Читать книгу The Open Sea: The World of Plankton - Alister Hardy - Страница 11

THE ANIMALS of the plankton are by definition those which are passively carried along drifting with the moving waters; those other inhabitants of the open sea which are powerful enough to swim in any direction—the fish, whales, porpoises and the squids or cuttlefish—are in contrast referred to as the nekton (see here). The vertebrates therefore, except for certain primitive relations, will only be represented in the plankton by the floating eggs offish and the young fish themselves up to the time when they become strong enough to migrate at will instead of being just helplessly transported.

In spite of this limitation, and the absence of insects, I believe it is no exaggeration to say that in the plankton we may find an assemblage of animals more diverse and more comprehensive than is to be seen in any other realm of life. Every major phylum of the animal kingdom is represented, if not as adults, then as larval stages with the partial exception of the sponges; the sponges do indeed send up free-swimming larvae but they are in the plankton for so short a time that they can only be claimed as very temporary components of it. In no other field can a naturalist get so wide a zoological education and in few others will he find a more fascinating array of adaptational devices.

It is this great variety of forms, and the unexpected finds which are always turning up, which make hunting in the plankton such an exciting occupation. Except for the jelly-fish and some of the larger crustacea, it is of course hunting with a lens. Nearly every member of the zoo-plankton can be seen with a ×6 hand-lens or a simple dissecting microscope, and the most effective searching can be done with these. Before transferring any specimen to a slide for examination under the more powerful compound microscope, it is well to watch it for a time swimming in its own characteristic way in a small glass dish under the simple magnifier. To anyone who has never seen this life before it is difficult to convey in words a picture of the delights in store for him. I am indeed lucky to have the privilege of having my account illustrated and enriched by the beautiful photographs of living plankton animals taken through the microscope by my friend Dr. Douglas Wilson of the Plymouth Laboratory; they are quite unique and many of them have been taken by that remarkable new device, the electronic flash, which has for the first time made the photomicrography of such small and rapidly moving creatures possible. The naturalist will soon forget the absence of the insects in the wealth of variously shaped and often beautifully coloured crustaceans which are to be seen swimming rapidly in all directions. Tiny pulsating medusae—miniature jellyfish—swim into view; and here and there can be seen the transparent arrow-worms Sagitta which remain poised motionless for a time and then dart forward at lightning speed to capture some small crustacean. Then there may be delicate comb-jellies propelling themselves by rows of beating iridiscent comb-like plates and trailing long tentacles behind them. These comb-jellies and the arrow-worms belong to two phyla—i.e. major groups of the animal kingdom—which are found nowhere else but in the marine plankton. There are many different kinds of Protozoa, among which one order (the Radiolaria) is also entirely planktonic. The segmented worms may be represented by beautiful pelagic polychaetes and the molluscs by the so-called sea-butterflies (pteropods) which are really small snail-like animals with the foot drawn out into wing-like extensions to assist in their swimming and support. Most of these animals are permanent members of the plankton, spending all the stages of their life-histories drifting in the open sea; in addition there are, however, a vast number of the young, or larvae, of the bottom-living invertebrates which ascend to live for a time in the plankton and so distribute the species far and wide. These temporary members present us with some of the most striking adaptations to this floating life. Some of them are nearly always to be found in a tow-net sample from our surrounding seas which have such a rich fauna on their floor. Group by group—flatworms, segmented worms, different kinds of polyzoa, starfish, sea-urchins and, of course, the bottom-living crustacea—each has its own characteristic way of solving the problems of pelagic life. The plankton indeed presents a paradise for the student of invertebrate development; we shall devote a special chapter (Chapter 10) to a consideration of these larval forms.

Hitherto only a small minority of amateur naturalists have shared the delights of exploring the living plankton. Preserved samples, such as are often obtainable from marine laboratories for examination, are certainly full of interest; they can, however, never give the observer the same satisfaction as seeing this teeming world all alive. The professional marine biologist, engaged in investigating the relationship between plankton distribution and the fisheries, finds it very tantalizing to be able only very rarely to find time to stop and look at his captures before he must kill them; he travels to and fro across the sea taking as many samples at intervals as he can, in order to get the most comprehensive picture of conditions in the time available. Usually he only just has time to deal with the concentration, labelling and preservation of one set of collections before the ship arrives at the next position where another set must be taken; for the sake of understanding the fisheries he must always hurry on. In the past the amateur has often had an even more disappointing experience: having obtained a tow-net and hired a boat to take him out in the bay, he has returned home only to find that the wonderful sample of plankton he collected is now just a mass of dead or dying creatures crowded together at the bottom of the jar. Two modern inventions have altered all this: the Thermos flask and the refrigerator. If you have a Thermos flask, or preferably two, you can go to the sea, travel back by train for several hours and still have your plankton alive; if you have a refrigerator, or know a kind neighbour who will allow you to keep one or two 4 lb or 7 lb preserving jars in his, then you can keep your animals healthy for several days to be studied at your leisure.

I believe there are a great many people—and not only those who would call themselves naturalists—who would like to see something of this strange planktonic world, or show it to their children, if only they knew how. Anyone who goes to the sea can catch plankton quite simply. Those who can take a yachting cruise are particularly fortunate; they can study the changes in the plankton as they move from one area to another, can see the difference between the animals at the surface at night and in the daytime, and can try and find out just what organisms are making the flashing lights around their vessel in the darkness. Those, however, who can only take out a rowing boat may make very good collections, especially if there is water from the open ocean bathing their coast. If there is a pier sticking out into the sea and sufficient tidal current, as there usually is at some time of the day, quite good samples may be obtained by streaming out a net on a line and allowing it to fish for a quarter of an hour or so. Some may even think this preferable to a boat if the sea is a bit choppy! If you can only collect from a pier, or from a confined area in a rowing boat, you need not be too envious of your friends in the yacht, for fortunately the water is always on the move; a sample taken at the pier today may be very different from one taken only a few days ago and quite different again from one you may get next week. I have taken very good samples from some of the many piers built out to receive the steamers plying in the Firth of Clyde area.

To help those who do not know how to proceed I will give a few instructions. It is a good thing to have at least two Thermos flasks, so that you can keep at least two different plankton samples separate from one another. If you can manage it, it will be an advantage to start out with one of your flasks filled with sea-water that has stood in a jar in the refrigerator over night. Half of this you can pour into the other thermos just before you add the plankton sample collected. Thus in each flask the animals will be added to sea-water that has been chilled; it will keep them cool, inactive and in good condition whilst they are brought home. Details of how to make and use a tow-net have already been given in Chapter 3. The net of very fine gauze suitable for collecting the small plants will also at the same time catch the very small animals, particularly the protozoa and small larval forms. For the capture of most of the zooplankton a coarser net having some 60 meshes to the inch is the most useful. If more of some of the larger animals are required, for example the larger crustacea and medusae, a still wider mesh net, say 25 meshes to the inch, should be used; this will filter much more water but let nearly all the smaller animals escape. The three nets of 200, 60 and 25 meshes to the inch will provide a very good equipment. Remember, as stressed in Chapter 3, to tow slowly, at a speed of not more than 1½ knots. It is best to tow only for short periods—not more than five minutes at a time—which can be repeated if too small a sample has been collected. If the plankton is very abundant a longer haul will give you much too much so that all the little animals will be far too crowded together to live healthily for more than a very short time. If you have too thick a sample, pour a lot of it away and only take home in your flask a small part of it, diluted as much as possible with more sea-water. It seems hard to pour most of it back, but you will be sure to have sufficient of the commoner kinds and a few kept in good shape will be better than a great many in poor condition.

I must now give some idea of the actual numbers of animals you may expect to get. Here I gave the figures for the diatoms and dinoflagellates taken in two 14-inch diameter tow-nets hauled for half a mile across the bay at Port Erin in the Isle of Man; they were averages for several hauls a week during the month of April over a period of fourteen years. For comparison I now give in the accompanying table the corresponding figures from the same source (Johnston, Scott and Chadwick, 1924) for the more important elements of the zooplankton in the same series of hauls.

The corresponding average totals for the months of June, August and October were 39,105, 38,812 and 35,631 respectively. Since it was calculated that approximately 8 cubic metres of water were filtered by the nets during towing, this gives an average of about 4,500 animals per cubic metre or some 120 per cubic foot of sea-water during the summer months. It must be remembered that these figures are averages and that individual samples may vary enormously from week to week. For comparison it may be interesting to give the average figures for the total plants of the plankton—the diatoms and dinoflagellates—recorded from the same series of net hauls for the four months April, June, August and October; they are in round figures 5,815,000, 6,674,000, 107,000 and 485,000 respectively. It must be remembered, however, that there will have been much larger numbers of the still smaller plants, the tiny flagellates referred to here, which will have passed through the meshes of the net and so not been recorded. To give the number per cubic metre we must again divide by 8.

If you have time, and the sea is calm enough, you should pour your plankton haul into a dish and examine it with a pocket lens as soon as it comes up; then with a wide-mouthed pipette you can pick out from it into another jar some of the rarer animals that you particularly want to study. After that you can more light-heartedly pour away most of the sample before putting the remainder, together with the rarities you have picked out, into your Thermos for transport home. The most useful dish from which to pick out specimens is one of the large oblong photographic dishes made of white porcelain and used for washing whole-plate negatives; half of the bottom of this can be covered with black paper so that you have a contrast of backgrounds to enable you to see both the darker and lighter forms more easily. All the jars, dishes and pipettes you use for living plankton must be kept thoroughly clean and never be used for samples that have been preserved with formalin or other chemical fixatives. These small animals are delicate in constitution as well as in form.

The majority of plankton animals tend to come up towards the surface at night and sink down into the deeper layers during the day (Chapter 11). Very rich samples of plankton may be collected by simply towing the net just below the surface at night; in the daytime, however, if you are over deep water you may have to send your net down to 15 to 20 fathoms to get a good haul. To reach this depth you will require a good length of line—50 to 60 fathoms—and you will also require a much heavier weight, say a 20 lb lead, to take your net and all this line down. Care must be taken, of course, to know just how deep the sea is at the point where you are working so as not to run the risk of trawling the bottom with your net and either bursting it by filling it with mud or tearing it to ribbons by dragging it over a rough bottom. If you have not a chart you can consult, you should take a sounding with your lead and line before starting.

It is often very interesting to take a series of samples from different depths at the same place as near together in time as possible to enable you to study the depth distribution of the various animals; if you repeat the series again at night you may be very surprised at the different results you will get. As you let the net run out on its line to a deeper level it will fish very little on its way down, for it is moving backwards with the water as it runs out and sinks; when you haul it up, however, at the end of a tow, it will of course fish all the way up. This difficulty is got over in modern oceanographic practice by having in front of the net a special closing mechanism which is operated by a brass messenger weight sent sliding down the cable; this releases the bridles when a trigger is struck and the net falls back to be closed and held by a throttling noose which passes round it behind the mouth. The net is thus hauled up to the surface closed like a sponge-bag with the strings drawn tight and you know that all the animals in it must have been caught at the actual level at which it was fishing. A simple example of this arrangement is shown opposite in Fig. 18. These devices, however, are perhaps rather elaborate to be practised by the amateur, especially as a smooth steel cable is required for the messenger and this means the use of a winch; they will not be further described but full information about them will be found in the descriptions of the equipment used on the Discovery Expeditions (Kemp and Hardy, 1929). To minimise the effect of catching whilst hauling up an open net, it is well to make rather longer hauls with it down; the time taken in coming up will then only be a small fraction of that during which the net was fishing at its proper level.

If you are going to take a number of such hauls for study you will soon accumulate far more material than you can hope to keep alive successfully; in this case it will be best to keep only a small part of one or two samples fresh and preserve the rest for study dead. The living plankton will give you the greatest pleasure in studying the swimming movements and behaviour of the animals; the dead samples may nevertheless give you interesting information about the depth distribution of the same animals, which you could not otherwise obtain. The best general preservative for plankton, and the easiest to use, is formalin, i.e. a 40% solution of formaldehyde in water; this, which can easily be obtained at any chemist, can be added to the sample in quite small quantities to give a mixture (about 5% formaldehyde) strong enough to keep it indefinitely in good condition. Remember always to reserve separate jars for preserved samples—never mix them with those used for fresh; a good plan is to stick a red label on them for danger! The dead formalined samples can of course be concentrated into a smaller space; 1lb honey jars with screw-on tops are convenient for their storage. If you are going to keep the samples for any length of time it is well to use what is called neutral formalin, i.e. that to which just sufficient borax—from 5 to 10 grams per litre—has been added to neutralise its acidity; ordinary formalin nearly always contains formic acid which if not so neutralised will very soon dissolve away the calcareous shells and skeletons of many of the animals.

FIG. 18

Sketches of a simple release mechanism for closing the mouth of a tow-net before hauling it to the surface. A, the rig of the net when towed; B, enlarged view of release gear about to be struck by messenger weight; C, the towing bridles released and the net closed by throttling rope.

What has just been said will have been sufficient to have corrected that very common misconception that the plankton exists almost entirely near the surface of the sea. Some people seem to have thought of it as existing as a kind of scum on the very surface itself; this is no doubt due to a misunderstanding of the expression often used that the plankton is the ‘floating life’ of the sea. The plants, as we have already seen, do in fact only flourish for a little way below the surface; but animal members may be found at all depths. Later on—in Chapter 12—we shall describe the plankton and nekton that is to be found at various levels in the ocean between the surface and the bottom, thousands of fathoms deep beyond the continental shelf. There is another erroneous impression about the plankton that is frequently held: the idea that it is more or less evenly distributed over quite wide stretches of the sea; it is often thought that if we used a tow-net in one place and another two or three miles away on the same day, the two samples would be almost exactly alike. This indeed may occur, but it is by no means always so.

A great many surveys have been made in the past, often in relation to some fishery problem, attempting to give some idea of the varying quantities of the major plankton organisms over a particular area. I have already described how a research ship will proceed in such a survey to traverse the area, stopping or slowing down to take tow-net samples at regular intervals. If the area to be covered is a big one, the stations—as the different points of observation are termed—cannot be very close together or the survey would take much too long; they are frequently spaced twenty miles apart. It has usually been assumed that a sample at one point, will, within a reasonable range of error, give a fair representation of the plankton in the area for ten miles around it; thus it has been felt that a series of such stations twenty miles apart will give an adequate quantitative survey. Some plankton organisms are much more patchy in their distribution than others; for some kinds such a method may give quite an adequate picture, but for others it may be hopelessly misleading. Very early in my career as a marine naturalist I had an experience which I will recall because it so well illustrates this very point; it was an episode which had a marked effect on much of my later work. In 1921, soon after leaving the University, I was appointed as Assistant Naturalist on the staff of the Fisheries Laboratory of the Ministry of Agriculture and Fisheries at Lowestoft and was delighted to be allowed to study the plankton in relation to herring. In March of the following year, through the illness of a senior, I found myself, at the last moment of sailing, as naturalist in charge of a cruise on that grand old research trawler the George Bligh.