Deep-Sea Dredging, an operation much resorted to by modern naturalists to investigate the bottom of the sea and its inhabitants. It has added vastly to our knowledge of the animal kingdom in general, of its distribution in depth, and of the important part it plays in the formation of the superficial layers of the earth. The first naturalist who appears to have made use of the dredge was Otto Frederik Muller, a Danish zoologist of the last century, who obtained by its means a large portion of the specimens described by him and figured so beautifully by his brother in the Zoologia Danica. The description of his sacculus reticularis, as he calls it, and of the alternate joys and disappointments of the dredge, is very amusing. A small figure of the instrument is given among the allegorical ornaments of the title page, and has been reproduced in Thomson's "Depths of the Sea." That same dredge was afterward purchased by Tilesius, the naturalist who accompanied Krusenstern in his journey round the world, and after his return it was deposited in the academy of St. Petersburg. It consisted of a square frame, each side being in the shape of a scraper. A bag of netting was attached to it, and four iron arms were hinged at the corners and met in a ring to which the rope was attached.
Little was heard of dredging after Muller's time until about 1838, when Dr. Robert Ball of Dublin introduced the modern dredge, commonly known as Ball's dredge. This is oblong, with only two scrapers and two arms. For dredging from a row boat 18 in. by 5 is a convenient size. The scrapers may be 3 or 4 in. wide, sharp at the forward edge, and pierced with holes at their hinder and thicker edge. To these holes the net, made of strong twine, and a protecting cover of canvas open at the bottom, are fastened with copper wire. The iron rings passing through the holes in the frame, and the iron rod running through the rings as used in recent English expeditions, appear to be unnecessary complications. The net ought to have small meshes, about half an inch to the side, and may be lined near the bottom with some open canvas to retain the smaller specimens. In the United States coast survey dredgings it was found convenient to have the net open at the bottom and to tie it up with a rope yarn; this gives greater facility for emptying it, and the numerous folds formed in the bottom of the bag have a great retentive power for small objects. A stout wire bent in a bow inside the net, and hooked by its ends to the frame, is useful in preventing the net from being turned inside out in going down.
The bag has also been made occasionally of strong canvas, with windows of copper or brass wire gauze on each side; but this construction seems to afford too much facility for washing out, unless the bag is of considerable length. Dredges have also been made of two sides of hide, laced all round with a small line; this works well on coral ground, but not elsewhere. Some of the Scandinavian and German naturalists use triangular dredges, having three scrapers and three arms. The two arms of an ordinary dredge are made each of a double or forked rod of iron, hinged on the small ends of the frame, and forming a ring at the outer end. The dredge rope passes through the ring of one of the arms only, the other arm being made fast or stopped to the first by a few turns of spun yarn. By this arrangement, in case the dredge is caught, the stop, being the weakest part, breaks first, and the rope pulling on one arm only, the dredge disengages itself and comes up endwise. For use from a steamer the dimensions of the dredge may be larger. Those used in the Porcupine expedition were 4 ft. 6 in. long by 6 in. wide.
Capt. Calver of the Porcupine, having observed that frequently objects came up adhering to the outside of the bag and even to the line, attached to the dredges some of the swabs used for washing decks. The frayed hemp swept up every rough or spiny object with which it came in contact; and among the dwellers of the deep, Crustacea, bryozoa, echinoderms, polyps, and sponges nearly all partake of those qualities. Smooth shells, and those which burrow in the sand, alone escape the " hempen tangles." In one case Prof. W. Thomson estimates that not fewer than 20,000 specimens of echinus Norvegicus came up in the tangles at one haul. These are best attached to an iron bar fixed transversely to the bottom of the bag, or in very rocky bottom they can be used without the dredge. In the expedition of the United States steamer Hassler they were successfully used attached to a long bundle of rattans, which yields when in contact with rocks, and adapts itself to the inequalities of the bottom; it is of course loaded sufficiently with lead. The salabre of the coral fishers of the Mediterranean is constructed on the same principle. It is a heavy wooden cross, carrying at the end of the arms large bunches of old tattered nets and swabs.
It is kept horizontal, and lifted and dropped alternately a number of times, thus breaking off the branches of coral which are entangled in the nets. In the recent expedition of the British ship Challenger it was found practicable to use in great depths the trawl net, with which European fishermen procure fishes living close to the bottom. It consists of a wooden beam 12 to 15 ft. long, provided at the ends with iron supports like the runners of a sled, which keep it one or two feet from the ground. A large bag net is laced to the beam by one edge of its mouth, while the other, kept down by leaden sinkers, drags on the ground. Besides fishes, many crustaceans can be procured with it, which either swim close to the bottom or rise from the sand at the approach of the beam. A miniature trawl net is used by some naturalists in small depths for the lesser Crustacea or radiata, the bag being made of bunting and the frame of hoop iron. The rope or line used in dredging must be made of the best materials, Russian or Italian hemp, and of a size to resist at times a very heavy strain. The deeper the dredge is to be used, the stronger the line must be. In the Porcupine, lines from 2 to 2 1/2 in. in circumference were used, the latter having a breaking strain of 2 1/2 tons.
In the United States coast survey, lines from 1 1/2 to 2 in., of Italian hemp, were mostly used. In dredging from a boat or a yacht depths from 100 to 200 fathoms can be reached. If the boat drifts considerably by the effect of the wind or current, it is necessary, unless the dredge is very heavy, to attach a lead to the line a few fathoms above the dredge; otherwise it is apt to be lifted clear of the bottom. The length of the line paid out is usually about twice the depth. - Deep-sea dredging, that is, dredging in more than 200 fathoms, necessitates the use of a steamer and of a donkey engine to bring up the dredge. The line passes over a large block suspended from a derrick or boom. The rope to which this block is attached passes through another block or a bull's-eye, and thence through an accumulator to the deck, where it is made fast. This accumulator is a combination of 30 or 40 strong India-rubber springs held apart by two wooden disks, and is very useful in indicating the strain on the line, which when too great can be relieved by a few turns of the engine. The line (which is marked for every hundred fathoms in the manner of a sounding line) is conveniently coiled in succession on a row of large iron pins projecting inward from the top of the bulwark.
On board the Hassler stout hooks of galvanized iron were hinged inside the bulwark in such a way that they could be turned flat against the latter when not in use. It is not usually practicable to pay out so large a surplus of line in deep casts as in more moderate ones. It is therefore well to attach a heavy weight to the line at a considerable distance above the dredge, say a fifth or a sixth of the depth; thus the traction on the dredge will be nearly horizontal. Another good practice in very deep casts is to steam slowly to windward during the descent of the dredge until the dredge rope is nearly perpendicular, and then let the vessel drift with the wind or current. The motion is thus communicated to the weight from the vessel, and the dredging performed as if from that and not from the latter. The contents of the dredge will generally be found to consist of mud or sand, out of which the specimens are to be sorted. For this a graduated set of sieves is used, constructed so as to fit in one another, the coarsest on top. The mud being placed in the latter, the whole set is moved up and down in a tub of water, after which the specimens can be selected with ease.
The best mode of preserving them is to put them in alcohol, the more delicate ones in bottles, the coarser ones in bags of muslin or bunting, which can be accumulated in larger vessels. Some very delicate objects, which are liable to lose their shape in pure alcohol, can be preserved successfully in a mixture of alcohol and glycerine. Superfluous duplicates of mollusks, Crustacea, echinoderms, corals, and sponges can be preserved by drying. Care must be taken not to place colored horny sponges in the same vessel with other specimens, as they are apt to stain everything as with ink. Abundant samples of the sand or mud ought to be preserved, as they afford an immense variety of microscopical specimens of great value. - An erroneous opinion prevails that the late American and English expeditions first revealed the existence of an abundant deep-sea fauna. The late Edward Forbes asserted that life becomes less and less as we descend, until it is almost or utterly extinguished. Guided by his own researches in the Mediterranean, he placed the limit of life at the very moderate depth of 200 to 300 fathoms. Norwegian fishermen, from time immemorial, have taken certain fishes in depths of 100 to 300 fathoms.
Some of these are quite large, as the halibut and the ling, and some have never been obtained in lesser depths. On the coast of Portugal also the fishermen of Setubal fish in similar depths for a peculiar shark, using lines of 500 fathoms. It is evident that where such fishes can live they must find abundant food, and this was proved by the contents of their stomachs, and by the specimens occasionally brought up on the hooks, but mostly rejected with superstitious dread by the fishermen. The Scandinavian naturalists Loven, Sars, Danielssen, Koren, and others, made extensive researches by means of the dredge in depths of 200 to 300 fathoms, bringing to light a great variety of animal forms peculiar to that region. Sir John Ross relates in his "Voyage," published in 1819, that in the preceding year he obtained a sounding in Baffin Bay in 1,000 fathoms, and brought up with the lead mud containing worms, and a living star fish (astrophyton) attached to the line. Sir James C. Ross, in his voyage to the antarctic regions in 1839-'43, obtained with the dredge in 270 fathoms "an abundance and great variety of animal life." He says : " Contrary to the general belief of naturalists, I have no doubt that from however great a depth we may be able to bring up the mud and stones of the bed of the ocean, we shall find them teeming with animal life.
The extreme pressure at the greatest depth does not appear to affect these creatures; hitherto we have not been able to determine this point beyond 1,000 fathoms, but from that depth several small fish have been brought up with the mud." At a later time evidences of the existence of animal life at great depths began to accumulate. Eh-renberg showed in 1851 that foraminifera from a depth of 2,000 fathoms still contained unchanged animal matter. Prof. Bailey of West Point confirmed these observations, although differing in the explanation of the fact. In 1853 the soundings of the United States coast survey revealed for the first time the great foraminifera bed which almost unmixed covers a large extent of the ocean floor. In a sample of scarcely two cubic inches, from a depth of 510 fathoms, Mr. Pourtales found specimens or recognizable fragments of at least 48 species, 20 of them mollusks. Dr. Wallich, accompanying the sounding expedition of Sir L. McClintock to Greenland, obtained living animals from depths of 1,000 to 2,500 fathoms, among them star fishes of considerable size. Doubt was still thrown on these results from the fact that some foraminifera are found floating near the surface, and it was even objected that the star fishes might have been swimming when entangled in the line.
No such objections could be made to the observations of Dr. Allman and A. Milne-Edwards, who found oysters and other shellfish, worms, and corals on a telegraph cable raised from a depth of 1,000 to 1,400 fathoms in the Mediterranean; or to the results of the Swedish expedition to Spitzbergen, under Thorell, who obtained, partly by dredging and partly by means of the Bulldog machine, extensive collections from depths down to 1,400 fathoms. In 1868 Prof. Sars of Christiania published a list of 427 species of animals living in depths of 200 to 300 and even as much as 450 fathoms. In the United States, dredging up to a recent date has received but little attention. The most successful worker in that field has been the late Dr. W. Stimpson, who however confined himself to quite moderate depths. In 1867 Professor Peirce, superintendent of the coast survey, commissioned Mr. Pourtales, one of his assistants, to make some dredgings in connection with the sounding out of a track for the submarine telegraph between Key West, Florida, and Havana. Although but a few casts were obtained in that year, and the depths reached did not exceed 500 fathoms, the results were so promising that dredgings were carried out systematically over the region comprised between the Florida reef and the axis of the straits of Florida, with occasional extensions to the coast of Cuba and the Bahama banks, during the two following years.
This is probably at this time the only example of a connected dredging survey of a defined submarine region. The results established several well defined zones characterized by peculiar faunas, the animals comprising them being generally new discoveries, or in several cases known only from high northern latitudes, thus bearing out in part Loven's theory that beyond a certain depth the ocean is peopled by a fauna extending from pole to pole. In these explorations the greatest depths reached were about 700 fathoms. One of the most notable examples of an animal found in Florida and off the northern coast of Norway is the little crinoid rhizocrinus Lofottensis, discovered the year previous by Sars. This seems to have been one of the principal incentives in determining the fitting out of the deep-sea dredging expeditions of the Lightning and Porcupine by the British government, under the direction of Profs. W. Thomson and W. B. Carpenter. The first cruise in the Lightning was made in 1868, between the British and the Faroe islands, with very promising results.
The next year the Porcupine was employed on the same service, off the W. and S. W. coasts of Ireland. The depth of 2,435 fathoms was attained by the dredge, which brought up quite a variety of living animals, ranging from mol-lusks and Crustacea downward. During the following summer the same researches were extended across the bay of Biscay, along the coast of Portugal, and into the Mediterranean, with great success generally, though in the latter sea the greater depths were found singularly barren, probably on account of the want of circulation in the water. (See Atlantic Ocean.) The British government was induced by the general interest which these researches had created to fit out a large ship, the Challenger, for a cruise of deep-sea exploration on a large scale, which in 1872 started, under the scientific leadership of Prof. W. Thomson, on a voyage of circumnavigation. In the earlier part of the voyage, in the Atlantic, the dredge was used with success in the depth of 3,895 fathoms. A few deep-sea casts were also made during the voyage of the United States steamer Hassler from Boston to San Francisco in 1872, but did not attain so great a depth.
In 1871 the United States fishery commission, directed by Prof. S. F. Baird, made extensive explorations of the sea bottom on the coast of New England, Prof. Verrill and Dr. Packard having charge of the dredging operations. Deep-sea dredgings were made off St. George's bank and in the gulfs of Maine comprised between Cape Cod and Cape Sable. The fauna, as might have been expected, was entirely arctic, with the exception of a single specimen of a small coral, deltocyathus, a West Indian deep-sea form, the presence of which north of Florida had not been detected before. - Dredging in deep water has greatly increased our knowledge of the animal world, and precisely at a time when the need of such a knowledge is most keenly felt. The great bed of foraminifera has already been mentioned. A gelatinous matter pervades this bed in most localities, which under the microscope seems to be endowed with a kind of motion or change of form, and has thence been raised to the dignity of a living organism under the name of bathybius Hoeckelii by Huxley. Its nature is really as yet but imperfectly understood.
The same may be said of small calcareous bodies which have been named coccoliths. (See Ba-THYBIUS, and Coccoliths and Coccospheres.) The sponges are represented by a number of forms, having generally a beautiful silicious skeleton. Then come a considerable number of small corals, generally simple. The echino-derms are abundant and peculiar; among them the most notable are the crinoids. The rhizocrinus Lofottensis has been found in the straits of Florida and in the seas of Europe. Another species has been recently discovered in the West Indies by the Hassler expedition, in company with two species of pentacrinus and the remarkable holopus. Another pentacrinus and bathycrinus gracilis belong to the European seas. The sea urchins or echinidoe of great depths are very remarkable, and among them several genera are represented much more nearly related to forms of the cretaceous or early tertiary period than to any recent ones. Of the mollusks we know less than of the other classes; partly because the collections of the American dredging expeditions were lost in the great fire of Chicago, and partly because the English collections have not yet been fully worked up.
What we know at present points to the same conclusions as derived from other branches of the animal kingdom. - Deep-sea researches are also extending the science of geology, particularly as it refers to sedimentary rocks. The dredgings off the Florida reef brought up large fragments of limestone rock in process of formation, and of different grades of compactness, formed by the various mollusks, bryozoa, serpulae, corals, foraminifera, etc, still living on its surface. On other parts of our southern coast we have the beds of foraminifera gradually undergoing transformation into glauconite or greensand, such as we find in the tertiary formation of New Jersey. This process, as yet unexplained, takes place in comparatively limited localities, while the great bed of foraminifera becomes compacted without alteration into a formation almost identical with the white chalk; and the continuity of that formation to the present time has been advocated by Huxley, Thomson, and others. A considerable proportion of the deep-sea foraminifera are un-distinguishable specifically from those of the chalk; but all the higher forms inhabiting the deep sea, polyps, echinoderms, Crustacea, and mollusks, are with one or two doubtful exceptions only generically related to those of the cretaceous and tertiary periods.
It may here be remarked that but little distinction has heretofore been made by geologists between formations deposited in deep and in shallow water, and that we are unduly struck by resemblances brought up suddenly before our eyes by the revelations of the dredge. Deductions will therefore not be safe until we acquire a fuller knowledge of the bathymetrical relations of fossil forms, so that we can compare those of a particular level with those of the corresponding level of the present seas.