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Depth of 60° Fahrenheit.
Surface. It appears, therefore, that among the causes limiting the range of corals in depth, light and hydraulic pressure must have great influence. The proportion of atmospheric air present may be another cause. Yet, according to Darondeau, the deeper waters contain more atmospheric air, and also more carbonic acid,—the difference being as much as idoth the volume of the water. *
Quoy and Gaymard were the first authors who ascertained that reef-forming corals were confined to small depths, contrary to the account of Forster and the early navigators. The mistake of previous voyagers was a natural one, for coral reefs were proved to stand in an unfathomable ocean; yet it was from the first a mere opinion, as the fact of corals growing at such depths had never been ascertained. It is now considered altogether probable that the bottom of the ocean in its deeper parts is mostly without life of any kind. The few Caryophylliæ, and other species which are met with in deep waters, have been shewn to be sparsely scattered, mostly of small size, and nowhere form accumulations or beds.
The above-mentioned authors, who explored the Pacific in the Uranie, under D'Urville, concluded from their observations, that five or six fathoms (30 or 36 feet) limited their downward distribution. Ehrenberg, by his observations on the reefs of the Red Sea, confirmed the observations of Quoy and Gaymard; he concluded that living corals do not occur beyond six fathoms. Mr Stutchbury, after a visit to some of the Paumotus Tahiti, remarks, that the living clumps do not rise from a greater depth than 16 or 17 fathoms. I Mr Darwin, who traversed the Pacific with Captain Fitzroy, R.N., gives 20 fathoms as not too great a range, and mentions reported instances of growing reefs in 25 or even 30 fathoms. He states that in the Red Sea, according to Captain Morehead, living corals occur at 25 fathoms. At Keeling Atoll growing corals are described by him as wholly disappearing beyond 20 fathoms; and at the Maldives and Chagos at a less depth. Other facts brought forward by Mr Darwin relate to Caryophylliæ, and those species which have a wide range beyond reef-forming zoophytes.
* Examination of sea water, collected during the voyage of the Bonite.Jameson's Edin. Jour., July 1838, p. 164.-Darondeau. Observations require confirmation.
† Afterwards also in the Astrolabe.
It thus appears that all recent investigators since Quoy and Gaymard have agreed in assigning a comparatively small depth to growing corals. The observations on this point, made during the cruise of the expedition, tend to confirm this opinion. The conclusion is borne out by the fact, that soundings in the course of the various and extensive surveys afford no evidence of growing coral beyond 20 fathoms. Where the depth was 15 fathoms, coral sand and fragments were almost uniformly reported. Among the Feejee Islands the extent of coral reef grounds surveyed was many hundreds of square miles, besides the more careful examination of harbours. The reefs of the Navigator Islands were also sounded out, with others at the Society group, besides numerous coral islands; and through all these regions no evidence was obtained of corals living at a greater depth than 15 or 20 fathoms. Within the reefs west of Viti Lebu, and Vanua Lebu, the anchor of the Peacock was dropped sixty times in water from 12 to 24 fathoms deep, and in no case struck among growing corals; it usually sunk into a muddy or sandy bottom. Patches of reef were encountered at times, but they were at a less depth than 12 fathoms. By means of a drag, occasionally dropped in the same channels, some fleshy Alcyonia, and a few Hydroidea were brought up, but no reef-forming species.
Outside of the reef of Upolu corals were seen by the writer growing in 12 fathoms. Lieutenant Emmons brought up with a boat anchor a large Dendrophyllia from a depth of 14} fathoms at the Feejees; and this species was afterwards found near the surface. Dendrophyllia, it may be remembered, is one of the deep water genera.
These facts, it may be said, are only negative, as the sounding lead, especially in the manner it is thrown in surveys, would fail of giving decisive results. The character of a growing coral bed is so strongly marked in its uneven surface, its deep holes and many entangling stems, to the vexation of the surveyor, that in general the danger of mistake is small. But allowing uncertainty as great as supposed, there can be little doubt after so numerous observations over so extended regions of reefs.
The depth of the water in harbours and about shores where there is no coral confirms the view here presented. At Upolu the depth of the harbours varies generally from 12 to 20 fathoms. On the south side of this island Lieutenant Perry, off Falealili, one hundred yards from the rocky shores, found bare rocks in 18 and 19 fathoms, with no evidence of coral. There is no cause here which will explain the absence of coral, except the depth of water; for corals and coral reefs abound on most other parts of Upolu. Below Falelatai, off the same island, an equal depth was found, with no coral. Off the east cape of Falifa barbour, on the north side of Upolu, Lieutenant Emmons found no coral, although the depth was but 18 fathoms. About the outer capes of Fungasar harbour, Tutuila, there was no coral, with a depth of 15 to 20 fathoms; and a line of soundings across from cape to cape, afforded a bottom of sand and shells, in 15 to 21 fathoms. About the capes of Oafonu harbour, on the same island, there was no coral, with a depth of 15 fathoms.
Similar results were obtained about all the islands surveyed, as the charts satisfactorily shew. There is hence little room to doubt that 20 fathoms may be received as near the range in depth for reef corals; and probably the limit lies between 15 and 20 fathoms, or not far from 100 feet.
It may be here remarked, that soundings with reference to this subject are liable to be incorrectly reported by persons who have not particularly studied living zoophytes. It is of the utmost importance, in order that an observation supposed to prove the occurrence of living coral should be of any value, that it be unequivocally determined whether the fragments, which a lead may bring up, are alive or not when broken off; for a dead fragment proves nothing. Even a strong impression upon the lead, shewing the form and character of the surface cells of a coral is not wholly satisfactory, as it may have been given by a mass not living. A living fragment, placed in water, will be seen to have a fleshy surface, even if the polyps do not expand. The best observations, with reference to this subject, would be made with a diving bell.
Much yet remains for further investigation. Mr Edward Forbes, in his Zoological Explorations of the Ægean, distinguished three separate regions of invertebrate species within 20 fathoms of the surface: the first, or littoral, extending to 2 fathoms in depth; the second from 2 to 10 fathoms; the third from 10 to 20 fathoms.* Similar subdivisions, or others on the same general principle, may yet be detected in the Pacific, indicated perhaps by zoophytes as well as molluscs. There is no evidence, however, that there are successive beds, composed of a distinct set of species, as has been sometimes suggested. The upraised reefs of Metia afford no proofs of such a mode of formation; on the contrary, they shew that the process is continuous and uniform in character through the reef-growing depths. The species in the lower part of the 16 fathoms are probably different from many of those above; but they pertain to the same genera in most instances, and moreover there are no abrupt transitions, consequently the resulting reefs should have a nearly uniform character, as here stated. This fact may be better appreciated after perusing the following chapter.
The Nullipore zone along the reef-line, between low and high tide, is clearly made out by Mr Darwin, and is one of the interesting results of his investigations. It performs a very important part, by the protection it gives the reef from abrasion. The exposed reef is thus gathering lime from the waters, and extending itself; when, if devoid of this protection, it would be constantly yielding to the sea. On the inner reefs, where the protection is not needed, it is not given. Some species of Nullipore, however, occur in these regions, and others are found at various depths.
* On the Ægean Invertebrata. E. Forbes, Rep. Brit. Assoc. for 1843,
As the Caryophyllia family extend into deeper waters than most other reef corals, it might be inferred that these at least may constitute a lower bed, or substratum. But this is by no means the case. As just stated, one species (the Dendrophyllia nigrescens) was found at 14} fathoms, and also of identical characters at low tide level. The Caryophylliæ are but sparingly distributed ; the species are few, and mostly small, and not a dozen different kinds were detected in the Pacific. Their contributions to reefs are therefore inconsiderable.
4. Rate of Growth of Zoophytes. The rate of growth of zoophytes is a subject but little understood. We do not refer here to the progress of a reef in formation, which is another question complicated by many co-operating causes; but simply to the rapidity with which particular species of coral zoophytes increase in size. There is no doubt that the rate is different for different species. It is moreover probable that it corresponds with the rate of growth of other allied polyps that do not secrete lime. The rate of growth of Actiniæ might give us an approximation to the rate of growth in a Mussa, which are coral animals of like size and general characters ; for the additional function of secreting lime would not retard necessarily the maturing of the polyp; and from the rate of growth of the same animals in the young state we might perhaps draw some inferences as to the rate in polyps of corresponding size. But no observations on this point were made by us while abroad.
Although the rapidity is undoubtedly far less than was formerly reported, the following facts from different sources, seem to shew that the rate is still greater than has been of late believed. Mr Darwin, citing from a manuscript by Dr Allen of Forres, some experiments made on the east coast of Madagascar, states, that in December 1830 twenty corals were placed by this gentleman apart on a sandbank, in three feet water (low tide), and in the July following each had