A Coral is a hard, cancerous, rock like substance formed either by the continuous skeleton or fused skeletons of members of a group of sedentary marine animals called polyps, that live in colonies only in clear warm, shallow seas or oceans.

A Reef is a mass of rock or coral, sometimes of shingles or and, occurring in the sea usually covered at high tide, but often partly exposed at low tide.

Corals make their skeletons by extracting limestone (calcium carbonate) from the sea and when they die, their fleshy parts decay, the skeletons remain attached to the rock at their base. As many millions of skeletons collect, the Coral rock grows. In this way, large banks of rock are gradually built up and they are called Coral Reefs.

Conditions which favour Coral growth

Coral Polyps do well in high temperatures of about 21^oc - 30^oc North and South of the Equator. Therefore their growth is confined to tropical and sub-tropical seas between 30^oN and 30^oS of the Equator.

Coral reefs are located on the Eastern side of continents where temperatures are relatively high compared to the Western sides of continents where cold currents exist.

Coral reefs require an environment of water since they are fundamentally Marine organisms which cannot grow outside. If Corals are found inland, it means that such an are was once under sea water.

Coral reefs require sufficient sunlight - Therefore prefer shallow usually not more than 60 metres in depth. Ideal conditions are down to between 45 - 55 metres. Very deep water does not allow penetration of sunlight for (photosynthesis in) phytoplankton on which coral live.

Coral polyps also do not survive in fresh water. They only survive in seawater with a salt content of at least 2.7 - 4.0%. The salt should make up at least 27 - 40 parts per 1000 mls. Corals do not grow at river mouths partly because river water is fresh and dilutes seawater at the rivers mouth.

Coral polyps survive in clean, clear and oxygenated water. Clear water allows sunlight to penetrate to a reasonable depth. This is because polyps feed on microscopic plants, which need sunlight for photosynthesis. Dirty water is not conducive to the growth of polyps corals can not grow in areas with dirty water such as where storm waves are dominant or at river mouths where silt or sediment brought from inland make sea water to be dark and can also suffocate the coral polyps.

Coral growth also requires relatively still or calm water. Turbidity quickly erodes developing corals, at the same time it stirs up dirt from the sea floor, which interferes with sunlight for photosynthesis in phytoplankton on which the coral polyps feed.

Presence of continental shelf and oceanic ridges and islands of rocks.

Corals are found in isolated places outside the tropics. For example, the Bermunda Island and along the coast of South Africa around Durban (both outside 30oN and 30oS). This is because of the warm ocean currents ( c....? stream for Bermuda and Mozambique current for South Africa) transport warm conditions to those areas making possible for the polyps to thrive.

Types of coral reefs in East Africa

Examples of coral formations in East Africa are along the coast extending from Lamu in the North, to the Mouth of river Ruvuma in the South./ Coral reefs also form large areas of the eastern sides of Zanzibar and Pemba Islands - the Wanda.

Three main types of Coral reefs exist.

(i) Fringing Reef: This is a flattish coral platform formed on continental shelf and separated from the coast by a narrow and shallow lagoon of upto 500-2000 meters wide. For instance at Diani beach, along the Tiwi coast and at Kilifi (Kenya) and at Oyster bay in Dar es Salaam, the reefs are attached to the shore. But in most places reefs are separated from the shore by narrow lagoons e.g. Andromache and Levin reefs t Mombassa, and off shelly beach on the south of Kenya coast.

(ii) Barrier Reef: A barrier reef is very similar to fringing reefs, but with much deeper and wider lagoons separating them from the mainland. Barrier reefs are not common on the East African coast because of the limited continental shelf which provide shallow platform for coral growth. But, a good example of a barrier reef is found along the south of the Kenyan coast offshores from Diani beach at Tiwi. Outside East Africa barrier reefs are also found on the East Coast of Australia the Great Barrier Reefs.

(iii) Atolls: Atolls are ring-shaped in plan. They enclose deep lagoons. The reefs may be broken by narrow channels. The best example of the Aldabra Atoll that lies both Zanzibar and Malagasy, about 700km from the African coast.

Mode of formation of coral reefs

Recent seismic exploration and drilling have revealed that fringing coral reefs simply grow from shallow continental shelves and advance seaward. But the formation of barrier reefs and Atolls has been explained by many theories, three of which are most important. These theories are discussed below:

(a) Subsidence Theory (Charles Darwin)

In 1942 Charles Darwin put forward this theory. He was later supported by Dana and Davis in 1928. Charles suggested that Island barrier reefs and Atolls began life on the edge of Volcanic islands (i.e. Evolution of species by natural selection).

He went ahead to state that when a volcanic island first erupts, its weight will destroy the isostatic equilibrium of the crust in that area. In time, due to isostatic adjustment, the volcanic will begin to sink under its own weight, and according to the size of the volcano, the crust may subside several hundred metres.

Darwins further proposed that if a coral had begun to grow as a fringing reef at sea level on the flanks of the volcano as seen in diagram A, the f first effect of isostatic sinking would cause the coral to transform to a barrier reef, provided the coral growth could keep pace with the sinking as shown in diagram B.

Later, if the volcanic island became completely submerged, the barrier reef would become an atoll as seen in diagram C.

Formation of Barrier Reef and an atoll by subsidence.

A.

B.

C.

(b) Antecedent Theory (Murray 1880).

Murray put forward this theory in which he suggested that for coral growth A occurs, then should be a still-standing marine platform which may be either submarine volcanoes, or accumulations of pelagic deposits upon which corals establish themselves.

According to Murray, a fringing reef begins to grow on the still-standing platform. Overtime, the reef builds further and further outwards and upwards becoming a barrier reef before passing into an atoll.

(c) Glacial control Theory (Daly)

This theory was proposed by Daly in 1934. Daly suggested that a rise in sea level during Pleistocene deglutition might be responsible for the formation of barrier reefs and atolls in the deep sea. He stated that during the peak of Quaternary glaciations, sea level must have been 90 metres lower than the present level. During that time all pre-glacial islands and reefs were eroded by waves down to sea level, thus providing good platforms for coral growth during postglacial times.

According to this theory, fringing reefs began to develop on these platforms, so that as sea level rose, the corals also grew upwards at the same rate as the level of the sea is rising.

Economic Importance of coral reefs.

Fringing reefs protect the coast against strong waves. The water above those rocks is shallow and this checks the spread of approaching waves. This contributes to the development of sheltered harbours.

The lagoons which are enclosed by coral reefs are good bathing and swimming places sheltered from attack by sharks.

Furthermore these sheltered lagoons can be used for fishing.

Coral rocks promote research and oil prospecting. As corals die, their partially decayed fats from their bodies seep into the sedimentary rocks below to form oil. Oil prospectors consider coral rocks as possible indicators of oil. Coral rocks are therefore of much interest to the researchers.

Coral limestone is an ingredient used in the manufacture of cement. Coral limestone at the Kenyan coast has led to the establishment of a cement factory at Bamburi near the Kenya Port of Mombasa

It should be realized that coral reefs are only found in the tropics (excluding effects of continental drift) and are therefore rare rocks. They are therefore tourist attractions and promote the tourist industries of Kenya and Tanzania.

Although corals are beneficial in the way they can contribute to economic development, coral may also hinder human and economic activities as outlined below:

Fringing and barrier reefs are major hindrances to navigation. Under high tide, they are submerged and therefore immediately unseen by sailors. They can cause devastating ship-wreckages.

Coral does not break down into good, deep soil. It forms poor soil which does not encourage much agriculture. This is seen especially in the Eastern Wanda of both Pemba and Zanzibar, where only mangoes and coconuts can reasonably do well. Along much of the mainland coast soil are also poor due to a lot of coral sand and coral platforms.

Coral growth on the continental shelves interfere with Marine fisheries. This is because they interfere with the Movement of the fishing vessels, and at the sometime, fishing nets also get caught and torn by corals.

The lagoons enclosed by fringing coral reefs are sometimes colonized by vegetation and turn into swamps. The swamps later become habitats for mosquitoes. This contributes to the spread of diseases. Coral reefs are therefore both a blessing and a danger to economic development of East Africa.