The Open Sea: The World of Plankton. Alister Hardy

that the tow-net was first used in 1844 by the German naturalist Johannes Müller, and I have myself been guilty of repeating this error. It is certain that Müller’s researches excited the scientific world and led many others to follow him; but our own great amateur naturalist J. Vaughan Thompson, when serving as an army surgeon in Ireland, was using a tow-net to collect plankton from the sea off Cork as early as 1828. It was there that he first described the zoëa, the young planktonic stage of the crab. A little later, 1833, he discovered the true nature of the barnacles and so solved an age-long puzzle. These enigmatic creatures, fixed to rocks or the bottoms of ships, had been thought to be aberrant molluscs. Thompson caught little undoubted crustaceans in his tow-net and found that they settled down to be transformed into barnacles. His classical discoveries were described in privately printed memoirs which he published in Cork; they are among the rarest items of biological literature. He showed that the plankton consisted not only of little creatures permanently afloat, but also of the young stages—larvae, as the scientist calls them—of many bottom-living animals; these latter more sedentary forms throw up their young in clouds to be distributed far and wide by the ocean currents, just as many plants scatter their seeds in the wind for the same purpose. Charles Darwin also used a tow-net before Müller, on his famous voyage in the Beagle; in his Journal of Researches (1845) under the date of 6 December 1833 he writes: “During our different passages south of the Plata I often towed astern a net made of bunting and thus caught many curious animals.” Today many forget that our famous T. H. Huxley, champion of Darwinism, began his career as did Darwin before him, as a great field naturalist; in 1846 he sailed for the South Seas as surgeon in H.M.S. Rattlesnake and by his use of the tow-net laid the foundations of our knowledge of those remarkable composite jellyfish-like animals, the siphonophores, which we will later discuss (see here).

      Another simple device, the naturalists’ dredge—a coarse netting bag on a rectangular iron frame—dropped and dragged along the bottom of the sea revealed another new world of life. It was first used by two Italian zoologists, Marsigli and Donati, in the middle of the eighteenth century, but it was another of our own great marine naturalists, Edward Forbes, who became the leading pioneer in this work; he began his dredging in 1840, both in British waters and in the Aegean Sea.

      How deep in the sea can life exist? This became the subject of much controversy among scientists following the discoveries made by the use of an ingenious device invented by just a boy—a brilliant young midshipman in the U.S. Navy—J. M. Brook. He hit on the idea of attaching a quill to the sounding lead used in plumbing the ocean depths and so bringing to light a sample of the ooze from the bottom into which it had penetrated. It gave only a tiny sample—but how exciting! That was in 1854, and soon from all over the Atlantic basin, from any depth over 1000 fathoms, came samples of oozy sediment containing minute calcareous shells. These were shells of animals belonging to the group of the Protozoa (single-celled animals) known as Foraminifera and nearly all belonging to one genus, called Globigerina on account of the spherical form of their shells. This form of deposit has consequently become known as Globigerina ooze. Did the creatures which made the shells actually live at these great depths, or did the shells fall from near the surface when their floating owners died? That was the problem. It is amusing for us now to recall that most of those who held the latter and correct view did so on quite false grounds: they believed that it would be quite impossible for life to exist at these great depths and that therefore the shells must have fallen from above. A drawing of a living Globigerina is shown in Plate 2.

      Edward Forbes had considered there was what he called a zero of life at about 300 fathoms—a boundary below which no life could stand the great pressure of the depths. This fallacy was soon to be exposed. The laying of submarine cables was just beginning. In the Mediterranean one of these after a little use had parted and was hauled up for repair in 1858; it came up encrusted with bottom-living animals, some of them at points on the cable which must have lain at a depth of over 1000 fathoms. Once it is pointed out, the truth of the matter seems obvious: an aquatic animal should feel no ill effects of pressure provided it has no spaces or bubbles filled with air or gas inside it. All liquids are only very slightly compressible. A body made up of fluid or semi-fluid protoplasm, and covered with a flexible or elastic skin, will contract only very slightly even under the greatest pressure; its contents too will be of course at the same pressure as the surrounding water. With the stresses inside and outside the body perfectly balanced in this way, the animal can have a most delicate structure and make the finest movements just as well in the great depths as can one living nearer the surface. Even the seemingly rigid armour-platings of such animals as crabs are in fact made up of a number of parts separated from one another by thinner flexible joints, so that changes of pressure are equalised inside and out; the same applies to the starfish and sea-urchins, whose armour is actually not strictly on the outside of the body, but just below the skin.

Simple as the explanation seems to us now, the discovery of these animals living under great pressure came as a real surprise to most people. This was all the more extraordinary, for actually there was in existence a thoroughly attested instance of a remarkable starfishlike animal (one of the Gorgonocephalidae with branching arms) being brought to the surface from a depth of 800 fathoms; it came up entangled round a sounding line on Sir John Ross’s expedition to Baffin Bay in 1818. It had been forgotten or overlooked by the naturalists of a later generation, who also did not appreciate the significance of the dredgings reported by his even more famous nephew Sir James Clark Ross. Accompanied by the young Joseph Hooker, he had made a number of rich hauls from depths down to 400 fathoms during those great south polar voyages in the Erebus and Terror from 1839 to 1843. Unfortunately these important deep-sea collections, which contained marine invertebrate animals in great variety, were subsequently lost to science.¹

      The waters immediately to the north and west of the British Isles may perhaps be regarded as the cradle of oceanography; they became the scene of the pioneer deep-sea dredging expeditions in the naval surveying ships Porcupine and Lightning led by Dr. W. B. Carpenter and Professor (later Sir) Wyville Thomson. During the summers of 1868–70 they made nearly 200 dredge hauls over a wide area and reached a depth of 2,435 fathoms; as far down as they went they revealed a wealth of life and opened up a new world to the naturalist. Thomson’s great book The Depths of the Sea (1873) is still fascinating reading. It was their remarkable discoveries, together with the interest taken in the new venture of laying transoceanic cables and the consequent need for a more accurate knowledge of the ocean floor, that led in 1872 to the dispatch by the British Government of H.M.S. Challenger on her famous expedition; under the leadership of Sir Wyville Thomson she sailed on a three and a half years’ voyage to explore all the oceans of the world. The results of this magnificent venture filled more than 50 large volumes with a wealth of information not only of the life of the ocean and of the nature of the sea-floor as revealed by tow-net and dredge, but also about the physics and chemistry of the sea at different depths. Oceanography as an organised branch of science had come into being. Other nations followed the example of the Challenger and sent out similar expeditions.

Having mentioned The Depths of the Sea I must also refer to another great book of similar title which I believe will always be a classic in the literature of Oceanography: The Depths of the Ocean by Sir John Murray and Professor Johan Hjort, published in 1912. Murray was on the Challenger with Wyville Thomson and later, when Thomson’s health failed, directed the Challenger Office, seeing to the completion of all the work and the editing of the great series of Reports; Hjort, who died as recently as 1948, was the great Norwegian marine biologist and Director of his country’s fisheries research. I shall be referring to this book again, particularly in Chapter 12, for it contains the results of a very successful expedition which the two authors made in 1910 in the Norwegian research ship Michael Sars to study the deepwater life of the North Atlantic. I draw attention to it here, however, because it is also a splendid introduction to our science in general, with a valuable chapter on the early history.²

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