Coral reef

“ Let the island continue subsiding, and the coral-reef will continue growing up on its own foundation, whilst the water gains inch by inch on the land, until the last and highest pinnacle is covered, and there remains a perfect atoll. A vertical section of this atoll is shown in the woodcut by the dotted lines;—a ship is anchored in its lagoon, but islets are not supposed yet to have been formed on the reef. The depth of the lagoon and the width and slope of the reef, will depend on the circumstances just referred to under barrier-reefs. ”

“ In the coral isle, with its blue lagoon, its circling reef and smiling vegetation, there is a wondrous fascination; while in the long reefs, with the ocean driving furiously upon them, only to be driven pitilessly back, all wreathed in white foam and diamond spray, there is enough of the sublime to transfix the most careless observer. The barrier reef that skirts the north-east coast of the Australian continent is the grandest coral formation in the world, stretching for a distance of a thousand miles, with a varying breadth of from two hundred yards to a mile. ”

“ In the first three chapters the three chief classes of coral formations—atolls or lagoon islands, barrier reefs, and fringing or shore reefs—are described. The fourth chapter deals with the distribution of coral reefs and conditions favorable to their increase, their rate of growth, and the depths at which reef-building corals can live. Darwin's theory of the formation of the different classes of coral reefs then follows. Coral polyps do not flourish below a depth of twenty or thirty fathoms, but reefs are found rising from much greater depths—how are these to be accounted for? ”

“ It is a peculiarity of coral-reefs that the outer side is usually nearly perpendicular, while the inner side is a gentle declivity. The cause of this may be better understood if we follow the development of the reef from its beginning. Upon a shore of sand or rock, and at a depth of less than 120 feet, the reef-builders attach themselves and commence to grow. The reef rises, but, as before remarked, corals grow most rapidly in the purest waters, and thus it is that reefs often seem to crowd against the waves without and assume a wall-like aspect. ”

“ A coral reef is a gigantic “plant-animal.” It is a community of countless billions of plants and countless billions of animals which act as a single organism, like the countless millions of specialized cells that make up the body of a man or a mouse. It is probably the most efficient of all earthly creatures. It is self-sufficient, creating its own constant food supply. It is essentially immortal. It is hungry like an animal. It is motionless like a plant. ”

“ This is the whole structure of the interior of such a reef as that which fringes the Red Sea coast, as seen e.g. during the excavation of the quay walls or slipway at Port Sudan. Great “stones” which are the more massive colonies, generally the genus Porites, are bedded in with smaller colonies whole or broken. In places are collections of grey mud and sand, also formed from coral by the action of boring organisms or perhaps as the residue left after partial solution by the sea.
“How fast does a coral reef grow?” is a question often asked, and never as yet truly answered. ”

“ When the coral reef has thus reached the surface and cannot grow farther in height, it spreads radially like a fairy ring on the talus formed by broken corals that have fallen down the slopes. The central parts, no longer protected by living organisms, are continually subject to the solvent action of the sea water penetrating the porous substratum, and sink to form the lagoon. ”

“ From this masses of coral, sand, stones or large boulders, are always falling on to the foundation slopes, forming successive sloping layers indicated by the dotted lines, upon which fresh growth takes its rise [57] . When the coral reef has become of some breadth [116] (and atoll rings may be 30 miles or more across) a boring at the edge might descend for a thousand fathoms and never meet with the original foundations, but would pass only through recent corals fallen from the shallow zone. ”

“ Oceanic currents carry planktonic larvae of coral reef associated animals to the shallow nearshore coral reef and sea grass habitats, where they transform into their juvenile stage. As they mature over months or years, they move offshore and take up residence in the deeper coral reefs, octocoral hardbottom, and algal plains. Between the monument's nearshore habitats and its shelf edge spawning sites are habitats that play essential roles during specific developmental stages of many reef-associated species, including spawning migrations of many reef fish species and crustaceans. ”

“ Thirdly, there was the fact that the coral reefs rise suddenly like a wall out of oceanic depths, soundings of a thousand fathoms and more being constantly found close up to the barrier-reefs. Fourthly, there is the fact that on the inner side, next the island or the continent which they inclose or protect, the lagoon or the sheltered area is often very deep close to the reef, not indeed affording oceanic soundings, but nevertheless soundings of twenty to thirty fathoms. ”

“ In the present state of our knowledge it seems reasonable to conclude that coral reefs are formed wherever the conditions suitable for growth exist, whether in areas of subsidence, elevation or rest. A considerable number of reefs, at all events, have not been formed in areas of subsidence, and of these the Florida reefs, the Bermudas, the Solomon islands, and possibly the Great Barrier Reef of Australia are examples. ”

“ John Murray's explanation is fully confirmed that the coral reefs often begin on shoals; that these shoals are due to elevations of the sea-bottom; that the reef when once established can and does grow seaward upon its own fragments broken and submerged; that these form a "talus" capable of indefinite advance until the farthest limit of the shoal is reached; that the rearward ranks of the coral animals die as they are left behind in the hot and shallow waters of the lagoon ”

“ The unevenness of the surface of the Bahama Banks (and the same is true also of the southern portion of Florida, the Yucatan peninsula, and of nearly all of the submarine plateaus or banks in West India waters) is largely due to the coral reefs and the coral rock formed on them. While the outer portion of the continental shelf, in most instances, is formed of soft, unconsolidated calcareous mud or ooze, in the tropical seas, where the depth, clearness of the water, etc., are favourable, reef-building coral-polyps become attached and form massive corals. ”

“ The coral reefs that grew around the shore line of the islands still remain after the islands have washed away, and thus the living reefs now mark the contours of the islands as they were. The currents flowing in and out of openings in the reef-rim have deepened the lagoons, but nevertheless there are many coral heads growing in the lagoon of every coral atoll. ”

“ Allan informs me in a letter, that the whole line of coast, from Tamatave, in 18° 12′, to C. Amber, at the extreme northern point of the island, is bordered by coral-reefs. The land is low, uneven, and gradually rising from the coast. From Captain Owen’s charts, also, the existence of these reefs, which evidently belong to the fringing class, on some parts, namely N. of British Sound, and near Ngoncy, of the above line of coast might have been inferred. Lieutenant Boteler (vol. i, p. 155) speaks of “the reef surrounding the island of St. Mary’s at a small distance from the shore.” ”

“ At depths greater than 160 feet they are rare. They die and increase again and again, and therefore the coral reefs grow in height and breadth, and only the height of water at ebb tide puts a limit to their upward growth. The continual surf of the sea and stormy waves often break off whole blocks of coral limestone, which roll down and break up into sand. With this all cavities are filled in, and thus the action of the sea helps to consolidate and strengthen the reef. Other lime-extracting animalculæ and also seaweeds establish themselves on the reef. ”

“ The Corals which form the Reef are not the only beings that find their home there: many other animals—Shells, Worms, Crabs, Star-Fishes, Sea-Urchins—establish themselves upon it, work their way into its interstices, and seek a shelter in every little hole and cranny made by the irregularities of its surface. In the Zoological Museum at Cambridge there are some large fragments of Coral Reef which give one a good idea of the populous aspect that such a Reef would present, could we see it as it actually exists beneath the water. ”

“ There is even a harbour for ships formed by the reefs, but it is at the mouth of a valley; red.—Atooi, in La Peyrouse’s charts, is represented as fringed by a reef, in the same manner as Oahu and Morotoi; and this, as I have been informed by Mr. Ellis, on part at least of the shore, is of coral-formation: the reef does not leave a deep channel within; red.—Oneehow; Mr. Ellis believes that this island is also fringed by a coral-reef: considering its close proximity to the other islands, I have ventured to colour it red. ”

“ Within the area of the bay the water varies in depth from a few feet to four fathoms, the rise and fall of the tide being about two fathoms. The fringing coral reef represents all stages of development and decay—live growth on the outer edge, ever encroaching on the deeper water, and comprising many varieties; dying masses on the shore-side, and a considerable extent of dead and denuded relics lying in mud. There are also weedy patches, bare sand-banks of limited extent, uncovered at low water, and muddy depressions both in the deep and shallow portions and clean sand. ”

“ I am, then, fully justified in repeating, that enormous spaces, both in the Pacific and Indian Oceans, are interspersed with islands, of which not one rises above that height, to which the waves and winds in an open sea can heap up matter. On what foundations, then, have these reefs and islets of coral been constructed? A foundation must originally have been present beneath each atoll at that limited depth, which is indispensable for the first growth of the reef-building polypifers. ”

“ In shallow waters, such as are the favourite haunts of mollusca, corals, &c., the tides and storms are frequently sufficiently powerful to shift the loose material on the sea bottom. A large part of a coral reef consists of broken coral rock dislodged from the growing mass and carried upwards to the beach or into the lagoon. ”

“ Every fact in geology proclaims that neither the land, nor the bed of the sea retain for indefinite periods the same level. As well might it be imagined that the British Seas would in time become choked up with beds of oysters, or that the numerous small corallines off the inhospitable shores of Tierra del Fuego would in time form a solid and extensive coral-reef. ”

“ But the margin of the reef, although the highest and most perfect part, from being most exposed to the surf, would generally during a slow rise of the land be either partially or entirely worn down to that level, at which corals could renew their growth on its upper edge. On some parts of the coast-land of Mauritius there are little hillocks of coral-rock, which are either the last remnants of a continuous reef, or of low islets formed on it. ”

“ There exist, also, scattered in the open ocean, some linear and irregularly formed strips of coral-reef, which, as shown in the last chapter, are probably allied in their origin to atolls; but as they do not belong to that class, they have not been coloured; they are very few in number and of insignificant dimensions. Lastly, some reefs are left uncoloured from the want of information respecting them, and some because they are of an intermediate structure between the barrier and fringing classes. ”

“ For should the outer and living margin perish, of any one of the many low coral-islands, round which a line of great breakers is incessantly foaming, the whole, it is scarcely possible to doubt, would be washed away and destroyed, in less than half a century. But the vital energies of the corals conquer the mechanical power of the waves; and the large fragments of reef torn up by every storm, are replaced by the slow but steady growth of the innumerable polypifers, which form the living zone on its outer edge. ”

“ The fact of reefs of the fringing class being always breached in front of streams, even of those which are dry during the greater part of the year, will be explained, when the conditions unfavourable to the growth of coral are considered. Low coral-islets, like those on barrier-reefs and atolls, are seldom formed on reefs of this class, owing apparently in some cases to their narrowness, and in others to the gentle slope of the reef outside not yielding many fragments to the breakers. ”

“ It has been a question with some naturalists, which part of a reef is most favourable to the growth of coral. The great mounds of living Porites and of Millepora round Keeling atoll occur exclusively on the extreme verge of the reef, which is washed by a constant succession of breakers; and living coral nowhere else forms solid masses. At the Marshall islands the larger kinds of coral (chiefly species of Astræa, a genus closely allied to Porites) “which form rocks measuring several fathoms in thickness,” prefer, according to Chamisso, [5] the most violent surf. ”