VOL. VII, No. 2
APRIL
1930
CHANGE OF PHOTOTROPIC AND GEOTROPIC
SIGNS IN DAPHNIA INDUCED BY CHANGES
OF LIGHT INTENSITY
BY G. L. CLARKE.
(Received 28th May 1929.)
(With Eight Text-figures.)
INTRODUCTION.
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.
1. MATERIAL AND METHOD.
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
av^ages of swarms of Daphnia (cf. Esterly, 1919, p. 80). For most of the experiV
one animal was used at a time and the same tests were repeated many times
JEB-VHH
8
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G.
L.
CLARKE
with the same individual. The other object was to avoid the effect of " shock *
much as possible in handling the Daphnia (cf. Esterly, 1919, pp. 66 and 77). ^ ^
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 " L u c " 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 gasflamewhich 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 o-150 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 toflowby passing between two zinc electrodes.
1
Daphnia may be easily cultured by keeping them in jars each containing about 150 c.c. of
water at room temperature and feeding them on a dilute solution of rotten egg. Normally feiT
produce broods every ten days. The young require four moults (about ten days) to become ai
Change of Phototropic and Geotropic Signs in Daphnia
111
These electrodes were thin triangular plates of zinc with dimensions of 45 x 45 x
^ n n . 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
1
The concentration of the electrolytes in Cambridge tap water is fairly constant at approximately 0-005 N.
^B£ For an account of the different effects of red and blue lights upon the movements of Daphnia,
^^pYisch und Kupelwieser, 1913.
ii2
G.
L.
CLARKE
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 w ^ P
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 i8° 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.
2. PRIMARY SIGN OF TROPISMS.
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.
3. CHANGE OF SIGN.
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^f
sign are, however, only temporary, and the temporary signs will be called S i
Change of Phototropic and Geotropic Signs in Daphnia
113
secondary signs. For soon after the light intensity becomes constant, the photo^ n s m 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,
Bright
Dim
Top
Bottom
A
r
\ /
r VV c
Time
\f.
V
\
1 \_
Light
Intensity
Position
Fig. 1.
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.
4. PHOTOTROPISM AND GEOTROPISM DIFFERENTIATED.
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
ctor of the lamp made it certain that the light entering the water was uniform
itensity and evenly distributed over the whole surface of the exposed side of
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G.
L.
CLARKE
the tank. As long as the light intensity was maintained constant, the Daphnia
always found at corner (i)—the result of their primary negative phototropism I
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 (i) 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
IYX
I \A V
!
\
1
1
•
1
t:1 1
1
1
i '
t
1
1
1
t
1
1
1
1
1
iB
t
I
1
1
0
Fig. 2 a.
Fig. 2 b.
The arrow indicates the direction of the light.
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.
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 threeeighths 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
1
For another example of the resolution of phototropic and geotropic forces, see Crozier
Wolf, 1928.
Change of Phototropic and Geotropic Signs in Daphnia
115
Jropism are changed. The result is that those Daphnia which were at the top
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.
Primary
Secondary
Primary
Secondary
Type B
1 + 1+
Type A
Geotropism
1 + 1+
Phototropism
Fast reaction
Slow reaction
r
•I.-*-
X
Case aa.
'"'ase ax.
Fig. 3 a.
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 al
there is no diagonal course as there was when one horizontal light was used; in
Case a% the two types of Daphnia are not separated out as they were when the
lower light alone was used.
is possible to separate the phototropic responses and the geotropic responses
t other ways. When the rate of dimming the light is made very slow, the
was dimmed at such a slow rate that the animal was never stimulated to
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.
! t
•:--«::
Case &,.
Case b2.
Fig. 3 b.
X
Bright
X
X
Dim
Dim
2 min. after dimming
30 min. after dimming
Fig. 3 c.
5. DURATION OF TROPISM REACTIONS.
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 alv
regained. If the light is extinguished, phototropism necessarily disappears,
Change of Phototropic and Geptropic Signs in Daphnia
117
ct that the majority of animals returned to the bottom even in complete darkness
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 4^ 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.
6. "TIME-CHANGE" AND "PLACE-CHANGE."
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 re( • i o n 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,
n8
G. L. CLARKE
as we have seen. Why is it that upon reaching this region of still lower
^^
the animal is not stimulated again to seek the light? It may be because the anirrral
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.
Sheet of tlim paper
P\ Region of
1 Reduced
/•Light
Q I Intensity
Fig. 4 a. Experiment 35. February 27th, 1929. Temperature n-2° C.
t!'
\ 1 '
\• /
Case 6,.
V
Case b2.
Fig. 4 6 . Experiment 33. February 26th, 1929, 100-watt blue bulb used. Temperature 13-7° C.
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. 4 a). 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.
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 ^m
a decrease to send them up. A square hole is cut in a black sleeve placed around
Change of Phototropic and Geotropic Signs in Daphnia
119
liddle of a museum jar standing in the diffuse daylight of the room. Whenever
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.
7. EFFECT OF INTENSITY AND DIRECTION OF LIGHT.
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.
assing the light through a cylinder of water, Moore (1909) succeeded in
ing the light in such a way that a "caustic " was produced in a second cylinder
K
120
G.
L.
CLARKE
in which nauplii of Balanus were swimming. He found that the animals
either directly toward the light or directly away from it (according to the
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.
8. ABSENCE OF ABSOLUTE OPTIMUM LIGHT INTENSITY.
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 ad^Ad
to the new conditions and the original primary tropisms come into play once more.
Change oj fnototropic and (Jeotropic Signs in Daphnia
121
situation is in no way altered by using different kinds of lights or different
dirVRons 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 (t.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
•o
e
US
O
140
120
100
80
60
40
20
Light
Intensity
90
§
§
eo
•°
70
<H
50
8
40
a
30
•|
20
11:35
Position
of
Daphnia
11:40
11:45
11:50
n:55
12:00
12:05
12:10
12:15
Time
Fig. s. Experiment 20. February 6th, 1929, Apparatus Type 1, vertical. Temperature 11-9° C.
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 4^t 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
122
G.
L.
CLARKE
the top of the tube. A further reduction from 33 to 9 keeps the animal at
But when the light is increased from 9 to 33, the Daphnia swims to the bd
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.
9. THE LATENT PERIOD AND THE EFFECT OF RATE OF CHANGE.
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:
Lag
Lag
DIMMING
s- PERCEPTION
s- MOVEMENT
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 denned as the time
which elapses between the start of the dimming of the light and the start of movement of the animal:
Latent Period
DIMMING BEGUN
> MOVEMENT BEGUN
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
^fc
satisfy all the facts. For, the Daphnia does not respond after a certain amouniof
Change of Phototropic and Geotropic Signs in Daphnia
123
intejgity change has occurred—on the contrary, the swimming is initiated at a
dif^^it 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
Bright
Intensity
for Y
Light
Intensity
Bottom
Time
Latent period
for ^Y
Fig. 6.
Latent
period
for Y
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
occurred, the light strength was 50 c.p. Contrast this with the succeeding
3.30 p.m. (Response H). In this case the rate of dimming was 37 c.p./min.,
6 CHI 20
55-|llo|— Light Intensity
3CH
25
9:00
1020
am
- Time
10.30
A
1040
10 50
TTTOO l t i O
B
11.50
C
11:30 TT40
Fig. 7-
11'50
D
12:00
12:10
1220
12:30 1.2:40
Change of Phototropic and Geotropic Signs in Daphnia
125
period 22-5 min., and the light strength was 23 c.p. From the data
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.
Light intensity
c.p.
Latent period
min.
i-6
7-S
63-5
3-7
6-6
7-S
16-0
230
230
22-5
Rate
c.p ./min.
Response
/
H
F
A
E
D
180
G
J
19-0
29-0
41-0
B
54-o
C
125
115
4-0
195
42-0
70-0
50-0
2-0
3-o
2-5
33°
65-0
84-0
I-O
°S
Experiment 14. January 23rd, 1929. The "Light Intensity" is measured by the strength in
candle power of the light source.
Intensity
- 90 C.p.
Time
min
70
•
j
60
Absolute Intensity
/kbsQ
Curve
50
40
/
J
30
20
m
V
to "1 \l_atent Perio
10
20 30
50
60 c.p./min.
Rate of change of intensity of light
Fig. 8. Experiment 14. January 23rd, 1929.
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
(fl^onses G and H) referred to in the preceding paragraph. Here although the
jEB-vnii
9
126
G. L. CLARKE
rate of dimming is very different, when once started the animal swims in bpth
cases to the very top and at its maximum speed. To be sure this is not alwaj^^he
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.
10. FATIGUE.
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. n o was used in a horizontal position the light was dimmed four times over the
same range (116-3 C-P-) a n ^ a t t n e 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 \ 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.
11. SPECIMENS WITH REVERSED PRIMARY SIGNS.
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.^t is
important to note that these individuals react to changes in light intensl^rin
Change of Phototropic and Geotropic Signs in Daphnia
127
the same way as the more usual forms of Daphnia except that the temsecondary 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.
12. ADAPTATION.
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^mionstrating conclusively that the light can be dimmed over its whole range
fr^m full bright to complete darkness without producing a response, although this
9-2
128
G.
L.
CLARKE
can probably be done when the difficulty of preventing the animal from
to other stimuli over such a long period of time is overcome. But that ar
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 /, 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.
CONCLUSION.
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 thoroi^My
investigated before the causes of diurnal migration can be conclusively ascertanrcd.
Change of Phototropic and Geotropic Signs in Daphnia
129
SUMMARY.
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
esponse takes place. Ordinarily, the response is maximal in respect to both
and magnitude.
t
130
G. L.
CLARKE
10. Fatigue will interfere with experimentation unless guarded against.
11. Specimens of Daphnia with reversed primary signs gain temporary
dary 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.
I am indebted to Professor J. 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.
REFERENCES.
ADRIAN, E. D. (1928). The Basis of Sensation. Christophers, London.
CROZIER, W. J. and WOLF, ERNST (1928). "Dark Adaptation in Agriolimax." Journ. of Gen. Phys.
Sept. 20, 12, No. 1, 83-108.
DICE, L. R. (1914). "The Factors Determining the Vertical Movements of Daphnia." Journal of
Animal Behavior, July—August, 4, 229—265.
ESTEBLY, C. O. (1919). "Reactions of Various Plankton Animals with reference to their diurnal
migrations." Univ. Coll. Pub. Zool. Berkeley, 19, No. 1, 1-83.
EWALD, W. F. (1910). "Ober Orientierung, Lokomotion und Lichtreaktion einiger Cladoceren
und deren Bedeutung fur die Theorie der Tropismen." Biolog. Centralbl. 30, No. 1, 1-16;
No. 2, 49-63; No. 11, 378-384; No. 12, 385-399.
(1912). "On Artificial Modification of Light Reactions and the Influence of Electrolytes on
Phototaxis." Journ. Exp. Zool. 13, No. 4, 591-612.
FRISCH, KARL V. und KUPELWIESER, HANS (1913). "Ober den Einfluss der Lichtfarbe auf die
phototaktischen Reaktionen niederer Krebse." Biol. Cent. 33, 518-552.
HOLT, E. B. and LEE, F. S. (1901). "The Theory of Phototactic Response." Amer. Journ. Physiol.
4, No. 9, 460-481.
LOEB, JACQUES (1918). "Forced Movements, Tropisms, and Animal Conduct." Monographs on
Experimental Biology, 1, 172 pp. J. B. Lippincott Co., Philadelphia and London.
MAST, S. O. (1921). " Reactions to light in the larvae of the Ascidians, Amaroucium constellatum and
Amaroucium pellucidum, with special reference to photic orientation." Journ. Exp. Zool. 34,
No. 2, 149-187.
MOORE, A. R. (1912). "Concerning Negative Phototropism in Daphnia pulex." Journ. Exp. Zool.
13, No. 4, 573-575MOORE, BENJAMIN (1909). "Reactions of Marine Organisms in Relation to Light and Phosphorescence." (Proceedings and) Transactions of the Liverpool Biological Society, 23, 1-34.
PARKER, G. H. (1901). "The Reactions of Copepods to various Stimuli, and the bearing of this on
Daily Depth Migrations." Bull. U.S. Fish Comm. 1901, 21, 103-123.
ROSE, M. (1924). "Recherches biologiques sur le Plankton." Bull. Inst. Ocian. 439.
(1925). "Contributions a l'fitude de la Biologie du Plankton. Le Probleme des
verticales journalieres." Archives de Zool. ixper. et gen. November 15, 64, 387-542.
Change of Phototropic and Geotropic Signs in Daphnia
131
, F. S. (1925). "The Vertical Distribution of Marine Macroplankton—An Observation on
rnal Changes." Journ. Mar. Biol. Ass. N.S. 13, No. 4, 769-810.
1926). " The Vertical Distribution of Marine Macroplankton. IV. The apparent importance
of light intensity as a controlling factor in the behaviour of certain species in the Plymouth
Area." Journ. Mar. Biol. Ass. N.S. 14, No. 2, 415-440.
(1927 a). "The Vertical Distribution of Plankton in the Sea." Biol. Rev. and Biol. Proc.
Comb. Phil. Soc. June, 2, No. 3, 213-263.
(19276). "The Vertical Distribution of Marine Macroplankton. V. The distribution of
animals caught in the ring trawl in the daytime in the Plymouth Area." Journ. Mar. Biol.
Ass. N.S. 14, No. 3, 557-608.
YEKKES, R. M. (1900). "Reactions of Entomostraca to Stimulation by Light. II. Reactions of
Daphnia and Cypris." Am. Journ. Phys. 4, No. 8, 405—422.
(1903). (Listed under G. H. Parker.) "Reactions of Daphnia pulex to Light and Heat."
Mark Anniversary Volume, Article 18, pp. 359-377.