and minerals. However, the degree to which the zooxanthellae are needed varies. Some corals of the branching type obtain up to 50 per cent of their food through photosynthesis while some more massive types only receive 5 per cent, the rest, in both cases, being obtained from animal prey. This, however, is not the whole story. The presence of the zooxanthellae is also vital for the production of the coral skeleton — and not merely because the coral needs food to deposit calcium carbonate crystals; the slowing of the deposition rate occurs immediately the light intensity decreases. The answer seems to lie in the fact that the zooxanthellae absorb carbon dioxide during photosynthesis (as do all green plants), which renders conditions more favourable for calcium carbonate deposition.
It is clear then that corals require light. Below about 50 metres depth there is very little light. In actual fact satisfactory coral growth is probably limited to about 35 to 40 metres depth, the most prolific growth being seen from 20 metres to the surface. On the other hand, polyps cannot tolerate being exposed to the air or fresh water. Cases have been observed in the Seychelles and elsewhere when exposure during extremely low spring tides, combined with rain, have devastated large areas of live coral. This intolerance towards exposure can readily be seen on the large Porites coral heads, common all along the coast. The tops are often flattened and covered in algae. This is because low spring tides in the past have left them uncovered and the polyps have died. Thus, it is no accident that they do not stick up out of the water.
The shape of corals is greatly influenced by the main method they use to obtain their food and the amount of light available. Those corals that rely mainly on food manufactured by their symbiotic zooxanthellae (autotrophic corals) tend to be highly branched, have very small polyps and are fast growing, while those which rely mainly on catching animal food (heterotrophic corals) tend to have large polyps and be massive, rounded and slow-growing. Sea water, with its many suspended particles, scatters light in all directions. The branching colonies, like those of Acropora, seem to have the most efficient light-absorbing surface while the brain coral type is most efficient at catching falling organisms which may settle on them. The shape of the colony can also be affected by in-creasing depth; branched corals tend to give way to plate-like types the deeper they go. When particular species are examined from different areas, it can be seen that the degree of exposure can change the form to such a great extent that the species can often only be recognised by careful examination of the individual polyp cups. Thus, specialists rely little on the overall form for identification.
The rate of growth of corals varies tremendously, In some staghorn corals about 20 centimetres increase in branch length per year has been reported. At the other extreme, the massive brain coral type can increase their diameter by 4 or 5 centimetres a year. An examination of the growth rings within one Porites coral head 5.8 metres in diameter showed it to be about 140 years old. Many live colonies are probably very much older than this. Some Caribbean Pocilloporabranched corals are thought to be several centuries old. Barring natural disasters and environmental changes, there is no reason why they should not live for ever. The skeleton is not alive; the living tissue consists of a thin skin over the surface, growing and dividing in a continuous renewal process. Barring natural disasters and environmental changes, there is no reason why they should not live forever. The skeleton is not alive; the living tissue consists if a thin skin over the surface, growing and dividing in a continual renewal process.
But natural disasters and environmental changes do occur. Extra low spring tides and rain have already been mentioned. Violent storms can wreak havoc, smashing up colonies. Rivers can emit large amounts of silt which settle on the corals and kill them; this is probably the main reason for there always being a break in the reef opposite a river. This is very marked at Malindi, near the Sabaki River. The whole main bay has no reef and allows large waves to reach the beach unhindered. Human pollution, causing turbidity, and oil-slicks can be fatal. Whether a damaged reef will recover depends firstly on the cause of damage being removed and secondly on time. After major storm damage in the Caribbean it seems to take between 25 and 100 years to re-attain the original, balanced state.
Animals and plants may also attack or damage live coral. A common green alga, Dictyosphaeria, which is small, green and rounded and is commonly found growing on the sides of rocks in the lagoons, has been seen growing over and killing corals in Hawaii. The cause of the unusually rapid growth of the alga was thought to be the washing into the sea of fertilisers — another example of man's unintentional modification of the environment.
Although many animals actually feed on the coral polyps, few seem to cause large-scale, irreversible damage, the situation is a balanced one and an essential part of the natural food-web of the reef. The most obvious of these predators (or grazers, as corals contain a large proportion of plant material) are the parrot fish, whose horny beaks are able to chew off lumps of coral, which are ground to a fine powder in a specialised part of the gut and the organic material digested. The indigestible waste is egested as a fine, white coral sand, probably contributing significantly to the white beaches of the Kenyan coast. The scrape marks are often clearly visible on Poritescoral. Other fish are more delicate feeders such as the long-mouthed file-fish, which pick individual polyps off the colony.
The infamous Crown of Thorns star-fish, Acanthaster plancialso occurs here but, luckily, only in small numbers. This starfish crawls slowly over the coral with its stomach turned inside out, digesting all the polyps in its path and leaving behind a bare, dead skeleton. In small numbers it will cause no more damage than the fish but in some parts of the world, such as the Great Barrier Reef, the numbers have suddenly increased dramatically, and huge areas of reef have been devastated. There are a number of theories as to why this has happened, including human interference in collecting one of their main predators, the attractive Tritonshells.
Exaggerated claims of several centuries have been bandied about as being the rate of recovery of the coral after an attack but more sober estimates, based on available information, indicate 20 to 30 years to reach an equilibrium, which is not a long time from an ecological point of view. Some corals are more susceptible than others to Crown of Thorns at-tack: the starfish prefers branching to brain corals as the latter are able to repulse the attacker by stinging their tube feet. Thus, after a large-scale attack, many brain corals still survive while most, or all, of the branching types may be destroyed. Repeated attacks lead to the situation where the brain corals occupy the sheltered areas and branching corals occupy the more exposed areas, which are avoided by the adult Crown of Thorns.
As well as being attacked by other creatures, it may come as a surprise to learn that corals can be aggressive towards each other. The most aggressive are the brain coral type and the least aggressive the branching forms. If this were not so, then the faster-growing branching forms would quickly overgrow and thus kill the slower-growing types. When two colonies come into contact with each other, the more aggressive one sends out thin threads from its polyps which penetrate and kill the neighbour. Other corals develop special, long tentacles which brush over the competitor with the water currents and destroy it. Soft corals seem to succeed simply by growing very quickly in the few empty spaces which occur; most would probably be edged out in time by more aggressive types.
The productivity of coral, in an environment generally considered to be of very low productivity, has been com-pared to that of grasslands. They are able to support a thriving community of animals with an enormous variety of types. When you take a close look at a large branched coral colony, such as Pocilloporathere is almost inevitably a number of other animals living among the branches. A sight which is commonly seen is that of a school of the turquoise damsel fish (Chromis caeruleus) cowering amongst the branches of the deep red Pocillopora eydouxi, seemingly quivering with fear, yet unassailable. Snapping shrimps,Alpheus sp. and a ferocious little crab, Trapeziumsp. will defend their coral colony against all-comers with great vigour; even a Crown of Thorns starfish has been reported to retreat under the onslaught. Many fish shelter in the coral at night; not only does this give them protection, but the coral polyps also gain nitrogen from their excretory waste, which is used by the zooxanthellae in making proteins.
The polyps are very sensitive organisms and the distribution of the different species is a result of centuries of adaptation and selection. When we see a beautiful staghorn coral growing in a particular area it is not there by accident but because the conditions, such as light intensity, temperature and food supply, are exactly right. It has succeeded in overcoming competition from other organisms fighting for its space and it has grown up in an area where the coral predators are in a balanced state with the coral prey. If any of these factors should change for the worse, then the coral will not survive. Much remains to be discovered about the ecological relationships of the corals but even for the casual snorkeler, a brief consideration of the complex interrelationships involved in coral life will, I am sure, enhance the fascination which most of us feel while drifting among the Kenyan reefs.
The reef community is sustained by the producer organisms such as the algae (1A), the sea grasses (I B), the phytoplankton (IC), and the symbiotic algae living in close association with animals, e.g. zooxanthellae within the coral polyps (1D) and algae in the giant clam tissues (1 E).
Feeding on these producers are the zooplankton (2A), herbivorous fish such as the surgeon fish (2B), invertebrates like the browsing starfish Linckia laevigata(2C), sea urchins such as Diadema setosum(2D), and molluscs like Strombus(2E).
The coral feeders can be considered omnivores, such as parrot fish (2F), the file fish (2G), the Crown of Thorns starfish (2H) and the egg cowrie, Ovula ovum (21), which feeds on soft coral.
Carnivores take many forms, from the coral polyps catching zooplankton to the barracuda (3A) feeding on large coral fish. Plankton feeders include giant anemones (3B), sponges (3C) and Chromis (3D). The tiny fan worms (3E) will take in any particles which are filtered out of the sea. Cleaner shrimps (3F) eat animal parasites off the skins of fish. More active carnivores are the cone shells (3G), which can eat fish. and trigger fish (3H), which eat anything including sea urchins and Crown of Thorns starfish. Nudibranchs (31) eat small encrusting animals.
Then there are the often omnivorous scavengers: goat fish (4A), brittle stars (4B), crabs (4C) and the saprophytic bacteria (4D).