1. Distribution of Actinozoa in Space. 2. Coral-reefs. 3. Distribution of Actinozoa in Time.
The Zoantharia malacodermata appear to have an almost cosmopolitan range, Sea-anemones being found on almost every coast; some of the tropical forms attaining a very large size. Whilst essentially littoral and shallow-water forms, a few of the members of this group have been found by the Challenger expedition to extend to great depths. Thus, as shown by Mr Moseley, Edwardsia has been found at 800 fathoms, and Cerianthus at no less than 2750 fathoms; while species of Actinia itself go down to over 1000 fathoms. A few forms also (such as Arachnactis) Nautactis, Plotactis, Oceanactis, and Minyas) are pelagic in habit, and live in the open ocean. The Antipathidae are principally inhabitants of warm seas; but have been found off the coast of Greenland; while they extend to great depths. The Alcyonidae are principally inhabitants of shallow water; but the Pennatulidae extend their range up to very great depths. The Gorgonidae are likewise mostly shallow-water forms, and they attain their maximum in the seas of the tropics. The Red Coral of commerce is a Mediterranean species, and occurs principally at depths of from 5 to 6 fathoms, though occurring at 120 fathoms or more. The Organ-pipe Corals (Tubipora) are confined to the warm seas of the "coral region;" and the genus Heliopora, the only recent representative of the family Helioporidae, is confined to the Pacific and Indian Oceans. The only living corals referred to the Rugosa are Guynia, which is found in the Mediterranean, and the Haplophyllia of the Florida seas. The Ctenophora are pelagic, free-swimming forms, and appear to be cosmopolitan in their distribution. Lastly, the Zoantharia sclerodermata are partly inhabitants of deep water, and partly shallow-water forms; and the latter, as commonly forming "coral-reefs," are so important as to demand special consideration.
The so-called "reef-building" corals have their distribution conditioned by the mean winter temperature of the sea, a temperature of not less than 66° being necessary for their existence. The seas, therefore, which possess the necessary temperature may be said to be all comprised within a distance of about 1800 miles of the equator on each side. Within these limits, however, apparently owing to the influence of arctic currents, no coral-reefs are found on the western coasts of America and Africa. They are found chiefly on the east coast of Africa, the shores of Madagascar, the Red Sea, and Persian Gulf, throughout the Indian Ocean and the whole of Polynesia, and around the West Indian Islands and the coast of Florida. The headquarters of the reef-building corals may be said to be round the islands and continents of the Pacific Ocean. A " coral-reef" is a mass of coral, sometimes many hundred miles in length, and it may be two thousand feet or more in thickness, produced by the combined growth of different species of coralligenous Actinozoa. As before said, a mean winter temperature of not less than 66° is necessary for their existence, and therefore nothing worthy of the name of a "coral-reef" is to be found in seas so far removed from the equator as to possess a lower winter temperature than the above.
According to Darwin, coral-reefs may be divided into three principal forms - viz., Fringing reefs, Barrier-reefs, and Atolls, distinguished by the following characters :
1. Fringifig-reefs (fig. 91, 1). - These are reefs, seldom of great size, which may either surround islands, or skirt the shores of continents. These shore-reefs have no channel of any great depth intervening between them and the land, and the soundings on their seaward margin indicate that they repose upon a gently sloping surface.
2. Barrier reefs (fig. 91, 2). - These, like the preceding, may either encircle islands, or may skirt continents. They are distinguished from fringing-reefs by the fact that they occur usually at a much greater distance from land, that there intervenes a channel of deep water between them and the shore, and that soundings taken close to their seaward margin indicate enormous depths. If the barrier-reef surround an island, it is sometimes called an "encircling barrier-reef," and it constitutes with its island what is called a "lagoon island."
Fig. 91. - Structure of coral-reefs. 1. Fringing-reef; 2. Barrier-reef; 3. Atoll, a Sea-level; b Coral-reef; c Primitive land; d Portion of sea within the reef, forming a channel or lagoon.
As an example of this class of reefs may be taken the great barrier-reef on the N.E. coast of Australia, the structure of which is on a perfectly colossal scale. This reef runs, with a few breaches in its continuity, for a distance of more than a thousand miles, its average distance from the shore being between twenty and thirty miles, and the depth of the inner channel being from ten to sixty fathoms, whilst the sea outside is "profoundly deep" (in some places over 1800 feet).
3. Atolls (fig. 91, 3). - These are oval or nearly circular reefs of coral, enclosing a central expanse of water or lagoon. They seldom form complete rings, the reef being usually breached by one or more openings, which are always situated on the leeward side, or on that side which is most completely sheltered from the prevailing winds. In their structure they are identical with "encircling barrier-reefs," and differ from these only in the fact that the lagoon which they enclose does not contain an island in its centre.
If a coral-reef be observed - say a portion of an encircling barrier-reef - the following are the general phenomena which may be noticed. The general shape of the reef is triangular, presenting a steep and abrupt wall on the seaward side, and having a long and gentle slope towards the land. The outer margin of the reef is exposed to the beating of a tremendous surf, whilst the soundings taken just outside the line of breakers always indicate great depths. The longer inner slope is washed by the calm waters of the inner lagoon or channel. The reef is only very partially composed of living corals, which are found to occupy a mere strip, or zone, along the seaward margin of the reef; whilst all above this, as well as all below, is constituted by dead coral, or "coral-rock."
As to the method in which such a reef is produced, the following facts have been established:
A. The coral-producing polypes cannot exist, at levels higher than extreme low water, exposure to the sun, even for a short period, proving rapidly fatal. It follows from this that no coral-reef can be raised above the level of the sea by the efforts of its builders. The agency whereby reefs are raised above the surface of the sea is the denuding power of the breakers which constantly fall upon their outer margins. These detach large masses of dead coral, and heap them up in particular places, until an island is gradually produced. The fragments thus accumulated are compacted together by the finer detritus of the reef, and are cemented together by the percolation of water holding carbonate of lime in solution. In this way the upper surface of the reef, along a line of greater or less breadth, is more or less completely raised above the level of high water. It is obvious, however, that the reef might be entirely destroyed by a continuation of this process - the sea being quite competent to undo what it had done - unless some counteracting force were brought into play. This counteracting force is found in the vital activity of the living corals, which form the seaward margin of the reef, and which, by their growth, prevent the sea from always destroying the masses of sediment which it may have thrown up.
B. The coral - producing polypes are essentially shallow-water animals, and cannot exist at depths exceeding some 15 to 30 fathoms. It follows from this that no coral-reef can be commenced upon a sea-bottom deeper than about 30 fathoms.
The question now arises - In what way have reefs been produced, which, as we have seen, rise out of depths of 300 fathoms or more? This question has been answered by Darwin, who showed that the production of barrier-reefs and atolls was really to be ascribed to a gradual subsidence of the foundations upon which they rest. Thus, if a fringing-reef which surrounds an island be supposed gradually to sink beneath the sea, the upward growth of the corals will neutralise the downward movement of the land, so far, at any rate, that the reef will appear to be stationary, whilst it is really growing upwards. The island, however, as subsidence goes on, will gradually diminish in size, and a channel will be formed between it and the reef. If the depression should be still continued, the island will be reduced to a mere peak in the centre of a lagoon: and the reef, from a "fringing-reef," will have become converted into an "encircling barrier-reef." As the growth of the reef is chiefly vertical, the continued depression will, of course, have produced deep water all round the reef. If the subsidence be continued still further, the central peak will disappear altogether, and the reef will become a more or less complete ring surrounding a central expanse of water; thus becoming converted into an "atoll." The production, therefore, of encircling barrier-reefs and atolls is thus seen to be due to a process of subsidence of the sea-bottom. The existence, however, of fringing-reefs is only possible when the land is either slowly rising, or is stationary; and, as a matter of fact, fringing reefs are often found to be conjoined with upraised strata of Post-tertiary age. Atolls and encircling barrier-reefs, on the other hand, are not found in the vicinity of active volcanoes - regions where geology teaches us that the land is either stationary, or is undergoing slow upheaval.
C. Different portions of a coral-reef are occupied by different kinds of corals. According to Agassiz, the basement of a coral-reef is formed by a zone of massive Astraeans. These cannot flourish at depths of less than six fathoms of water, and consequently when the surface of the reef has reached this level, the Astraeafis cease to grow. Their place is now taken by Meandrinas (Brain-corals) and Pontes; but these, too, cannot extend above a certain level. Finally, the summit of the reef is formed by an aggregation of less massive corals, such as Madreporidae, Millefioridae, and Gorgonidae.