1. A method is described for studying the responses of Daphnia to changes of light intensity with special attention to the behaviour of the individual and to the avoidance of “shock” effects. The types of apparatus used provide for rigid control of the temperature, for illumination from any direction, and for an adjustable rate of change of the light intensity by means of a chemical rheostat.

  2. The great majority of Daphnia magna and Daphnia pulex were found to be primarily negatively phototropic and positively geotropic. That is, they always exhibited those tropistic signs under constant conditions of illumination.

  3. A reduction of the light intensity causes a temporary reversal of the tropism signs. The secondary signs thus produced are positive phototropism and negative geotropism.

  4. The presence of both phototropic and geotropic forces is proved by experiments in which illumination is (1) from one side, (2) from beneath, and (3) from two opposing sides or from above and below simultaneously. In these tests and in others in which very slow and very fast rates of dimming are used the phototropic and geotropic forces are resolved, antagonised, and neutralised in succession. The responses of the Daphnia indicate that there are two types of animals which exhibit exactly the same tropisms, but in one type phototropism is the stronger while in. the other geotropism is the stronger.

  5. In this material it was found that the temporary secondary tropistic signs persisted only a few minutes while the primary signs persisted for hours, although this effect was somewhat less marked in weak light or in darkness.

  6. The difference between “time-change” and “place-change” of light intensity is pointed out. Daphnia is stimulated by both types of change if the rate of change is sufficiently great.

  7. That photosensitive animals are stimulated to respond to changes in the intensity of light only and are merely orientated by the direction of the light is shown in the work of previous, investigators as well as in this paper. The rigidity of this mechanism is indicated by experiments in which the light is graded in intensity at right angles to its direction and in which the light is rendered converging and diverging by a lens.

  8. Evidence is given for believing that there is no “absolute optimum” light intensity for Daphnia but that a “relative optimum” exists which is the intensity to which the animals are adapted at the moment.

  9. The interval between the inception of the reduction of the light intensity and the beginning of swimming movements in response is called the latent period. The faster the rate of dimming, the shorter is the duration of the latent period. A minimum, amount of intensity change is required to produce any response, at any speed, but beyond that the slower the rate of dimming, the greater is the amount of change required and hence the lower is the absolute intensity at which the response takes place. Ordinarily, the response is maximal in respect to both rate and magnitude.

  10. Fatigue will interfere with experimentation unless guarded against.

  11. Specimens of Daphnia with reversed primary signs gain temporary secondary signs following an increase of light intensity ; otherwise they behave like the more usual forms.

  12. The possibility that the processes of adaptation in Daphnia may account for the photic responses observed is discussed. Support for this theory is derived from the fact that it is possible to dim the light over a given range at such a slow rate that no response is produced.

The importance, as well as the difficulty, of applying the results of laboratory experiments of this type upon the responses of Daphnia to the general problem of the behaviour of plankton animals in vertical diurnal migration is stressed.

The following series of experiments was undertaken in order to obtain further information in regard to the part played by light in the diurnal migrations of planktonic organisms. It is the opinion of many investigators (e.g. Russell, 1926, 1927 a) that light is one of the most important factors regulating the vertical position of the plankton. One method of attacking this problem is observational; another is experimental. In the former, observations upon the vertical distribution of the plankton at different times of day are made by means of tow nets. Then as much information as possible is obtained on the light intensities at those depths and at those times of day, and attempts are made to correlate the movements of the plankton with the changes in light intensity. In the experimental method, on the other hand, plankton animals are brought into the laboratory where the environment can be carefully controlled. Here they are subjected to different conditions of illumination, and the resulting movements of the animals may be studied. It is in the belief that investigations of this experimental type can be devised in such a way as to reveal the fundamental mechanisms of phototropism in particular and of vertical migration in general that I have attempted the work about to be described. Although neither my experiments nor those of previous investigators (cf. Parker, 1902; Ewald, 1910; Dice, 1914; and Rose, 1925) are sufficiently large either in number or in scope for generalisations to be made, they indicate the type of problem susceptible to experimental attack.

In selecting the material and method for these experiments, two primary objects were kept in view. The one was to make a study of the behaviour of the individual. In this way it was hoped to reach a truer understanding of the basis of light responses and to do so more directly than by drawing deductions from averages of swarms of Daphnia (cf. Esterly, 1919, p. 80). For most of the experiments one animal was used at a time and the same tests were repeated many times with the same individual. The other object was to avoid the effect of “shock as much as possible in handling the Daphnia (cf. Esterly, 1919, pp. 66 and 77). This was accomplished by employing laboratory-raised animals which therefore had been accustomed to aquarium conditions all their lives1. Unless otherwise stated, the experiments which follow were performed on adult specimens of Daphnia magna reared in the laboratory. It was found, however, that wild specimens of Daphnia pulex reacted to light substantially in the same manner.

The type of apparatus designed for the more careful control of the environment consisted essentially of a long glass tube filled with tap water, kept at constant temperature by a water jacket, and illuminated at one end by a Sheringham Daylight Lamp (150 watts). The glass tube was 6-3 cm. in diameter and 96 cm. long. It was sealed at the end towards the light with a strong piece of plate glass and closed at the other end by a rubber bung provided with a disc of black ground glass to prevent reflection. Inside this experiment tube two thermometers were secured by means of copper wire coated with insoluble “Luc” cellulose paint in such a way that they registered the temperature at the ends of the tube. The water jacket consisted of a cylindrical museum jar 17 cm. in diameter and 96 cm. long and provided with a tap at each end. Against the bottom of the museum jar inside was placed a piece of black ground glass, while the top end was provided with a thick plate glass cover held firmly in place by six screw clamps. When a rubber washer coated with vaseline was placed between the glass cover and the lip of the museum jar, the apparatus was found to be quite water tight and could be used in either a vertical or a horizontal position. The water which was circulated through the water jacket was piped from the laboratory tap. Its temperature could be kept remarkably constant at any desired level by passing the tap water through a copper vessel heated by a gas flame which was regulated by a thermostat device in the usual manner. The temperature of the water within the experiment tube remained for hours within a degree or two of its original value and there was rarely a difference of more than 0·15° C. between one end of the tube and the other. The position of the Daphnia was noted every half minute by taking readings in centimetres from the bottom on the scale marked on the outside of the water jacket opposite the experiment tube. A red lamp, too dull to affect the movements of the Daphnia and placed upon an adjustable slide behind the apparatus, was employed to make the animal and the calibration visible. The whole apparatus-was set up in a dark room, but experiments could be carried on in the lighted laboratory by enclosing the museum jar and contents in a light-proof box with a sliding front. A peep-hole cut in this movable front was adjusted precisely opposite the red lamp and the two were made to move together in following the movements of the Daphnia.

The light intensity of the Sheringham Daylight Lamp could be reduced or increased by means of a chemical rheostat. This consisted of a tank of tap water through which the current was made to flow by passing between two zinc electrodes. These electrodes were thin triangular plates of zinc with dimensions of 45 × 45 × 10 cm. and held 0·5 cm. apart. Two additional tanks, one above and one below, connected to the central tank by glass tubing fitted with stop-cocks, made it possible to fill the central tank or to empty it at any desired speed. When the central tank was full of tap water1, it added almost no resistance to the electric circuit ; but as the water was drained away the light became dimmer and dimmer until the resistance caused by the reduced electrode area resulted in stopping the flow of electricity altogether. Accordingly, the experimenter could obtain a steady change of light intensity at any speed he wished by merely adjusting the stopcocks. The resistance of the water could be diminished by adding small quantities of washing soda. The light intensity was measured by an A.C. ammeter in the electric circuit which had been calibrated to show candle power for the daylight lamp used. It was found possible to cut the zinc plates in such a shape that the rate of change of light intensity was linear. It is true that the composition of the light changed as it was dimmed : at low intensity values there was noticeably more red in the light emitted by the lamp, but this slight change of colour did not appear to interfere with the general results of the experiments in any way2. A skylight provided with a light-proof door immediately over the apparatus made it possible to use daylight instead of electric light when required. In this case dimming could be produced only by shutting the door of the skylight, but the same general results were obtained as with artificial light (cf. Yerkes, 1900). It should be noted that in both cases all ultra-violet light was excluded by the glass through which the rays of light must pass. Ultra-violet light of wave-length shorter than 3341 A.U. has been shown to be specific for causing negative phototropism in Daphnia pulex (Moore, 1912), but it is doubtful whether enough ultra-violet light reaches planktonic animals in nature to affect their vertical movements.

A second type of apparatus was designed particularly to permit lighting from any direction. It consisted of a glass battery jar (30 cm. long, 20 cm. wide, and 30 cm. high) placed upon brackets screwed to the wall with an electric light fixture on each side, above, and below. Either one or two lights in any position could be used. Sixty-watt bulbs were employed for the most part and dimming was accomplished by means of the same chemical rheostat.

The usual method of procedure was as follows. A healthy specimen of Daphnia magna was selected and transferred to the experiment tube by means of a large pipette. The water in this tube was tap water which had been allowed to stand for at least several hours. This precaution permitted the escape of the bubbles of excess gas which would otherwise form upon the appendages of the Daphnia and prevent it from swimming normally. Moreover, it is of the utmost importance that the animal be subject to no sudden changes of temperature. Hence by allowing the tube water to reach room temperature before the transfer of the Daphnia was made, “shock” effects could be avoided and any desired lower or higher temperature could be reached gradually after the tube had been placed in its water jacket. If these precautions were carefully observed, experimentation could begin almost immediately—otherwise several hours of waiting were necessary before the organism would behave normally. No change in the behaviour of Daphnia was observed for experiments carried out at temperatures of 8° to 18° C., but elaborate tests of the effects of temperature change were not made. No provision for the aeration of the water confined within the experiment tube was necessary since the volume of water was so comparatively large. Daphnia would live sealed up inside for a week or more and even produce broods of young, but ordinarily no animal was used for more than two or three days at a time.

The great majority of the Daphnia used were primarily negatively phototropic and positively geotropic. That is, these animals swam away from any light regardless of its intensity and swam or sank to the bottom of their container in any light or in darkness. It will be shown that the signs of the tropisms may be temporarily changed experimentally, the term primary sign of phototropism or of geotropism signifying that sign which the organism always exhibits under constant conditions. There was always a minority of animals which exhibited unusual primary phototropic and geotropic signs, and I have also observed a few cases of reversal of primary sign. For example, one animal seemed to be permanently positive to light but a week later it became primarily negative to light. Other specimens appeared to be indifferent to light or to gravity or to exhibit rapid changes of sign for no apparent reason, but such forms were exceptional. When Daphniapulex was used, it was found that the same primary signs existed (i.e. negative phototropism and positive geotropism), although some investigators have found their experimental animals to be permanently positively phototropic (e.g. Yerkes, 1903) or to be positive to weak light and negative to strong light (Dice, 1914). But whatever variation in tropisms there may be, we shall deal here only with that type of Daphnia, by far the most numerous in my material, which is primarily negatively phototropic and positively geotropic under any constant conditions of illumination.

The hypothesis which seems best to explain the results of the experiments described below is that changes in the light intensity induce changes in the sign of the tropisms. Such changes of tropism signs have been observed by previous investigators (cf. Ewald, 1910; Frisch und Kupelwieser, 1913; Dice, 1914; and Mast, 1921). The Daphnia under consideration are primarily negatively phototropic and positively geotropic. Following a reduction of the light intensity, the phototropism becomes positive and the geotropism becomes negative. These changes of sign are, however, only temporary, and the temporary signs will be called the secondary signs. For soon after the light intensity becomes constant, the phototropism and the geotropism regain their original (primary) signs. An increase in light intensity strengthens the primary tropisms, or, if the secondary tropisms are still operative, the return to the primary signs is hastened. Fig. 1 shows the movements of a normal animal resulting from such changes in tropism signs when the illumination is from above. While the light intensity remains constant, the Daphnia is to be found close to the bottom as a result of its primary negative phototropism and positive geotropism. When the light is dimmed and the change to the secondary tropism signs occurs, the animal is stimulated to swim to the top of the tube (see A, Fig. 1). But soon after the light intensity is again steady, the return to the primary signs occurs and the Daphnia goes back to the bottom (see B, Fig. 1). Increasing the light intensity to its original value now produces no further effect since the animal is already at the end of the tube (see C, Fig. 1), but if the light is made bright again while the animal is still at the top, the return to primary signs, hastened by the increase of light intensity, can be demonstrated (see D, Fig. 1). When the tube and light are employed in a horizontal position, the same responses are observed, but now the movements occur in a horizontal plane. Following a dimming of the light, the Daphnia swims to the end of the tube towards the light source, and after a short time or when the light is made bright again, it returns to the far end of the tube. These reactions are phototropic only since geotropism, although present, obviously cannot act when the experiment tube is horizontally placed.

To show that both phototropic and geotropic forces are present, experiments were performed using the battery jar apparatus in which the Daphnia were free to move in any direction. In this case illumination was provided by the Sheringham Daylight Lamp directed horizontally as shown in Fig. 2 a. The large size of the reflector of the lamp made it certain that the light entering the water was uniform in intensity and evenly distributed over the whole surface of the exposed side of the tank. As long as the light intensity was maintained constant, the Daphnia were always found at corner (1)—the result of their primary negative phototropism and positive geotropism. When the light was dimmed, however, the tropism signs were reversed : the phototropism became positive and the geotropism became negative. As a result the animals swam to corner (3). Soon after the light intensity again became constant, the phototropic and geotropic responses regained their original primary signs, and consequently the Daphnia returned to corner (1)1. The path taken from (1) to (3) depends of course upon the relative strengths and speeds of the two forces acting upon each animal. The positive phototropism tending to cause the organism to move towards the light usually acts sooner than the negative geotropism producing the upward movement. Course B is followed by those animals in which the positive phototropism is much stronger and faster in its effect. Course A is followed by those in which these forces are about equal, or more frequently, in which the geotropic reaction is the stronger. The curves of Course A upward and downward, although slight, are in the same direction as in Course B. This is because in Course A the phototropic effect is felt first and masks to some extent the fact that the geotropic response is really stronger as shown by the next experiment.

Fig. 2.

The arrow indicates the direction of the light.

Fig. 2.

The arrow indicates the direction of the light.

That these differences in relative strengths do in fact exist is substantiated by experiments in which the light source is placed below the tank. The phototropic and geotropic forces are now directly antagonised. Those animals (about three-eighths of the total number) in which the phototropism is stronger than the geotropism (these followed Course B in Fig. 2,a) will be found at the top of the tank, because their phototropism is primarily negative, while those specimens in which the geotropism is stronger than the phototropism (these followed Course A in Fig. 2,a) will stay near the bottom because their geotropism is primarily positive. If the light intensity is now reduced, the signs of both the phototropism and the geotropism are changed. The result is that those Daphnia which were at the top swim to the bottom, and those which were at the bottom swim to the top. And, as we should expect, increasing the light sends both types back to their original positions (see Fig. 2 b).

The situation is represented in Table I. In each case the stronger tropism is shown in heavy print :

Table I.
graphic
graphic

When two opposing lights are thus employed, all phototropic effects are neutralised and phototropism may be considered as absent. Since geotropism alone remains as an effective force, all the Daphnia move directly upward when the light is dimmed (secondary negative geotropism) and directly downward when it is made bright again (return to primary positive geotropism) (see Fig. 3 a). In Case a2 there is no diagonal course as there was when one horizontal light was used; in Case a2 the two types of Daphnia are not separated out as they were when the lower light alone was used.

It is possible to separate the phototropic responses and the geotropic responses in yet other ways. When the rate of dimming the light is made very slow, the geotropic response disappears. This is shown in Fig. 3 b., Case b1, below. The light was dimmed at such a slow rate that the animal was never stimulated to leave the bottom. In Case b2 the light was switched out. Accordingly, the Daphnia underwent an instantaneous change from bright illumination to complete darkness. Since there was no light present to orientate the animal when it began to swim, a phototropic response could not occur. The result was that the animal swam directly upwards, and then back to the bottom when the light was switched on again. The geotropic response alone was effective.

As we have seen, the Daphnia respond to a dimming of the light by swimming upwards, and towards the light source. Soon after dimming has ceased and the intensity is held at a constant low value, the primary signs of the tropisms are regained and consequently the Daphnia swim to the bottom of their container and move as far away from the light source as possible. If the intensity of the light is very low, these responses are weak, but although not as marked as with stronger illumination, a negative phototropism and a positive geotropism are always regained. If the light is extinguished, phototropism necessarily disappears, but the fact that the majority of animals returned to the bottom even in complete darkness shows that geotropism is present and positive as before. Fig. 3 c expresses these facts in diagrammatic form.

The effect of light upon geotropism in various planktonic organisms has been studied by many investigators. Attention is called particularly to the work of Dice (1914) on Daphnia pulex. Following Experiment 12, “Persistence of Negative Geotaxis in Darkness,” in which data are given to show a persistence of 4I hours’ duration, Dice says: “We have shown that in Daphnia pulex increase of light intensity causes a tendency toward positive geotaxis, while decrease of intensity causes a tendency toward negative geotaxis. This tendency seems to be stronger the greater the change in intensity. It seems also that these tendencies are persistent for a considerable length of time.” And in the summary he says: “The diurnal movements of Daphnia pulex are caused chiefly by variations in geotaxis induced by changes in light intensity.” My results agree with Dice’s as far as the effects of increased and decreased illumination are concerned, but I never observed the secondary negative geotropism to persist for more than a few minutes. It is true that in my material the fraction of the total number of animals which did not return to the bottom was larger in D. pulex than in D. magna, yet with both forms I found that the majority of animals always returned to the bottom in light or in darkness. Hence it is doubtful how far this photo-geotropic effect will be found to explain the diurnal migrations of plankton in general.

Enough evidence has now been given to establish the fact that Daphnia is stimulated by a change in light intensity. It is to be noted next that this change in intensity may occur in two different ways. The organism may be subjected to a change in time or to a change in space. If the Daphnia is stationary in one place and the source of light is dimmed, then the animal experiences a “time-change”— the light intensity at its particular position in the water changes with time. This is the type of light intensity change we have been considering thus far, and, as we have seen, the animal is caused to move by such a stimulus. But if the intensity of the illumination is held constant, and the animal swims about, then as the animal moves to or from the light source or swims into shadows or bright regions, it experiences a “place-change” in the light intensity. Whether or not Daphnia can perceive and is stimulated by low gradient “place-changes” is a matter for discussion. It is hard to believe that “time-change” and “place-change” do not come to the same thing. If an animal swims from a region of high intensity to a region of low intensity, why does it not receive the same stimulus as if the light is dimmed in time over the same range? But in the experiments described, the Daphnia do not appear to be stimulated by the change in intensity which they must experience in swimming from one end of the tube to the other. Following a reduction of the light, the Daphnia swims towards the light source. After a short time at a constant low intensity the animal swims back again to the far end of the tube, as we have seen. Why is it that upon reaching this region of still lower intensity, the animal is not stimulated again to seek the light? It may be because the animal is never able to swim fast enough to produce an effective change of intensity (see “Rate of Change” below). The fact is that the animal ordinarily remains quietly at the far end of the tube. This matter needs further investigation and careful measurements of the rates of change involved.

But if an abrupt “place-change” of light intensity is encountered, the Daphnia will receive a stimulus. Experiment 35 will serve as one example of this (see Fig. 4a). Following a dimming of the light, the Daphnia swims diagonally upwards. The abrupt decrease in light encountered at Q speeds up the reaction while the abrupt increase in light at P holds the response in check for some time. The same phenomena are observed during the return trip following a brightening of the light source.

Fig. 4a.

Experiment 35. February 27th, 1929. Temperature 11·2° C.

Fig. 4a.

Experiment 35. February 27th, 1929. Temperature 11·2° C.

Fig. 4b.

Experiment 33. February 26th, 1929, 100-watt blue bulb used. Temperature 13·7° C.

Fig. 4b.

Experiment 33. February 26th, 1929, 100-watt blue bulb used. Temperature 13·7° C.

Another example of the perception of “place-change” is taken from the geotropic responses. As we have seen above, the geotropism of Daphnia is such that any increase of light (regardless of direction) tends to send the animals down and a decrease to send them up. A square hole is cut in a black sleeve placed around the middle of a museum jar standing in the diffuse daylight of the room. Whenever any of the Daphnia swam in front of the opening from any point, they were immediately stimulated to swim directly downward although in many cases they could have reached darkness much sooner by returning in the direction whence they had come.

Now that reactions due to changes in the light intensity have been considered, we may deal with the effect of the direction of the light. Holt and Lee (1901) have shown that the distinction sometimes made between photopathy (sensitiveness to intensity of light) and phototaxis (sensitiveness to direction of light) as different forms of irritability is unwarranted. It is made clear that the intensity of the light determines the sign of the response (positive or negative), while the part of the body stimulated—determined by the direction of the light—decides the ultimate orientation of the animal. Enough experiments have already been given to establish the fact that a change of light intensity stimulates the Daphnia to move. The direction in which the animal responds is determined by the force of gravity and by the direction of the rays of light falling upon it. The response to gravity we have already considered. Up to this point the phototropic responses discussed have been simple movements directly to or from the light source. Other experiments demonstrate clearly that the direction of the light rays does not stimulate the organism, but merely orientates it after it has been urged to move by a change in the light intensity. If the animal is negatively phototropic, it turns in the direction from which it is receiving the least amount of light until it comes to be moving directly away from the light source. Conversely, a positively phototropic animal is orientated by the light falling upon it to move directly towards the light source.

The famous experiments of Loeb (1918) showing the mechanical nature of this phototropic orientation in various animals are well known. Holt and Lee (1901) devised experiments with Infusoria using a trough of water through which a band of light passed. A prismatic screen was placed perpendicularly to the beam of light so that a grading of the light intensity from bright at one side to dim at the other was produced at right angles to the direction of the light. Yerkes (1903) working on Daphnia pulex produced a similar optical condition by another type of apparatus. In both investigations it was found that negative animals moved into the dark end of the trough and positive animals into the bright end although in both cases the animals were required to move at right angles to the direction of the light. The mechanism by which this result is produced and its agreement with the theory that intensity stimulates and direction orientates are clearly set forth by Holt and Lee. That this mechanism of orientation functions in planktonic animals regardless of the fact that it may lead them into even more unsuitable conditions is shown by experiments using converging and diverging beams of light. By passing the light through a cylinder of water, Moore (1909) succeeded in focussing the light in such a way that a “caustic” was produced in a second cylinder in which nauplii of Balanus were swimming. He found that the animals swam either directly toward the light or directly away from it (according to the sign their phototropism) although for part of each journey the light intensity was increasing and for part it was decreasing. He says on p. 18:

At first sight it looks proven from this that intensity of light is of no effect, and the direction of incidence the whole matter, because the organisms appear to swim in one direction indifferently, whether the illumination is increasing or decreasing. In reality, however, such a conclusion would be fallacious, for in order that, say, a positive organism should turn when it began to swim in light of gradually decreasing intensity, it would be necessary for it to turn its sentient surface away from the light, and that would plunge it into darkness.

And on p. 33 :

Movement in converging and diverging light is shown to be explicable on the basis of intensity of light alone, and that direction produces its effects in a secondary manner on account of the light and shade effects of the animal’s own body.

In my investigations with Daphnia magna a similar experiment was performed as shown in Fig. 4 b. A lens 16 cm. in diameter was used to produce a cone of light in the experiment chamber. When the light was dimmed the Daphnia moved towards the light source although it was thereby moving into a region of weaker light, and when the light intensity was increased, the animal moved away although this meant swimming into a region of greater and greater intensity. Following a reduction of the light intensity, the phototropism becomes positive as we have seen. Presumably this means that the organism is stimulated to seek again the same light intensity. This would ordinarily mean moving towards the light and the response to that stimulus has come to be a turning towards the direction of the light and hence a movement toward the light source. In this experiment the same reaction occurs. The Daphnia is stimulated to turn towards the side of its body which is most strongly illuminated, that is, the side towards the light since the other side is in shadow. The animal may perceive that it is getting into weaker and weaker light, but the mechanism of the response is such that it is forced to move in that direction. Exactly the same argument holds for the return trip. When the light is made brighter, the animal is compelled to move away from it. As it reaches regions of greater and greater intensity, it may be stimulated to swim faster and faster, but it cannot turn about and move back to a more favourable position.

There is, then, no “absolute optimum” light intensity for these Daphnia. They do not seek any particular intensity of illumination. The animals become adapted to the light intensity which exists at that time and place—this is for them a “relative optimum.” If the intensity rises or falls below the value to which they are then accustomed, the organisms are stimulated to move accordingly. Soon after the light intensity becomes constant at its new value, the Daphnia have become adapted to the new conditions and the original primary tropisms come into play once more. This situation is in no way altered by using different kinds of lights or different directions nor by making the water less transparent (through the agency of mud, “Aquadag,” Bismark brown, etc.) and thus increasing the rate of change of light intensity in space (i.e. “place-change”).

Many previous investigators have discussed the possibility of there being an absolute optimum light intensity for planktonic organisms. Of those who give evidence for believing such an absolute optimum to exist, special attention is called to Russell (1927 a, pp. 247 and 253). Evidence against such a belief is given by Yerkes (1903, p. 362), Moore (1909, p. 32), and Ewald (1910, p. 15 and 1912, p. 594). Special attention is called to the experiments of Yerkes already discussed in the preceding section of this paper. It will be remembered that the Daphnia was placed in a band of light of graded intensity, the grading being at right angles to the direction of the light. Since the animal under consideration was positively phototropic, it moved towards the highly illuminated end of the trough. If there had been an absolute optimum of light intensity, stimulation would have ceased when that absolute value had been reached and the animal would have stopped swimming in that direction. The fact that the Daphnia continued to move into greater and greater light intensities and even into regions of lethal thermal conditions proves that there is no absolute optimum in this case.

Proofs that the relative intensity and not the absolute intensity is important in determining the position of Daphnia are to be found in most of the graphs of recorded experiments. Attention is called particularly to Experiment 20; a graph of Part of this is shown above in Fig. 5. Here it will be seen that when the light intensity is reduced from 126 c.p. to 33, the Daphnia responds by swimming to the top of the tube. A further reduction from 33 to 9 keeps the animal at the top. But when the light is increased from 9 to 33, the Daphnia swims to the bottom. Since the illumination at the top of the experiment tube is necessarily much brighter than that at the bottom (due to absorption in the water), the organism cannot be said to be in an optimum light intensity in both places. Yet with a light source of 33 c.p. the animal is first at the bottom and then at the top. The effect such an intensity has upon the movements of the animal depends not upon its absolute value but upon whether it is reached by a dimming or a brightening of the light. The sign of the stimulus received depends upon the history of the environmental changes and not upon the situation at the moment.

Fig. 5.

Experiment 20. February 6th, 1929, Apparatus Type 1, vertical. Temperature 11·9° C.

Fig. 5.

Experiment 20. February 6th, 1929, Apparatus Type 1, vertical. Temperature 11·9° C.

These changes of tropism signs do not take place immediately following a reduction of the light—there is always a certain latent period or “lag” which is interposed between the stimulus and the response. There may well be two such latent periods concerned with every response. First, there is the interval between the initiation of light reduction and the perception by the Daphnia of this dimming, and second, there is the interval between the perception and the response (swimming movement) which follows :
formula
In view of the fact that we have no information upon the integral parts of the whole reaction, but can observe only the times of beginning and ending, the two possible reactions are taken together, and the latent period is defined as the time which elapses between the start of the dimming of the light and the start of movement of the animal :
formula
This latent period always exists whatever the absolute light intensity and the rate of change may be. But the duration of the latent period depends upon the speed at which the light intensity is changed. Thus, the slower the dimming of the light, the longer the latent period lasts—in other words, the slower the rate of change of light intensity, the more time elapses between the beginning of the dimming and the beginning of the swimming movements in response. It might be supposed that a certain amount of intensity change was required to stimulate the Daphnia to respond—that, starting from a certain high value of illumination, a certain low value had to be reached before the animal was caused to move. Obviously, such a situation would produce qualitatively the same effect because at a slower rate of dimming a longer time would be required for the requisite amount of intensity change to occur—for the certain low value to be reached. But when the data from actual experiments are consulted, it is seen that such an explanation does not satisfy all the facts. For, the Daphnia does not respond after a certain amount of intensity change has occurred—on the contrary, the swimming is initiated at a different intensity value for every different rate of dimming. In general, the faster the rate of intensity change, the smaller the amount of change required and hence the greater the absolute intensity existing when the organism responds. These facts are represented diagrammatically in Fig. 6. The two responses shown, X and Y, occur after a dimming of the light over the same range but at different speeds. A glance at this diagram will make clear that response X, resulting from a slow rate of dimming, occurs after a long latent period and at a low intensity value, whereas response Y, resulting from a rapid rate of dimming, occurs after a short latent period and at a relatively high intensity value.

Fig. 7 is the graphical expression of actual observations—the data of Experiment 14. In this experiment the responses of one individual Daphnia were watched during a long series of tests in which the light intensity was changed many times over the same range but at different speeds. Although such a long series of observations upon the responses of one animal under these circumstances was carried out only once, nevertheless the results seem trustworthy since the animal was still behaving normally at the end of this experiment and since other shorter experiments confirm the general conclusions. The conditions were most carefully controlled. The experiment tube was placed within a light-proof box in addition to the usual light leakage precautions, rest periods of at least 15 minutes each were allowed between tests, and the temperature was delicately regulated as shown by the values given at the top of the graph.

From a study of the graph of Experiment 14 Table II has been drawn up showing the duration of the latent period (in minutes) and the absolute light intensity (measured by the candle power of the light source) at the time of response as functions of the rate (c.p./min.) at which the light is dimmed. For example, consult the graph at 2·30 p.m. (Response G). Here the light intensity is reduced at the rate of 19 c.p./min. Three minutes after dimming has begun, the animal responded by starting to swim directly to the top of the tube. At the moment when this response occurred, the light strength was 50 c.p. Contrast this with the succeeding test at 3·30 p.m. (Response H). In this case the rate of dimming was 3·7 c.p./min., The latent period 22-5 min., and the light strength was 23 c.p. From the data shown in Table II the curves of Fig. 8 were constructed. This graph shows that as the rate of dimming is increased, the duration of the latent period is decreased, while the magnitude of the absolute light intensity at the time of response is increased (i.e. the amount of intensity change required is decreased).

Table II.

Intensity and latent period as functions of rate.

Intensity and latent period as functions of rate.
Intensity and latent period as functions of rate.

The graph of Experiment 14, as well as many other observations I have made, shows that in general when once the stimulus has taken effect, when once the reaction has started, the response proceeds at a maximum rate and continues to a maximum magnitude. Take, for example, the two cases in Experiment 14 (Responses G and H) referred to in the preceding paragraph. Here although the rate of dimming is very different, when once started the animal swims in both cases to the very top and at its maximum speed. To be sure this is not always the case, as can be seen in the earlier part of this same graph where submaximal and irregular responses are to be found. Usually, however, the response is maximal. Moreover, the Daphnia generally swims straight to the top of the tube even if the dimming of the light is stopped soon after the upward movement begins. In some cases, however, a continuation of the dimming is necessary to send the Daphnia to the very top. If the rate of change is very fast, however, the dimming may have ceased before the response has even begun. Whether or not the animal would respond while the light intensity is held constant following a dimming of the light down to an intensity just above the value at which a response is usually evoked at the same rate of reduction has not been determined. Such an experiment would give us information upon the nature of the latent period and the possibility of there being two “lag” periods for each response. But the experiments appear to show definitely that a certain minimum amount of intensity change must take place in order to produce a response; a change of intensity over a range smaller than this minimum will not stimulate the organism to move, no matter how fast the rate of change may be. Many more observations are needed in this field and experiments especially designed to test each particular problem must be performed before conclusions can be drawn.

The question of fatigue is an important one and has been discussed by other workers (e.g. Yerkes, 1900 and 1903). In all the experiments described thus far ample time between the various tests has been allowed for rest. But in Experiment 21 (graph not reproduced here) in which the apparatus described on p. no was used in a horizontal position the light was dimmed four times over the same range (116-3 c.p.) and at the same speed without any interval between the tests. The experimental animal responded maximally and in practically an identical manner after the first three stimuli. But the fourth response was of approximately only 13 magnitude. Further stimulation evoked even smaller responses and a rest of about 20 minutes was not sufficient to restore the animal to its normal condition. But a 3-hour period of complete darkness (the animal remaining at the bottom during this time) is followed by a normal response when the light is subsequently increased and dimmed.

Thus far I have been dealing exclusively with animals which are primarily negatively phototropic and positively geotropic. A small number of Daphnia magna are found to be primarily positively phototropic and negatively geotropic. it is important to note that these individuals react to changes in light intensity in exactly the same way as the more usual forms of Daphnia except that the temporary secondary tropism signs are evoked by an increase of the illumination instead of by a decrease. When the light intensity is reduced, the primary signs are strengthened, but increasing the light intensity results in the temporary establishment of the secondary signs. When experimenting with these animals, then, a dimming of the light produces no effect as they are already as close to the source of light as possible, due to their primary positive phototropism, but as soon as the light intensity is increased they become temporarily negatively phototropic and start swimming away from the light. As before, moreover, this change of sign does not last long and the Daphnia soon come back to their starting-point close to the light source. In complete darkness the animals are found at the top of the tube as a result of the primary negative geotropism existing in this type of Daphnia.

In all this work, adaptation suggests itself as the explanation of the phenomena observed (see Crozier and Wolf, 1928, and Adrian, 1928). The optimum light intensity for an animal at any given moment is that intensity to which the organism is then adapted. If the environmental conditions are changed, that is if the illumination is subsequently increased or diminished, the Daphnia is stimulated to swim either away from the light source or towards it. But as soon as the animal has become adapted to the new intensity value, it responds to further stimulation in exactly the same way as it did at the old intensity value. That adaptation is not a slow process is indicated by the uniform results obtained after each of three similar reductions of the light with no interval allowed for rest between. This is shown clearly in Experiment 21 referred to above. If adaptation were slow, one would expect to find a progressive change in the three responses (such as summation, etc.) instead of the identical curves obtained in the graph. But on the other hand, adaptation is not instantaneous for the animal is stimulated to move towards the light the whole time until the process of adaptation is complete. Evidently, adaptation has not been accomplished during the time which elapses while the Daphnia is swimming towards the light and while it remains at the top of the tube— that is, several minutes.

By means of experiments in which the light intensity was switched instantaneously from one value to another it has been shown that the rate of change of intensity cannot be too fast for the Daphnia to be stimulated by it. Further support of this theory of adaptation is found in the fact that it is usually possible to dim the light at such a slow rate that no response is obtained, although the animal does react to a change of light intensity over the same range when this change takes place at a high speed. This suggests that at the slower rate adaptation in Daphnia is able to keep pace with the change of light intensity: the organism is always adapted and therefore never stimulated to move. I have never succeeded in demonstrating conclusively that the light can be dimmed over its whole range from full bright to complete darkness without producing a response, although this can probably be done when the difficulty of preventing the animal from reacting to other stimuli over such a long period of time is overcome. But that any given part of the light range can be traversed without causing a movement of the animal by such a slowing of the rate of change has been shown many times—for example, see Response I, Experiment 14, Fig. 7. One important phase of the observed responses of Daphnia for which this theory of adaptation does not seem to provide an adequate explanation is the return of the primary signs of the tropisms. Following a reduction of the light intensity the animal swims towards the light apparently stimulated to move again into a region of intensity to which it has been adapted. No matter how small a light change has taken place, the Daphnia usually swims to the very end of the tube, as we have seen, although in so doing, in the case of very small reduction, it must pass by the intensity value which had existed at its first position. When the animal reaches the end of the tube nearest the light source, it remains there supposedly until it has become adapted to the new intensity., When this adaptation is complete, the Daphnia starts swimming away again. Apparently, this return to primary negative phototropism is not a phase of adaptation, but is due to a different stimulus produced by a steady light.

A discussion of the possible bearing of the results of this paper upon the problem of the vertical migration of plankton animals in general would be premature. In the first place, many more experiments must be performed upon Daphnia itself. Secondly, observations upon the behaviour of a large number of different genera of plankton animals must be made. And thirdly, some method must be devised for proving that results obtained in the laboratory may legitimately be applied to the behaviour of the organisms in nature. Still the responses of Daphnia to changes of light intensity are suggestive. Diurnal migration may be due simply to a positive phototropism and a negative geotropism produced by the rising of the sun in the morning and to a reversal of those tropisms when the sun sets at night. But we have seen that Daphnia exhibits complete power of adaptation to light—it is primarily negative to any intensity of illumination. What would prevent such an animal from swimming down and down indefinitely? How can these responses be reconciled with the fact that the plankton has been observed to be distributed at definite levels in the water? If it could be shown that plankton organisms in nature, unlike laboratory Daphnia, have no power of adaptation to light, or have a very limited power, it would be conceivable that they be stimulated to seek a region of fixed light intensity which was for them an absolute optimum. Were this found to be the case, a theory that the plankton follows an optimum light region as that region moves down into the depths and up to the surface again during the course of each day would be tenable. As it is, experimental results do not seem to agree satisfactorily with inferences from field observations. Evidently the mechanism of the responses of these animals must be much more thoroughly investigated before the causes of diurnal migration can be conclusively ascertained.

I am indebted to Professor S. Gardiner and to Mr J. T. Saunders for their sustained assistance and advice during the progress of this investigation at the Zoological Laboratory of Cambridge University. I am also indebted to Dr C. G. Lamb of the Engineering Laboratory for many helpful suggestions and the loan of apparatus.

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1

Daphnia may be easily cultured by keeping them in jars each containing about 150 c.c. of tap water at room temperature and feeding them on a dilute solution of rotten egg. Normally females. produce broods every ten days. The young require four moults (about ten days) to become adult.

1

The concentration of the electrolytes in Cambridge tap water is fairly constant at approximately 0·005 N.

2

For an account of the different effects of red and blue lights upon the movements of Daphnia, see Frisch und Kupelwieser, 1913.

1

For another example of the resolution of phototropic and geotropic forces, see Crozier Wolf, 1928.