The more delicate of coral skeletons, such as those of the porous branched madrepores, rarely survive the death of the polyps that formed them. On losing its coat of living flesh the coral is exposed to the action of boring animals, as well as to the direct solvent action of the sea-water, and many are thus destroyed. Partly they go back into solution, but the greater portion is broken down to mud and sand. In shallow water branched colonies are broken up into pebbles and coarse sand by the waves, and these materials serve to fill in the spaces between the larger colonies and pack the whole together into a solid mass.
There are other constituents of coral reefs of not very much less importance than the corals themselves. Large masses are formed of the bivalve shells which live in the coral mud, and which, by the solidification of this mud, form with it a limestone, such as that of which the houses of Port Sudan are built. Although among the very numerous and conspicuous fossils of this stone coral branches are not the commonest, yet the mass of shells and hardened mud is every bit as much a part of the reef as anything else is. In some reefs too, even where coral is growing abundantly, the shells of the great clam Tridacna are so abundant as to make up a considerable part of the total mass. Others again contain quantities of certain peculiar seaweeds (of which the “coralline” of British seas is one) which, though true plants in every detail, have the property of taking up limestone from the sea and forming therewith a skeleton, even harder and more compact than that of the corals. [Plate XXX] shews the appearance of these plants, and will enable the reader to identify some of those he meets with. These sometimes form a cement, by which the coral colonies and fragments are held together, and in some others the whole reef is formed of them[47]. Other organisms assist, but I observe my principle of dealing only with the most important features and desist from enumerating all.
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. Probably each of the hundreds of species of coral has its own maximum rate of growth, which is however rarely attained, as it is certain that the rate of every colony of each species varies very widely with its position on the reef and its immediate surroundings. So taking the rate of growth of a few samples would go a very little way towards giving that of the corals in any given square yard of the reef edge. Although individual colonies may grow quite rapidly this is but half the question. We must know also full details of the action of eroding and transporting sea currents, solution, boring organisms (in coral, coral sand and mud) and subtract the total from that of the deposition of stone by living polyps, to obtain the net increase.
Plate XXX
Figs. 64 and 65. Stony seaweeds, massive and branched. Lithothamnia
The boring animals mentioned as reducing coral stone to mud are very easily found and examined. Almost any old worn piece of coral, and many still living colonies, are found to be studded with small slit-like holes with slightly raised borders. On breaking into the stone each hole is found to lead at once into an oval cavity, say an inch long by three-eighths in diameter, containing a bivalve shell of about the same size, Lithodomus by name, from its appearance known as the “date shell,” which has made the hollow and is continually enlarging it. Other colonies when broken across instead of shewing pure white limestone are found to be honeycombed with yellow or red spongy matter. This is the sponge Clione, which has the property, especially astonishing in a sponge, of boring into any limestone, whether coral or shell, making it quite rotten and so, finally, reducing it to mud and sand.
Certain worms live in the same way. The largest species of these, by name Eunice siciliensis, attains a length of a yard or so, and the thickness of a quarter of an inch, but so intricate is its boring that it is practically impossible to extract a full-grown specimen entire. The head end is at the innermost part of the burrow, and when extracted the two white gouge-like teeth, by which the burrow is cut out, are easily seen.