427
THE REACTIONS OF NORMAL AND MUTANT TYPES
OF GAMMARUS CHEVREUXI TO LIGHT
BY A. WOLSKY AND J. S. HUXLEY.
(Received 4th May, 1932.)
(With Five Text-figures.)
CONTENTS.
PAGE
I. Introduction .
.
I I . Material and methods
.
.
.
.
427
.
.
429
I I I . Results with normal animals .
.
430
IV. Results with mutant types
.
436
.
V. Discussion
VI. Summary
439
440
V I I . References
440
I. INTRODUCTION.
THE reaction of various species of Gammarus to light has been investigated or touched
upon by various authors. S. J. Holmes (1901) found that Amphipods in general
were negatively phototactic. This held for the two species of Gammarus which he
investigated, G. mucronata and G. annulatus (the latter species stated in the original
paper to be G. locusta; corrected in Smith (1905)). He added that the phototactic
reaction was frequently interfered with by thigmotaxis. High temperature will
generally cause a reversal of the sense of the phototaxis, but most aquatic forms die
before this temperature is reached. Other authors have independently confirmed
Holmes' general statement as to the negative phototaxis of Gammarus, e.g. Langenbuch (1928) for G. locusta and G.pulex, and Barnard (1927) for the South African
species of G. capensis and G. auricularius; this latter result is of great interest, as
both species mentioned by Barnard have highly degenerate eyes.
A few years after Holmes' work, Smith (1905), working with G. annulatus, was
the first to attempt to give a numerical measurement of the phototaxis of Gammarus
by statistical methods. His results, however, were quite different from those
Holmes gives. He states that animals exposed to light for a long period such as an
hour are insensitive for about 10 min., but later show a marked positive phototaxis.
He expresses the view that this behaviour is correlated with migration of pigment
in the ommatidia as a result of exposure to light, which had previously been
described for G. locusta by G. H. Parker (1899).
The applications of the subject were further increased owing to the important
work of Loeb (1904, 1906) on Crustacean phototaxis. Loeb states that Gammarus
428
A. WOLSKY and J. S. HUXLEY
is normally insensitive to light; he used one fresh-water species (G. pulex) and one
marine form, but unfortunately omits to give the specific name of the latter. His
most interesting results concerned the effect of various acids in inducing negative
and positive phototaxis; e.g. he found that fresh-water Gammarus becomes positively
phototactic when various acids (e.g. carbonic, oxalic and acetic), were added to the
medium, but states that this effect lasts only a few seconds. No positive phototaxis could be induced in the marine species by means of acids, although the results
still held good for the fresh-water species put into sea-water. On the other hand,
negative phototaxis, again lasting for a few seconds only, can be obtained by shaking
the animals or, better, by means of irradiation by ultra-viole"t light from a quartz
mercury lamp.
Later, Moore (1913) confirmed the fact that acid induces positive phototaxis on
another Crustacean, Diaptomus, and obtained the opposite result (negative phototaxis) with various drugs, notably caffein, strychnine and atropine.
Finally Phipps (1915) has made a study of the reactions of light of three Amphipods. We will only refer here to his results with G.fasciatus. He employed an
elaborate "light grader," described by Shelford (1914), which permits of making a
separate analysis of the animals' reactions to the intensity and to the direction of
light. He worked statistically, being thus able to express his results in numerical
terms. He states that normal animals gather in the region of low light intensity;
however, the difference between his figures and those to be expected in a chance
distribution appear to be within the limits of statistical error.
Agencies which depress general metabolism (captivity, cyanide, chloretone, reduced Ot, and starvation), tend to reduce the animals' sensitivity to light, though
here again the results were not very clear-cut. He also stated that the animals
showed a moderate degree of negative phototaxis, but his method was not well
adapted to give quantitative expression to this. In general, he found that the
direction of the light was more important than its intensity. He states on p. 219,
" In none of the experiments was there evidence of orientation either to light intensity or to direction of rays." But (p. 212) he later states that "if the animals
entered the field of intense light they were plainly stimulated and usually darted
back quickly to the dark area."
These various observations need clearing up in several respects. We first have
the contradiction between Smith's and Loeb's results and those of other authors
who found negative phototaxis in normal conditions. Secondly, no brackish-water
species have, so far as we know, been investigated. As Loeb found that his marine
species behaved differently from the fresh-water species which he used, the use of
a brackish-water species, G. chevreuxi, should be interesting. Finally, the discovery
(Huxley and Wolsky, 1932) that the albino and colourless mutants of G. chevreuxi
possess no retina or optic nerve prompted us to see whether these types, as expected, lacked all phototaxis. We decided also to study the phototactic reactions of
the normal wild type and of its red-eyed mutant.
Reactions of Normal and Mutant Types of G. chevreuxi to Light
429
II. MATERIAL AND METHODS.
The wild type G. chevreuxi were derived from stock brought in from the wild
from Chelson Meadow, Plymouth, in October, 1931. The mutant types, red-withwhite (bb), red-no-white (bbww), albino (cc) and colourless (ccvnv) were from stocks
bred at Oxford for a number of years. We have to thank Mr E. B. Ford for supplying
these.
A word may be said about these various types. The wild type has an eye with
normal black facets separated by a moderate amount of white interfacetary pigment.
The black facets contain both the black melanin and the red carotinoid pigment.
The red-with-white (bb) differs from the normal by the mutation of a single gene
(B-b) which reduces the rate at which melanin is formed: in early life the facets are
bright scarlet, gradually darkening to a brownish colour.
The no-white mutation (tow) prevents the appearance of interfacetary white
pigment. The eyes of such animals contain only the coloured facets, which touch
each other. The red-no-white types used in these experiments are produced by a
recombination of the red and the no-white mutations. The albino type (cc) is due
to a single mutation of the colour-controlling gene (C-c); the eyes of such animals
are irregular in shape, consisting of a mass of white pigment with a few colourless
facets rather irregularly scattered over it. Investigation has recently shown (Huxley
and Wolsky, 1932) that in albinos the facets represent only the crystalline cones of
the ommatidia; the retinulae are entirely absent, as also the optic nerve. As the
black and red pigments are deposited exclusively in the retinulae, the eyes show no
colour. The colourless type is produced by a recombination of the albino and nowhite mutation, and has therefore the constitution ccww; in such animals the scattered facets representing crystalline cones are present, and are of course colourless,
but there is no white pigment. Excellent surface views of these different types of
eyes can be seen in the paper by Allen and Sexton (1920), and sections in Huxley
and Wolsky (1932).
As regards methods, we soon found that owing to the constant active swimming
of the animals, only statistical results could be trusted. To obtain these, a long tube
was prepared with a section of its length marked off at either end by means of a
narrow rubber ring. The brackish water and animals were inserted, and the tube
illuminated from one end. At definite intervals, the number of animals in the two
end sections was recorded. The tube used was 48 cm. long, with a bore of 19 mm.
The end sections were each one-sixth of the total length of the tube, leaving a
central portion two-thirds of the total length. It was found necessary to close the
ends of the tube with glass instead of cork, since otherwise the cork surface aroused
a thigmotactic response which often interfered markedly with the animals' reactions
to light. One end had a piece of glass sealed on to it, the other end was ground, and
closed for each experiment with a piece of glass which was pressed against it by a
screw in a wooden frame. The water and animals were inserted and removed
through this end of the tube. In the centre of the tube, a short piece of 6 mm. tubing
was sealed in, in order to provide for aeration during protracted experiments, and
43°
A. WoLSKYan^J. S. HUXLEY
to permit of the introduction of reagents such as acids during the course of the
experiments.
Illumination was provided by a 20 watt Osram electric light bulb, the bottom of
which was 100 mm. vertically above one end of the tube. With this arrangement
there was no shadow inside the tube. The apparatus was constructed by Mr D. A.
Kempson.
Before the beginning of each experiment, the animals were usually placed in
complete darkness for about 5 min.
The counting of the animals was done at i-min. intervals, but in certain experiments (especially after acid treatment) at half-minute intervals. For the purpose of
plotting, the counts have been grouped by threes.
For certain purposes it was found advisable to reverse the direction of the light
at definite intervals. This was done by changing the position of the light relative to
the tube.
III. RESULTS WITH NORMAL ANIMALS.
(a) Results in normal animals in untreated medium.
Results of six experiments of this type are presented in Table I, and have been
shown graphically in Fig. 1 (WN). They show quite clearly that there is a marked
difference between the number of animals at the two ends of the tube: the average
percentage for all experiments of animals noted towards the dark end is more than
four times as great as the number towards the light end. In no experiments was the
ratio of animals at the dark end to tbat at the light end less than 2. Owing to the
rapid swimming of the animals there were of course considerable fluctuations from
minute to minute of the numbers in the two ends. To give an idea of this we reproduce graphically the details of one experiment (Fig. 2 (1)).
If the animals were distributed at random through the tube there should be
16-7 per cent, at either end, and 66-6 per cent, in the middle section. However, the
mean number of animals in the light end is only 8-6, and never reached over 13 per
cent, in any one experiment, while in the dark end the percentage was never below
25 per cent. The percentage of animals in the centre of the tube averaged 55-6 per
cent., which is rather less than that to be expected under random distribution, and
again indicates negative phototaxis.
The fact that the light end contains on the average about half the percentage of
animals to be expected with a purely random distribution through the tube indicates that the phototaxis of G. chevreuxi is weak, but the results are conclusive in
showing that when other environmental factors, such as those making a thigmotactic reaction possible, are excluded, phototaxis is definite.
(b) Experiments with one eye varnished.
G. Frankel (1931), in an exhaustive discussion of the light reactions of animals,
distinguishes between various forms of phototactic reactions. The only one which
is characterised by circus movements if one eye is blinded is what he calls tropotaxis,
Reactions of Normal and Mutant Types of G. chevreuxi to Light
431
which corresponds almost precisely with tropism as defined by Loeb (1918). One
eye was varnished with a quick-drying celluloid varnish of the so-called cold type
similar to the commercial brands of Robbilac, which was applied with a fine brush
to one eye of the animal as it lay on a glass plate, and dried in a few seconds. The
animals were always left undisturbed for at least 6 hours before being used for ex90 T
•H- ~1
80-
70
++ 1
+
60
50
t
+
t\
i
40
D
30
S
o
o
D
•
•c
0
s
+
O
D
cm
•D *o
o
20
C
>
D gj
10
ACN
ACr
Fig. 1. Graph giving the results of experiments tabulated in Tables I, II and III L (o), D ( • ) , and
C (+) indicate the mean percentage of animals in the light, dark and central sections of the tube
respectively. W indicates experiments with wild-type specimens, R with red-eyed specimens, AC
with albino and colourless specimens. The suffixes N and T denote experiments in normal conditions
(without acid) and treated (with acetic acid) respectively. (In the RJI series the results of Exps. Ra
and i?10 are not inserted, on account of their short duration.)
periments. By the end of 1 or 2 days the varnish had usually been removed by the
animal scratching at it. As result, distinct circus movements were induced. However, the results were not simple, since the eyes of Gammarus are concerned in the
so-called "dorsal light reflex" (Lichtruckenreflexe), i.e. reflex orienting the animal
in space with reference to the direction of incident light. Accordingly, unilateral
blinding causes not only a tendency to circus movements, but a disturbance of
orientation, manifested generally by a rolling movement round the long axis.
432
A. WoLSKYaw^J. S. HUXLEY
Furthermore, G. chevreuxi, like some other animals, has a thigmotactic tendency
which prompts it to lie frequently on one side on the bottom of the vessel, and
swimming in this position instead of vertically through the water. However, it is
1
9
12
15
18
21
12
15
18
21
24
27
Fig. 2. Graphic records of typical single experiments in normal environment (without acetic acid)
(i) with wild-type specimens (Exp. W,); (2) with red no-whites (Exp. Rg); (3) with albinos (Exp. At)
(4) with colourless (Exp. Q). Ordinates, percentages of animals in the light section of the tube (o)
and in the dark section ( • ) . Abscissae, time in minutes, from beginning of experiment. Vertical lines
marked R indicate the times when the light was reversed, by being moved from one end of the tube
to the other. The fact that the light section of the tube after reversal becomes the dark section is
indicated by dotted arrows. (1) and (4) show that while wild-type animals react promptly to reversal,
colourless do not.
quite clear that the reactions induced by blinding on one side included circus movements. In the first place, they were induced immediately on the light being turned
on, thus excluding the possibility that the cause was irritation due to the varnishing
(a conclusion confirmed by the fact that the animals mated normally even when
blinded).
Reactions of Normal and Mutant Types of G. chevreuxi to Light
433
Neither in phobotaxis, which depends on a trial and error reaction, and the more
complicated forms of directional reactions that Frankel calls telotaxts and menotaxis
would circus movements occur after blinding.
Further, the circus movements always took the animals round the source of
light, naturally with the blinded eye on the inner side; if the position of the light
was changed, the centre of the circus movements was also changed. Fig. 3 records
some of the typical circus movements of animals with one eye blinded, swimming
Fig 3. Routes of wild-type G. chevreuxi with one eye varnished, illuminated from above during
30 sec. (see text). The movements tend to be circular, and the radii of the circles to remain nearly
constant for each individual. The scale is indicated on the figure.
freely in a large vessel. They were obtained by tracing the path of the animal in ink
on a glass plate placed over the vessel.
In a number of cases animals swimming through the water showed striking
corkscrew movements. These appear to be a combination of circus movements with
a rolling over to one side caused by the disturbance of the dorsal light reflex. When
the animals are swimming touching the bottom of the vessel, the movements are
more difficult to analyse. One common tendency is for the animals to describe
more or less circular movements of a much smaller radius than those executed by
unblinded specimens. The centre of the circle may be either towards the ventral or
434
A. WOLSKY and J. S. HUXLEY
the dorsal side of the animal. It is thus clear that circus movements do occur as the
result of one-sided blinding, and that the negative phototaxis of Gammarus is
therefore a true tropism, in Loeb's sense.
(c) Effect of chemical change in the medium {acetic acid and caffein).
It was thought advisable to try whether G. chevreuxi, like the species used by
Loeb and Moore, showed positive phototaxis in the presence of acetic acid, caffein,
etc. The results with acetic acid were clear-cut. We added 5 c.c. of a 1/10 Absolution
of glacial acetic acid to a tube with animals in it, which contained 130 c.c. This gave
a solution about 25 per cent, stronger than that employed by Loeb. As shown by
Table I and Fig. 1 (WT), there is a marked reversal of the phototaxis in acid, the
average percentage of animals in the light and dark ends relatively being 38-4 and
16-6 in the acid, as against 8-6 and 35-8 in water without acid. (The averages for
the acetic acid experiments excluded the first 3 min., for reasons mentioned below.)
Table I. Experiments with normal animals.
No. of exp.
No. of animals
Length of exp. (min.)#
L%
D%
c%
Length of exp. (min.)J
L%
D%
w1
w,
18
18
24
3°
130
34-2
52-8
—.
—•
—
—
c%
•
+
t
§
6-3
30-1
636
—
—
—
—
•
18
6
wt
wt
w.
12
12
18
12
12
12
10 6
25-9
II-7
292
63-S
59-i
7-S
46-6
96
43 8
2-2
77
46-8
SI'O
5
47-2
4S-i
44'4
33'3
250
417
II-2
44'4
5
6
29-3
20-7
500
Average
8-6f
35-8t
55-6t
—
38-4§
i6-6|
45 o§
Animals counted every minute.
Untreated.
Animals counted every half-minute.
With acetic acid treatment (first 3 counts omitted).
However, since one of the striking effects of the acid is to effect a change in the
sense of the animals' reaction, it is essential to state the time relations of the change
as well as the final results of it. For this purpose graphs are appended showing the
details of the process (Fig. 4). They demonstrate that after a certain time, usually
about i\ min., the sense of the phototaxis begins to change, and that the change is
usually fully completed after about 3 minutes. By that time the number of animals
in the light end of the tube is always greater than that in the dark end. The change
is usually very marked, but there is considerable variability in detail.
Our results are of some interest, since according to Loeb the change of sense is
effected in a few seconds in the species with which he worked. However, he gives
no precise details concerning the time relations of the process.
Further, it might suggest that in his fresh-water Gammarus species the effect
only lasts for a few seconds in all, in contradistinction to other Crustacea such as
Cyclops and Daphm'a, in which it proceeds for some time.
We have not found this with G. chevreuxi, the effect lasting at full intensity for
the whole length of the experiment. This, however, cannot be prolonged beyond
Reactions of Normal and Mutant Types of G. chevreuxi to Light
435
7-10 min. owing to the ill effect of the acetic acid on the animal, which was manifested by slowing or cessation of movement. Usually, the experiments were prolonged for 6 min. only, and the animals then replaced in normal brackish water. It
will be recalled that the marine species of Gammarus used by Loeb did not show
positive phototropism under the influence of acetic acid. It is interesting to find a
positive reaction in a brackish-water species such as G. chevreuxi.
18
70
21
AC
60
50
40
30
20
10
Fig. 4. Graphic record of typical single experiments with acetic acid treatment. Single vertical
lines (R), ordinates and abscissae as in Fig. 2. The double vertical line (AC) indicates the moment
when the acid was added. (1) With wild-type animals (Exp. WJ. Acetic acid causes prompt reversal
of the phototaxis; the positive phototaxis is retained after reversal of the direction of light (R).
(2) With red-eyed stock (Exp Rlo). The first portion of the experiment is omitted. The result is in
general similar to that of (1), with a slightly slower response to the acid. (3) Another type of result
with red-eyed animals (Exp. R^), showing markedly slower change of phototaxis on addition of aad,
and a failure of response to reversal of light.
Experiments were also conducted with caffein, which Moore found increased
the negative phototaxis of Diaptomus. Our experiments, however, were inconclusive : although in a number of cases the percentage of animals at the dark end was
slightly increased, in other cases it remained constant or was even diminished.
Further, it was found that caffein was more harmful to the animals than the acetic
acid, and the experiments were soon discontinued.
JEB'KIV
28
A. WoLSKYaniJ. S. HUXLEY
436
IV. RESULTS WITH MUTANT TYPES.
(a) Red-eyed types.
These experiments were mostly carried out with red-no-white stock (bbww), but
in some cases red-with-white stock (bbW) was used to see whether there is any effect
of interfacetary pigment. As this did not appear to be the case the results have been
summarised together (see Table II and Fig. i (R)). As was to be expected, the
animals were found to show the same general reactions as wild type (black eye)
animals. However, there are definite indications supporting the view that the
partial absence of melanin in the eyes of the red-eyed forms makes them seem more
sensitive to light, although we have not succeeded in making this conclusion perfectly certain. As shown in the tables and graphs, there is a distinct tendency for
the percentage of animals at the light end of the tube to be smaller with red-eyed
than with wild-type specimens. The average percentages of all experiments are 8-6
and 7*3 respectively, which means a decrease of over 15 per cent, in the percentage
of the red-eyed. The percentage of animals at the dark end appears to be of relatively
little significance.
Table II. Experiments with red-eyed animals. (All animals used were redno-white except in Exp. Ra, where they were red-with-white.)
No. of exp.
Ri
R,
R,
No. of animals
Length of exp. (min.)*
18
16
10
12
3°
9
6-4
6
24
60
82
L%
D%
c% of exp. (min.)!
Length
L%
D%
c%
24-4
67-4
—
—
• — •
—
•
+
I
§
R<
80-5
16-0
78-0
io-8
ib-3
72-9
—
—
—
—
—
—
—
—
—
—
—
—
131
R,
R,
R.
R.
Rio
12
18
3°
12
12
9
12
18
15
0-4
18
18
3'2
162
806
—
2-4
119
204
4«'3
Si"3
48-4
49'2
24-3
63-8
6-5
36-3
I7-S
46-2
53-7
2S'9
10-5
40-8
iS-4
43 •«
• — •
—
—
3'4
45-4
51-2
10-5
21-7
28-7
90
II-O
16-7
40-7
49-6
426
i»-3
29-6
S2-i
Average
6
7-3t
3O-6+
62-I +
26 8§
26- 3 §
46-g§
Animals counted every minute.
Untreated.
Animals counted every half-minute.
With acetic acid treatment (first 3 counts omitted).
In certain cases, notably Exps. Rg, R7 and R8, showed a sensitiveness much
greater than that found in any normal experiments; e.g. in Exp. R, the proportion
of animals in the light section was only 0-4 per cent., and in fourteen out of fifteen
counts there were no animals at all in this region of the tube. At the same time the
proportion of animals in the dark end was much above normal (nearly 50 per cent.).
In spite of this, on the light being twice reversed during the experiment, the light
end of the tube was completely empty before the first count was made after one
further minute.
This conclusion appears to be confirmed by the results of the experiments in
which acetic acid was added. As seen by the graphs (Fig. 4 (3), Fig. 5(1)), red-eyed
animals usually take longer to show full intensity of reversed phototaxis than do
normals, e.g. in Exp. R&, 6 min. as against the normal 2-3 min.
Reactions of Normal and Mutant Types of G. chevreuxi to Light
437
Further, in some cases after reversal of the source of light, there was little
change in the proportions of the animals in the previous dark (now light) end; this
might indicate that the phototactic reactions of red-eyed animals were more easily
fatigued than those of normal stock. This effect was, as was to be expected, more
marked when reversals were made more frequently than normal, e.g. at i^-min.
intervals. In such cases the percentage of animals at the two ends often remains
approximately the same throughout the experiment (e.g. Exp. R^).
Fig. 5. Experiments with acetic acid treatment, (i) With red-eyed stock (Exp. Rf). The animals were
reacting slowly, as in Fig. 4 (3), and frequent reversals prevented their arriving at the "correct"
distribution until after 9 min. (2) and (3) Failure of response with colourless stock (Exps. C% and C s ).
There is no effect either of the acid or of light reversal.
Finally, whereas in normal animals treated with acetic acid the percentages in
the light and dark ends were 38-4 and 16-6 per cent, respectively, when red-eyed
animals were used the percentage was almost identical for the two ends, namely
26-8 and 26-3 per cent. It was further noteworthy that the retardation and diminution of the acetic acid effect was most pronounced in those experiments previously
mentioned (R^, R7 and i?8) in which the animals were exceptionally negative in
normal water.
It might be imagined that these facts were conclusive. Unfortunately, however,
later experiments undertaken to confirm and extend these did not always give concordant results. In some cases the animals were less sensitive to light and more
sensitive to acetic acid than wild-type specimens, although some of the wild-type
28-2
A. WOLSKY and J. S. HUXLEY
438
experiments went further in this direction than any of the experiments with redeyed types. It appears, therefore, that the precise degree of phototaxis, negative or
positive, is extremely variable in G. chevreuxi, and so far our attempts to discover
the cause of this variability have been unsuccessful. We have carried out experiments
which show that neither temperature nor diet seems to have an effect sufficient to
explain the variation found; further, long exposure to direct sunlight, which might be
expected to fatigue the eyes, also appears to have no marked effect either on normals
or red-eyed specimens. Further analysis with animals from an inbred strain and with
more delicately controlled conditions might yield conclusive results. Meanwhile,
we can only record here that there is considerable presumption for believing that the
red-eyed types are slightly more sensitive on the average to light than normal
specimens.
(b) Experiments with albino and colourless types.
Casual inspection of stock animals might indicate that colourless and albino
types were negatively phototactic as well as normals. This, however, is not the case;
as Table III and Fig. i (AC) show, no trace of phototactic reaction can be obtained
in such types. The appearance of preference for dark situations apparent to casual
inspection must be due to thigmotaxis. In these experiments the effect of chance
can be clearly seen, since in addition to numerous experiments in which the distribution in the three sections of the tube corresponded closely to a random one, in
other cases there was a preponderance of animals at one end or the other. Further,
the number of cases in which this was the light end was about equal to the number
in which it was the dark end.
Table III. Experiments with aMno and colourless
animals.
No. of exp.
A,
4.
A,
AK
As
A,
No. of animals
Length of exp.
(min.)
18
30
18
3°
J
4
24
14
24
12
12
12
12
14-4
214
64-2
13-4
24-4
622
188
60-3
14-5
—
—
—
15-0
28-3
56-7
9
—
30-6
176
5i-8
L%
c%
Length of exp.
(min.)
L%
D%
16-3
24-2
59-5
—
c%
• Untreated.
—
E
—
200
76-4
st
2§- 7
36-1
c,
c,
c.
c5
c6
Average
18
21
18
12
12
12
12
12
IO
—
J7-5
20-4
62-1
184
139
677
16-0
—
—
—
—
4'St
I5'2
17-2
676
16-3*
69 6
64-2*
4-St 30t
24-1
30'6
4S"3
147
45'3
40-0
io-5t i3-5t
191
179
63-0
15-2
37-4
674
22"0l
25-01
53-oj
f Animals counted every half minute (in all other cases counted every minute).
\ Acetic acid treatment.
In no case was the percentage of animals found at the dark end to be double that
found at the light end, and in four out of nine cases the percentage in the light end
was higher, which never occurred with normal or red-eyed types. The mean difference between the two ends shows a difference of 3-2 per cent, in favour of the dark
end, a difference much less than the limits of probable error.
Equally convincing were the results of experiments with acetic acid; this has no
effect upon the light reactions of the animals. The only effect to be noted is that
Reactions of Normal and Mutant Types of G. chevreuxi to Light
439
animals treated acetic acid as a noxious stimulus, and since it was added through
the inlet in the centre of the tube they tend for a short time to be crowded towards
the two ends of the tube in their efforts to avoid high concentrations of acid.
This result can be expressed numerically; the percentage of animals in the middle
section of the tube decreases on the addition of acetic acid from 64-2 to 53 per cent,
(mean of all experiments).
Further, when there was a high concentration of animals at one end of the tube,
reversal of the light had no effect upon it, showing that it was merely due to chance.
V. DISCUSSION.
There is no need of any extended discussion of these results. We will merely
mention one or two points.
First, the complete absence of phototaxis in the cc types (albino and colourless)
is to be expected on the basis of our anatomical findings (Huxley and Wolsky, 1932),
which showed that such animals possess only the crystalline cones, the retinulae and
optic nerves being entirely absent. The species worked on by Barnard (1927) had
highly degenerate eyes, with no crystalline cones or retinulae, and only a certain
amount of interstitial tissue. However, the optic nerve was intact, and it is extremely
interesting to note that the animals still showed negative phototaxis. Secondly,
possibly the greater sensitiveness of the red-eyed (bb) type is perhaps what is to be
expected owing to the small amount of melanin pigment in their eyes, which we can
suppose exerts a protective effect upon the photosensitive substance in the retinula
in strong illumination. Further, it should be mentioned that sections of the eyes of
wild type and red-eyed Gammarus, including specimens kept in the dark for some
time and those exposed previously to rays of strong light, show little or no evidence
of " dark adaptation " as evidenced by changes in the position of the black pigment.
This is curious, since changes of this sort have been recorded for numerous other
Crustacea, including other species of Gammarus (see Parker, 1899; Smith, 1905;
Bennitt, 1924). This point will be treated more fully in a forthcoming paper on the
anatomy and development of various mutant eye types of G. chevreuxi.
Further, it should be remembered that the red-eyed mutants do not completely
lack melanin. The formation of this pigment is merely delayed, and its final level is
somewhat reduced (see Ford and Huxley, 1927). Adult animals of the red-no-white
stock used in these experiments after being bred at standard temperature of 230 C.
all showed an eye between stages 7-11 on the red-black chart used by Ford and
Huxley (loc. ctt.), full black being taken as 14. It seems clear from consideration of
the previous work of Loeb and others that a comparative investigation of the phototactic reactions of different species of Gammarus would yield interesting results.
44°
A. WOLSKY and].
S. HUXLEY
VI. SUMMARY.
1. A method is described for obtaining statistical results on the phototaxis of
G. chevreuxi. A long tube is illuminated from one end and the numbers of animals
in two arbitrarily delimited end-sections counted at regular intervals.
2. Wild-type specimens in normal conditions show a moderate degree of
negative phototaxis.
3. Animals with one eye varnished show circus movements; hence the phototaxis is true tropotaxis (Frankel, 1931).
4. The sense of reaction can be reversed and the animals made to show a
moderate positive phototaxis by the addition of acetic acid. Caffein has no effect.
5. Red-eyed mutants, which lack most of the melanin eye pigment, behave
similarly to the wild type, though there are indications that they are often rather
more sensitive to light, as shown by stronger negative phototaxis in normal conditions, weaker positive phototaxis after addition of acid. The variability of the
results, however, is too great to permit of definitive conclusions being drawn.
6. Albino and colourless mutants, which possess neither retinulae nor optic
nerves, show no phototaxis.
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