Fig. 69
Fig. 70
Borings of molluscs and sponges
Pholas, another boring bivalve mollusc, common in shales on some British coasts, is less often seen. Its burrow is deeply buried in a solid living colony separated from the outside by a comparatively long passage. But it is not nearly so common as the preceding forms, and lives more solitary, Lithodomus generally occurring in numbers together.
After the coral has been broken down by these means, the sand is further reduced to fine mud by the action of those animals which live by burrowing in it, and passing large quantities through their guts, after the manner of earthworms. Just as there is a great fauna which lives on the nourishment filtered from large quantities of sea-water[49] so there is another great and varied community of sand eaters. There are first of all the worms, next, but more important in the tropics, great numbers of large holothurians or “sea slugs” (though slugs they are not), some of which crawl on and eat only the surface sand, but one species burrows deeply and raises casts like an earthworm, but a hundred times the size. Considering the large effects produced by the ordinary earthworm in a year, that resulting from the presence of animals hundreds of times their bulk, whose casts in many lagoons entirely cover the bottom, must be very considerable indeed.
One observation that can be made by anybody is to note how long it is before corals reappear once a reef has been cleared out, e.g. for the foundations of a quay wall. Two small portions of an apparently growing reef at Port Sudan were buried under a pile of stones for the foundations of the east and west Customs landings, and four years later there was no growth of coral on the artificial slopes thus made, though every condition apparently remains as favourable as before. Again at a point inside Dongonab Bay, where coral growth is luxuriant in shallows, the coral was some years ago collected from one spot and a sea wall built with it. A few small colonies have established themselves upon the sides of the wall after an interval of twelve years; they are perfectly healthy, yet their bulk is an infinitesimal fraction of that removed from the wall by solution and attrition. This shews how even among growing coral one cannot be sure that the degradation of rock to sand and mud is not in excess of aggradation, i.e. its building up by coral organisms, and how a lagoon may be rapidly eating away its encircling reefs and yet contain comparatively luxuriant coral gardens.
Diagram 1. Section across a Coral Reef, fringing the shore
It is in its external form that a coral reef shews features which give it an individuality above that of a mere heap of stones. Generally it rises with a steep slope from the sea bottom which ends in a low precipice, above which is another and more gentle slope to the highest point of the reef, a foot or two above lowest water, which is near its outer edge. Passing landwards the reef level is lower again, and we may have a boat channel or series of lagoons, where the native canoes can travel on calm water however the sea may rage outside. This is succeeded by a flat of bare rock, which rises slowly up to the base of the undercut coral cliffs as in imaginary section in Diagram 1 and the Photograph on [Plate XXXII].