Popular Science Monthly/Volume 60/April 1902/Contributions to Biology from Investigations on the Breeding Salmon
| CONTRIBUTIONS TO BIOLOGY FROM INVESTIGATIONS ON THE BREEDING SALMON. |
By YANDELL HENDERSON, Ph.D.,
YALE UNIVERSITY.
IN science as in history, it often happens that facts bare and dry in themselves are infused with a sympathetic human interest, when viewed as events in the lives of men. Such an interest, focused in a single devoted investigator, and intensified by the pathos of his death, attaches to the name of Friedrich Miescher, late professor of physiology in Basel. In 181)5 by more than twenty years of laborious investigation, he had brought to a successful conclusion one of the most brilliant and important researches ever attempted in biological chemistry. Up to that time few and incomplete reports of these results had appeared in print, the personal modesty and scientific caution of the investigator tempting him to delay publication. So it happened that the great mass of his researches was recorded in notes which only their author could decipher, when Miescher was seized by a mortal disease, and after a lingering illness, with the chapters describing his work completed in his brain, but with no strength to transfer them to paper, he died in the bitterness of a life work completed, but unpublished.
It may indeed be doubted whether the value of Miescher's work would have received full recognition twenty years ago, since many of his investigations then completed bear on problems which had formed themselves in the minds of few biologists of that time. To-day, however, these problems are of universal interest. The recent publication of two volumes[1] containing the few papers which Miescher had published, both on the salmon and on other subjects (which by themselves would entitle him to a high place among physiologists), together with extracts from his letters to his colleagues describing his work, and such of his notes as could be utilized, dates Miescher's work from the present time rather than the period of its performance.
For a proper understanding of the extraordinary tissue changes which he discovered in the breeding salmon, Miescher found it necessary to investigate fully the life and habits of these fish during their sojourn in fresh water. Of this subject little more was known than such practical knowledge as fishermen had developed. The position of Basel close to the head waters of the Rhine, the breeding ground of the salmon, gave Miescher peculiar opportunities for this investigation. The salmon spawn during the latter half of November and the early part of December, or just before the shallows of the lakes and streams, where the eggs are deposited, are frozen over. These eggs are hatched by the warmth of the next spring. During the following year the young salmon live in the lakes and streams, and when a year old and from seven to nine centimeters in length swim down the river and out to sea. These facts were easily established. To determine the probable period of the salmons' return to fresh water was more difficult. By measurements of more than two thousand of the fish caught at Basel, together with statistics of those taken in the lower reaches of the Rhine and in other rivers of Germany and Sweden, Miescher found that the greater number both of males and females could be classed under one or other of three sizes. Contrary to what might have been expected the most numerous of these classes is that of the largest and oldest fish, while the least numerous is that of the small 'St. Jacobs' salmon. Unless it be supposed that some of the younger fish go to other rivers, which is unlikely, it is evident that not all the salmon of the proper age participate in every migration to fresh water. These considerations and others, based on the probable rate of growth of the fish, led to the conclusion that from two to three years after entering the ocean, many of the males and a few of the females make their first journey up the river to spawn. This accomplished, the fish return to salt water for another period of about three years. Then occurs a second migration to fresh water by a large proportion of both sexes. The males of this migration average seventy centimeters in length, the females seventy-six centimeters. Probably all of the salmon that have survived the dangers of marine life unite in the third migration. The average males of this age have attained a length of eighty-two centimeters, the females ninety centimeters. These differences in size Miescher regarded as the result of another period of life and growth in the ocean, between the second and third migrations, of at least two years. As only a few salmon of ninety-eight to a hundred centimeters length are taken, and these are all males, it is probable that the females as a rule make only two journeys to the spawning grounds during their lives, while the males ascend the river three or even four times. Within considerable limits of error the probable age of the largest salmon taken in the Rhine is therefore fourteen years.
By a careful comparison of the statistics of the number of the salmon taken in Holland and at Basel, Miescher found that any marked increase in the catch in the lower reaches of the river was regularly followed, after an interval of eight or nine weeks, by the appearance of an unusually large number of the fish in Switzerland. Basel is five hundred miles from the mouth of the Rhine, and if the average current were only twenty-five miles a day, the fish must make their way through two thousand miles of water. In fact, however, the Rhine is throughout much of its course a swift flowing river, leaving Switzerland at a speed of four or five miles an hour; and the spawning grounds of the salmon lie above the falls and rapids south of Basel. Indeed even these considerations can scarcely be taken as a measure of the physical exertions of the salmon, since the swifter the current the greater becomes their activity. The heaviest runs of the salmon occur during June and July, although many fish whose condition shows them fresh from the sea are taken at Basel even during January. All these fish remain near the head of the river through the spawning season, and in the following December unite in a headlong rush back to salt water. The average duration of their stay in the Rhine is therefore from six to nine months, while in some cases as many as fifteen months must elapse from the day the salmon enter the mouth of the river until their return to the sea.
In view of these facts it would seem almost beyond belief, had not Miescher established it by absolutely complete demonstration, that the salmon never feed in fresh water. From the day they leave salt water until they return to it they maintain an absolutely unbroken fast. Careful examination of more than three hundred fish caught at Basel at all periods of the year, and of many taken in the lower reaches of the river just after the fish had left the sea, showed that not only was the alimentary canal empty of all food material, but the digestive apparatus was in no condition to handle nutriment even if offered it. The gastric mucosa was in a more or less desquamated condition, and alkaline in reaction. The gall bladder was empty, and the pancreas shrunken. Only two exceptions were noted, and these more apparent than real. In the stomach of one fish was found a large winged insect—quite undigested; in another a minnow—only partially digested. Of the latter case, however, Miescher records that the fish was caught near Basel in January, long after the spawning season, and was so extremely emaciated as to suggest that it must have been prevented in some way from escaping back to the sea.
The development of the genitalia (or sexual glands), occurring almost wholly after the salmon have entered fresh water, becomes of the greatest interest in the light of these observations; for the material built up in these organs must be drawn from the other tissues of the animal itself. To the problems involved in these tissue changes, as well as to those resulting from the expenditure of energy by the fish in their long journey up stream, Miescher devoted himself especially. His examinations and analyses of the salmon caught in the lower reaches of the Rhine at all periods of the year, and of those taken at Basel from January to May, show that the fish on their way up stream are always in a well nourished condition. The intestines are surrounded by masses of fat; the muscles in all parts of the body are full and firm, and exhibit innumerable globules of oil within and between the fibers. The sexes are practically indistinguishable. The genitalia amount to only three-tenths of one per cent, of the weight of the animals. During the months from May to December a gradual but steady change takes place. The jaws of the males develop a beak three to five centimeters in length. The skin of both sexes changes color, losing much of its lustar, and becomes loose. The fat about the intestines disappears first; then that contained in the muscles. Certain muscles of the back, which are less important in swimming, diminish to nearly half their original size; and their content of solids to an even greater extent. The muscles most important in swimming, however, are maintained in full vigor; and even in those drawn upon, it is significant that no loss of structure occurs; for on the salmon's return to the sea, this material can be replaced without a reconstruction of the tissue. Just before spawning, the average weight of both sexes is ten per cent, less than that of fish of equal length (eliminating the difference due to the growth of the jaws of the male) caught in May. In contrast with these changes is the growth of the genitalia. The testes or spermaries of the 'ripe' males amount to as much as six per cent, of their body weight, and the ovaries or roe of the females to twenty or even twenty-five per cent., and contain thirty per cent, or more of the total solids of the fish.
While the available information on the subject is as yet by no means so complete as is desirable, on the whole it indicates that the habits of the salmon in other parts of the world, and of other species, are at least similar to those of the Rhine. It is the belief of many of the guides in Maine and New Brunswick that the salmon in their streams do not feed after leaving salt water, and that the 'fly' used to catch them must appeal to their curiosity rather than to their appetite.[2] The salmon in the rivers of Scotland certainly resemble closely those of the Rhine according to a recent and very thorough investigation[3] by the Fishery Board of that country. The salmon in the rivers of Alaska, however, acccording to the report upon the subject by the United States Fish Commission,[4] exhibit certain differences which are significant of the physiological purpose of the habits and tissue changes in the breeding salmon. The principal runs of these salmon from the sea to their spawning grounds occur after the ice has broken up, and through a great part of the brief Alaskan summer. The number of fish swimming up stream at such times is so enormous that at places the current is almost choked by the struggling mass. A man wading through the shallows can kick the salmon out upon the shore by scores. The distances which these salmon must ascend is much less than in the Rhine—often only a few miles. Yet the difficulties overcome in struggling over shoals, leaping up waterfalls, and in attempting to pass the barriers of heavy timber recently erected by the canneries, are enormous. During this journey the males develop formidable looking beaks, and the genitalia grow to a size even greater perhaps than do those of the Rhine salmon—although exact figures are not available. At the same time both sexes become emaciated to an extreme degree, all the muscles and organs of the body being drawn upon apparently to supply material to the genitalia. It is certain that the fish do not feed after leaving salt water; indeed it is doubtful whether the beaks of the males would allow it even if they wished. But all the Alaska salmon—of which there are several species—differ from those of the Rhine in one respect. No adult salmon has ever been seen swimming down stream, and those which are washed down by the current die after reaching salt water. The greater number after spawning remain, near the spot where the eggs are deposited, driving off intruding salmon that would disturb their nests, and marauding trout that would devour their eggs, until overcome by starvation and the exhaustion entailed by their journey and tissue changes, they die. They afford a striking example certainly of the sacrifice of the individual to the good—or the only 'good' that Nature seems to recognize—the perpetuation of the species; and this example is none the less striking because it can scarcely be supposed that these fish have any consciousness of the object for which they thus struggle and die.
The reason for these habits and tissue changes is probably to be found in. the advantages which they confer upon the salmon in the struggle for existence. The risks to which the eggs are exposed are indeed enormous. Many fail of fertilization; many are devoured or otherwise destroyed. Nor do the dangers to which the salmon are subject end with their hatching. Even if the growing fish escape the voracity of the trout in their natal streams, a host of enemies await them in the ocean. If the number of salmon from year to year remains fairly constant, it is evident that the chances for any one egg surviving to become a mature fish, to replace one of its parents and in turn have offspring, would be represented by two, while the chances of destruction would equal nearly the total number of eggs produced by the female, which amount to many thousand. Yet compared to the dangers which a deep-sea fish like the cod must escape, the hatching and early life of the salmon in the lakes and streams probably offer great advantages. Were the salmon to remain in fresh water, however, where the food supply is limited, their numbers would be at least as limited as are the trout. The same explanation probably holds good for their fasting in fresh water. Were they to feed during the spawning season they would leave nothing for the newly hatched fish,—and indeed the spawn of the preceding year and their own eggs would form, as in the case of other fish, a large part of their diet.
In the salmon nature offers to science, on a scale far exceeding the resources of any laboratory, an experiment in the metabolism of hunger—a demonstration that the energy liberated within the animal body comes not directly from the combustion of the carbonaceous substances of the food, as the energy of a steam engine from its fuel, but from the breaking down of the tissues themselves. Furthermore the conditions are reduced to their simplest terms. In the warm-blooded animals the maintenance of a temperature many degrees above their surroundings, necessitates a continual drain on the potential energy stored in their tissues. In the cold-blooded salmon, on the other hand, lying quietly, for weeks or months together, between the stones on the bottom of the Rhine, the processes of oxidation sink to a minimum, involving little else than the movements of the gills and the beating of the heart. Thus the energy latent in the fats, carbohydrates and albuminous substances, which the fish brings from the sea, is utilized almost wholly in the contractions of the muscles; and within the limits of the dynamic efficiency of these tissues this force is expended in the mechanical work of swimming. For physiologists the especial importance of this experiment lies in its bearing on the recent revival, in modified form, of the theory of Liebig, that the heat of the body is maintained by a combustion of the fats, but that the albuminous substances or proteids are the source of muscular work. Unfortunately the data upon this subject collected by Miescher are only partially available. The conclusions, however, at which he arrived, are supported by the thorough investigation of Dr. Noël Paton and his coworkers of the Scottish Fishery Board on the salmon of their rivers. They have determined the amount of the various fuel materials which disappear from the bodies of the salmon during their journey up stream. For this purpose they have analyzed all the tissues and organs of numerous salmon taken from the estuaries in the spring and early summer, just as the fish were starting up the rivers. They also found the amount of fat and albuminous material remaining in the tissues of the fish taken from the head waters of the rivers, a month or so later. By calculating from the numerical results of these analyses the equivalent figures for a fish of 'standard length'—arbitrarily taken at one meter—a fairly accurate basis of comparison was obtained. On this basis the difference in the composition of the tissues of the fish of the estuaries, and those of the head waters, reveals the number of grams of fat and albuminous material which the journey up stream costs the fish. The results show that, varying with the length of their journey—which in no case approaches the distance up the Rhine—the swiftness of the current overcome, and the other exertions necessary, the salmon in passing from the estuaries to the head waters, expend three hundred to six hundred grams of fat, and only sixty to a hundred and twenty grams of albuminous material. In the animal body the combustion of the fats and sugars is complete. They leave the system in the form of carbonic acid and water, after liberating within the tissues precisely that amount of energy which they would yield as heat, if burned in a perfect lamp or the most accurately constructed calorimeter. For the albuminous substances the combustion is less complete; but the amount of energy which comes from each gram decomposed within the body is determinable with no less accuracy than for the fats. To find the energy which the salmon expend in the ascent of the Scottish rivers, it is only necessary, therefore, to multiply the amount of fuel material expended by the number of calories which one gram of fat or albuminous material yields in a calorimeter. Thus it is found that the same six hundred grams of fat and one hundred and twenty grams of proteid, which the journey up the longer rivers costs the salmon, would heat sixty-five liters of water from the freezing point to boiling, or, to express the same amount in terms of mechanical work—would, if theoretical conditions in this regard were attainable, lift fourteen kilos (the weight of the 'standard fish' of the Scottish investigations) to the height of a hundred and eighty kilometers. But it must be borne in mind that the dynamic efficiency of few engines devised by man exceeds fifteen per cent.; and the investigations of physiologists have shown that the contracting muscles develop an efficiency only five to ten per cent, greater. From such data as these, together with the rate of flow of rivers, their length, and the time consumed by the salmon in accomplishing the distance, it would be easy to reach fairly exact conclusions in terms of distance traveled, weights transported and fuel expended by the salmon, and to compare these results with those accomplished by a steamship; or to calculate the resistance of the water overcome by the salmon, and from this compare the relative advantages of the 'lines' of a salmon and those of a racing yacht. There can be little doubt that in the first case the advantages would be heavily on the side of the salmon, and that the yacht would show little if any superiority. But leaving aside such theoretical considerations—for which, it is only just to say. Dr. Noël Paton and his coworkers are in no way responsible—the investigations on the Scottish salmon show that eighty to ninety per cent, of the energy liberated by them in the muscular work of swimming is derived from the fats. The hungering salmon, like a hungering man or dog, reduces to a minimum the waste of protoplasm—that peculiar jelly of albuminous substances which constitutes the chemical framework and essential mechanism of the living cells of the body. In the salmon ascending a river, as in a man ascending a mountain, the energy liberated in the work done is supplied by a vigorous oxidation, and this is evidenced by an increased absorption of oxygen and excretion of carbonic acid. The elimination of nitrogenous substances from the waste of the tissue proteids is, however, only slightly increased.
No less interesting are the processes by which the genitalia develop, since they afford an example of constructive activity almost without parallel among animals; processes so characteristic of plants, on the contrary, that they were long supposed to exhibit the generic differences in the vital mechanism in the plant and animal kingdoms. Modern research has, indeed, shown that these great apparent differences are matters of degree, not kind. Plants can not now be considered as devoted solely to absorbing carbonic acid, and by means of the heat and light of the sun synthesizing carbonaceous material. They can, and when need arises, they do draw on their store of fuel, exhale carbonic acid, and even liberate measurable quantities of heat. On the other hand, physiologists have come to admit that the cells of the animal body, although wholly dependent on the vegetable kingdom for their materials and energy, yet possess wide powers of transforming the food substances to their needs. Uncertainty has, however, attended the efforts of the investigator of metabolism in man and the higher animals. Generally when the subject of the experiment fasts, growth stops. If on the other hand the subject is fed, the origin of the substances shown to appear or increase in any tissue—for instance the fats—may be assigned with almost equal chances to any one of the constituents of the food, or to a transportation from other tissues of the body itself. In the salmon, on the contrary, the conditions are of extreme simplicity and clearness. The constituents of the genitalia are essentially different chemical compounds from the substances of the muscles out of which they are manufactured; yet the fact that their formation takes place wholly during the period of the animals' fast, leaves no other source for them. In the synthesis of these complex organic substances, the phosphorus—to mention only one element—can be traced with certainty back to the simple phosphates stored in the muscles of the salmon of the estuaries.
The mechanism by which this material is transported and transformed was the subject of much careful investigation by Miescher. lie noted a marked increase in the functional activity of the spleen—an organ as enigmatic in the salmon as in man. He observed also a decrease in the blood supply to those muscles at the expense of which the genitalia grow, and an equal increase to the genitalia themselves. The special significance of these conditions lies in the fact that a very small blood supply, continued through the summer and autumn, would transport many times the amount of material which actually becomes a part of the genitalia. As these organs are not motile, and are apparently the seat of no marked oxidation, their respiratory needs would appear small. Nature, however, follows closely the principle of 'least action'; and Miescher advanced the theory that, aside from its part in respiration, the oxygen brought to the tissues by the blood exercises a tonic influence upon their constructive processes. Thus in the liquefying muscle the lack of oxygen causes a stoppage and reversal of nutrition. The cell contents are absorbed into the blood. In the genitalia on the other hand the excess of oxygen in which the cells are bathed stimulates their nutritional processes, and results in vigorous growth. This view is, of course, a pure hypothesis. Yet it is interesting on account of its close similarity to theories advanced now, a generation later, by pathologists to explain the causes of abnormal growths in the tissues of man.
Important as are the results of the study of the salmon thus far mentioned, none rank in value with Miescher's investigations on the chemical nature of the sperm and ova. These researches were practically the first in which there was an attempt to lay bare the chemical processes involved in fertilization and the formation of the embryo. The clue which Miescher furnished has been followed by others, until to-day we seem to be approaching a determination of the structure of the proteid molecule—the first step necessary to solving the problem of the chemistry of living matter. Modern histology has shown that the fertilization of the ovum, from which the animal body develops, consists essentially in the entrance of the nucleus of the spermatozoon or male cell, and its fusion with the nucleus of the ovum. The nucleus of the spermatozoon must therefore be regarded as the carrier potentially of those characters physical and mental which the individual inherits from its male parent; as the nucleus of the ovum must be for the female parent. In the series of divisions by which the fertilized ovum separates into the cells which form the tissues of the body, the material or chromatin of the fused nuclei is with the utmost exactness divided equally to each, so that every unit of the system receives its share of this chemical endowment. This much the study of structure has taught us. But to the chemist the ultimate explanation of these processes seems to lie in the nature of the chemical substances composing the chromatin. In the salmon the ova and sperm are easily obtainable, and afford the unfertilized eggs and spermatozoa almost free from admixture with other matter. Accordingly the methods which Miescher employed for the separation of the constituents of these cells were simple, yet yielded the material sought in quantity and purity. The nuclei from the sperm proved to be composed mainly of a peculiar compound or series of compounds of the nature of ethereal salts. The acid radical was nucleic acid—a substance previously discovered by Miescher in the white corpuscles of the blood of mammals; the other component was an organic base which he called protamin. To the question as it presented itself to Miescher's mind, whether protamin may be regarded as the essential element in fertilization, the answer must be in the negative. Miescher found that protamin is not present in the sperm of the frog or the testes of a bull; and subsequent investigators have shown that it is absent even in some fishes. Yet in these cases it is apparently replaced, and the same functions performed by another substance of basic character known as histon. Miescher was led by these facts to the view that there is no one substance which can be regarded as the essential element in fertilization; but that the chemical processes involved are probably a series of reactions.
To attempt even an outline of the investigations and discoveries to which Miescher's study of the chemistry of the cell has led, would necessitate reviewing one of the most fertile fields of biological chemistry, and would involve much that is important in pathology as well. rt is sufficient here to point out that what has been accomplished is in great measure the ripening fruit of the seed planted by Miescher in the quiet of his laboratory a generation past. In the final solution of the profounder biological problems, these investigations on the salmon and the researches to which they have led will probably be found to have contributed in large measure toward that object, which Miescher in one of his letters—almost the last before his death—expressed in the words—"There remains then this great question to be fought out by the biologists of the future—Is it chemical composition or cell structure to which we must look as the ultimate basis of vital phenomena?"
- ↑ Friedrich Miescher, 'Histochemische und Physiologische Arbeiten'; Leipzig, F. C. W. Vogel, 1897.
- ↑ During the past summer Dr. C. W. Green, of the University of Missouri, has been carrying out investigations in conjunction with the U. S. Fish Commission on the salmon of the rivers of the Pacific coast. As these researches have not yet been published the writer is personally indebted to Dr. Green for this statement: "Concerning the question whether or not the 'King' salmon takes food in its run up the Sacramento, I have reached the tentative conclusion that it does not, in fact can not. Every salmon examined by me at the U. S. Fish Hatchery at Baird, Cal., and I examined many, not only had no food in the stomach and intestine but these organs were so much atrophied that only the smallest object could have been swallowed. Salmon a meter in length have an intestine not larger than a small lead pencil, and the stomach is reduced to less than seven centimeters in length. On the other hand in the stomach of one salmon taken direct from salt water at Monterey I counted eighteen squid and several small fish"
- ↑ Report of the Scottish Fishery Board on the Investigations on the Life History of the Salmon in Fresh Water, from the Research Laboratory of the Royal College of Physicians of Edinburgh; Edited by D. Noël Paton, M.D., 1898.
- ↑ Bulletin of the U. S. Fish Commission, Volume XVIII, 1898.