Desert and water gardens of the Red Sea

by Cyril Crossland

From what has been said of the formation of the coral cups it is clear that the quantity of living matter going to the formation of these great cylinders is very small, a mere gelatinous film over the surface.

The fundamental simplicity of structure, which is common to every coral, does not preclude the evolution of an amazing variety of forms. In the course of time as many species have been evolved as there are possible combinations of the conditions, animate and inanimate, which affect coral growth and survival. In form these range from huge and solid stones, weighing many tons, to tiny delicate things like petrified lace or ferns, some of substance nearly as hard as shells, others so spongy as to be easily cut into by a knife. I have been enabled to give two plates illustrating a few out of this amazing variety. Both massive, hemispherical or dome-shaped, and more delicate branched species are shewn, but lobed growths, such as that shewn at the bottom of the first group, intergrade the two divisions. The latter specimen is of particular interest, being a species of the genus Porites, already referred to as forming great cylinders of solid stone. This small specimen was taken from shallow water, near lowest tide level, so that the polyp cups, which are too small to be visible in the photograph, were intact only on its sides. Above they were killed by the air and sun, and the stone they had formed, being exposed to the action of the sea, has been dissolved away slightly, leaving a narrow rim round the edge, where the part that was protected by living flesh shews the height the colony originally attained.

In the middle of the plate are small dome-shaped colonies, of species which, though rarely growing to the size of the Porites cylinders, may form very considerable boulders. Notice the different shapes of the polyp cups, with their radiating plates, and the varying beauty they impart to the surface of the stone, a beauty which is enhanced by examination under a lens.

Plate XXVIII

Fig. 60. Stony corals
All are forms of genus Madrepora except the lowest, which is a species of Symphyllia

Most of the branched kinds belong to one great genus Madrepora, one of the most conspicuous of all the forms seen in a living reef, and which contains a very great number of species. In spite of the wide variety of outer shape, the structure of the coral polyps is almost identical throughout this genus. The colonies may be quite small and are generally of moderate size, but one tremendous growth has been recorded, which covered an acre of the sea bottom and sent its branches of stone to the height of 50 feet. In this case a single coral equalled in size a plantation of large trees, but what is usually seen is a network of branches springing from one thick stem and spreading horizontally, and covering a fan-shaped or circular area of a square yard at most.

Not only are the growth-forms of corals varied as those of plants, but the details of the polyp cups are well worth attention. Typically the surface of the coral is covered with round depressions, which may be minute, as in Porites, or half an inch across as in Caryophyllia and the dome-shaped species shewn on [Plate XXVI]. All are partially filled up by complicated series of radial plates, and a central core, best seen in the illustration of Caryophyllia, and the arrangement and ornamentation of these form an endless variety of patterns. In other cases the depressions, instead of being round, are elongated, forming meandering grooves over the surface, which, from their superficial likeness to the convolutions of the human brain, give the name “Brain coral” to certain kinds. In others again the walls of the cups disappear, and the system is reduced to a network of plates, converging to the centres of the polyps, or these may be so thickened and flattened that the spaces between them appear as fine lines, tracing a lace-like pattern on the surface of the stone.

One is tempted to write a whole book on the beauties of corals and coral animals but must refrain; one other form is however so interesting, and at the same time so common, that a short special description is given.

In many sheltered water gardens may be seen numbers of what look like overturned, stalkless, mushrooms. On handling they are found to lie loose on the sand and to be stony corals. They are in fact single polyps of phenomenal size, being up to six inches across, and the radiating plates, which so resemble the “gills” of a mushroom (hence the name of this genus, Fungia) correspond to those already seen in the other corals illustrated. The cup wall is however absent.