RIDDLE OF THE REEFS

Timaru Herald, Volume CLVIII, Issue 23321, 3 October 1945, Page 7

RIDDLE OF THE REEFS

Coral Mystery Found In All Seas Of The World During the war, thousands of people who had their first sight of a coral reef and a coral atoll, were not only im?l eS t S . et f- by their beauty. but marvelled at their structure. When seen from the air one of the first things about hese coral atolls that strikes one is , samel J es s. Not in the sense of monotony—but their uniformity—the way in which they conform to a more or less standard pattern Yet, in spite of the uniformity o* their general outline—the outer reef zone where the rollers break, the coral ; ' eac!1 .' the sandy island ringed coconuts, blue lagoon in the centre—there have been many very {Merent theories as to their formaltTcre I s . ? n as P e ct of marine oiologv upon which doctors differ, writes a Hpram P “ ld A e /r nl - in t!le Morning Heiald. Many points about the formation Of coral atolls still remain an unsolved riddle. It is not so tong a “ that one heard the organism responsible for the building of coral reefs referred to popularity as “cora sects.” It is true that the corals a"e living organisms, but they are creatures closely related- to the sea anehisects and ln U ° WlSe related t 0 the The actual hard formation generally n.!(T*T l , ( a - s 1 “ Co T al ” —such as specimens brought back from the Barrier fteef by tourists—is a mass of coral limestone and represents actually the dead skeleton, built up by thousands of living polyps, as the coral organisms are called, and within the pores of which the polyps dwelt. These are lowly organisms which, after a brief larval period in which they are free-swim-mmg, settle down to a sedentary existence and start reef-building on their owm. 6 Each of these coral masses is the work of countless thousands of little organisms, which at high tide when covered by water, expand and wave their fiqwer-like tentacles in the water These living corals are very beautiful and vary in colour from yellow and orange to bright red and blue Corals live in all the seas of ’h= world, intemperate and cold zones as well as in the tropics. But it is only tropical seas where the temperature does not fall far below 20 degrees centigrade at any time in the year, that the reef-building corals flourish. It is well known among zoologists that besides requiring warm seas, reefbuilding corals live only in relatively shallow waters. The fact is important for in it lies one of the great difficulties m explaining certain types of coral formations. . Of coral reef formations, three distinct types are recognised—fringing reefs, barrier reefs, and atolls. The first two types of reef grow in more or less proximity to islands or continental land masses. Great Barrier Reef A fringing reef is a coral lying close off-shore; but a barrier reef, of which the Great Barrier Reef of Australiamore than one thousand miles in length—is the finest example in the world, runs parallel with the coast,* but may be a considerable distance away The Great Barrier Reef, while it approaches to within a few miles of the Queensland coast in many' places, may lie as much as 40 miles off the coast separated by a deep channel. As it is an established fact that reefbuilding corals can only work in relatively shallow waters, in depth not greater than 30 fathoms, it was difficult to explain the fact that in many places on the Great Barrier Reef the coral limestone extended to a far greater depth than that in which the coral poiyp can flourish. It is now an accepted fact that the Great Barrier Reef has been graduallybuilt up on a subsiding continental shelf—the coral animals working at their best on the seaward side of rhe reef where the food supply is most abundant. But easy as it may be to explain the development of barrier reefs, the development of coral atolls was for a long time a riddle to marine zoologists. The belief was held that coral atolls grew up on the tops of submerged volcanoes, and the characteristic form of the central lagoon of most atolls was taken as evidence of this. The lagoon was said to be the crater of the old volcano. Charles Darwin, however, recognising that corals grow best at a depth of about 15 fathoms, and that they will not live below 30 fathoms, propounded his “subsidence theory.” He examined known coral atolls and produced evidence that these were situated in an area of known, or of probable subsidence. This subsidence theory is still the most important and widely accepted explanation of coral reef formation. It is now known that corals thrive best on the seaward slope of the reef where food and oxygen are most abundant, and that the organisms will not live in muddy, discoloured water, which often occurs on the leeward side of the reef. This explains the central lagoon of the typical coral atoll. In the Cocos Keeling Island, Professor Wood-Jones made long and painstaking observations on the formation of coral islands, and showed graphically the influence of tide and wind direction on the formation and shape of coral islands. Wood-Jones demonstrated that fragments of coral rock and boulders which offer an obstruction to waves, tend to deflect these, so that they flow to either side of such obstruction, depositing their burden of rock or sand on the embryonic island, causing it to develop in crescentic form—the arms of the crescent turned away from the direction of prevailing wind and tide