Physiology & Behavior, Vol. 36, pp. 445--449.Copyright©PergamonPress Ltd., 1986. Printed in the U.S.A.
0031-9384/86$3.00 + .00
Absence of Light Response
in Eyeless Planaria
E D W A R D A. A R E E S
D e p a r t m e n t o f Psychology, Northeastern University, Boston, M A 02115
R e c e i v e d 13 F e b r u a r y 1985
AREES, E. A. Absence of light response in eyeless planaria. PHYSIOL BEHAV 36(3)445-449, 1986.--Planaria Dugesia
dorotocephala were bilaterally enucleated, decapitated caudal to the auricles, or sectioned at the level of the pharynx.
None of these worms either altered or elicited any movement responses to stimulation by horizontal or overhead light even
though they made such responses to mechanical stimulation. By comparison, all intact planaria responded vigorously to
light stimulation but not to an elevation of water temperature. These results suggest that light receptors in these planaria
might have evolved away from the body surface and are located in the ocelli.
Planaria
Dugesia dorotocephala
Eyeless planaria
Light stimulation
Mechanical stimulation
Movement responses
Evolution
R E L A T I V E L Y little information is known on the evolvement of the eye from the simple receptor found scattered in
the body wall of the hydra and sea anemone [1] to the compound eye of the vertebrate. The formation of a bilateral
depression in the head end of an organism, in which the
simplest eyes first developed, occurred with the evolvement
of the flatworm [6]. However, the evolutionary period, in
which other photoreceptors on the body surface have disappeared, has not been firmly established. The purpose of this
study was to determine if such photoreceptors exist on the
body surface of planaria as measured by a negative phototaxic response.
Studies, dating from the latter part of the last century,
have shown that the movement of planaria through water is
altered by the sudden presentation of a light stimulus [2-5,
7]. When stimulated directly with artificial light, planaria,
with or without eyes and already in motion, altered their
path in a water-filled petri-style dish. This response occurred
even if the worms were moving away from the light source
when it was initially presented [5], although negative phototaxic responses in eyeless planaria were made at a significantly slower rate and with less precision than those made by
intact worms [5,7]. These results suggest that there are
photoreceptors other than the cephalic ocelli which, when
stimulated by artificial light, also alter the movement of
worms away from the light source. However, the location or
mechanism of action of such receptors has not yet been
identified.
In our laboratory, similar experiments did not produce a
negative phototaxic response in eyeless planaria. Further,
direct light stimulation did not elicit any movement response
in stationary, eyeless planaria, although these worms were
able to respond to mechanical stimulation in a similar but
somewhat slower manner than intact planaria.
METHOD
Animals
One hundred and forty planaria, Dugesia dorotocephala,
445
Heat stimulation
purchased from the Connecticut Biological Supply Co., were
tested. They were housed in plastic petri dishes 85 mm dia.,
10 mm deep, in dechlorinated water 17-2°C, 10 planaria to
each dish. They were fed raw liver 48 hours before the start
of the experiment.
Procedure
The planaria were divided into four groups: (1) a control,
intact group (N=30); (2) a bilaterally enucleated group (Fig.
1 A, N=30). Here, the planaria were temporarily immobilized by placing them in carbonated water [7] and a No.
11, surgical stainless steel scalpel was used to remove the
ocelli receptors. The lesion was made at an angle so as to
minimize damage to nerve cells just ventral to the receptors;
(3) a group (N=40) decapitated just caudal to the auricles
(Fig. 1 B); (4) a group (N=40) sectioned transversely at the
level of the pharynx. All worms in the experimental groups
were lesioned approximately 24 hours prior to their first day
of testing.
The light stimulus came from two sources. The first was
generalized, horizontal light from a 60 W incandescent bulb
located 25 cm from the center of the petri dish. This generated 70.3 ft 1 of illumination on the water surface as measured with a Macbeth illuminometer. The second was a
hand-held, overhead 3 V penlight which produced a concentrated beam approximately 2 cm in diameter of 135.2 ft 1.
Between test sessions the petri dishes were kept under a
black oil cloth and presented one at a time for testing. The
tests were conducted in a darkened room which was indirectly illuminated by stray light from an adjoining room. To
minimize any effects of stray or reflected light each petri dish
was kept on a black cardboard dull-finished surface, during
testing.
The first experiment was designed similar to an earlier
study [5]. Ten worms from each of the four groups were
tested individually, one trial a day for four consecutive days,
in petri dishes identical to the holding dishes. Each worm
was transferred by a plastic dropper to the center of the test
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FIG. 2. Trajectory of intact worms on 4 trials without direct light
stimulation in an almost straight-line (A) or erratic (B-D) trajectories
(T=Trial, Time in parentheses). Changes in direction sharpened for
illustration.
FIG. I. Lesioned worms, bilaterally enucleated (A) or.just caudal to
the auricles (B). In (B). the lesion was made three days before
photograph. Notice the start of regeneration. Magnification 32 ~.
dish. When intact worms are transferred from one holding
dish to another, they always m o v e through the water until
they reach the c i r c u m f e r e n c e where they generally direct
their dorsal surfaces towards its center. Thus, the time and
path taken to traverse the water b e c a m e appropriate dependent measures.
After each w o r m was transferred, a translucent
protractor, 10 cm dia., was placed on the rim of the test dish,
with its center directly o v e r the worm. Each planarian took a
few seconds to orient itself before it started to m o v e through
the water. At this moment, the horizontal light was presented, in random order, either, from the 0 ° or 180 ° positions.
The time taken to cross the water was noted and a verbal
recording was made during each trial of the path taken by the
worm until it reached the circumference. After each trial, the
tape was replayed and the path diagrammed onto recording
paper. Prior to obtaining these data, an identical control
study was done without light stimulation, in which 8 randomly selected w o r m s from each of the four groups were
tested, one trial per day for two successive days.
In the next experiment, the horizontal light stimulus was
presented to stationary planaria from the four groups for five
two-minute test trials with two rain intertrial intervals each
day for five c o n s e c u t i v e days. They were tested in the holding dishes and all worms showed no m o v e m e n t prior to the
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FIG. 3. Number of lesioned worms stopping in each quadrant before
reaching circumference after being transferred to test dish without
direct light stimulation.
presentation of the light stimulus. The w o r m s were o b s e r v e d
visually through a handheld 5 × magnifying glass. After each
trial each petri dish was rotated 180 ° to gather most of the
intact w o r m s on the side nearest the horizontal light source
for the following trial. F o r control, a 180 ° rotation was also
done for planaria in the three experimental groups.
E a c h day, following exposure to the horizontal light
stimulus, three planaria w e r e r a n d o m l y selected from each of
the 10 petri dishes and presented the o v e r h e a d light stimulus
for l rain. The 2-cm wide b e a m was large enought to shine
o v e r the entire dorsal surface of each worm. Again, the
planaria showed no m o v e m e n t prior to being tested.
W a t e r temperatures w e r e r e c o r d e d three times daily during testing; just prior to Trial l and during Trial 5 in
NO LIGHT RESPONSE IN EYELESS PLANARIA
447
LIGHT
0o
0
TABLE1
EXPE~MENTAL GROUPS
5
Bilaterally
enucleated
Lesion
at
auricles
Lesion
at
pharynx
Total
7
8
2
6
0
5
9
19
43.5
52.4
84.4
Days
1.2
3,4
Reaching
Circumference
LIGHT
FIG. 4. Trajectory of intact worms on 2 trials with direct light stimulation, (A) direct movement away from light, (B) initial movement
towards light, then abrupt turning away.
Mean time
(4 days, sec)
Number of planaria already in motion when stimulated by light
and reaching circumference. (Total possible: 20/group/2-day period.)
horizontal-light exposure, and during the 60-sec overhead
exposure. To determine if any movements were being made
to increases in water temperature instead of to the light
stimulus, a household clothes iron was used to raise the
water temperature 2°C, following which the worms were observed for 1 min.
Finally, to insure that the surgery did not interfere with
the motility of the planaria in the experimental groups, 5
randomly-selected worms from each of Groups 2, 3, and 4
were transferred to other petri dishes on Days 1, 3 and 5. A
glass rod provided mechanical stimulation and all movement
responses were noted through a stereomicroscope.
RESULTS
Without direct light stimulation, the eight intact worms,
after being transferred to test dishes, continuously moved
through the water in an apparent random manner until they
reached the circumference. On only 3 of the 16 total trials,
over 2 days, the trajectory was almost a straight line (mean
time 22.7 sec; Fig. 2 A) while on the other 13 trials, their
movement was in a pronounced erratic manner (mean time
85.4 sec; Fig. 2 B-D). Of the 48 total trials for the three
experimental groups without direct light stimulation, worms
on only five trials (all from the bilaterally enucleated group)
reached the circumference of the test dish before stopping.
On all other trials, planaria moved a mean distance of 29.5
mm from the center, before stopping. I f a worm did not show
any movement after 2 min, the trial was considered completed. The final locations appeared randomly selected (Fig. 3).
Following light stimulation, the l0 intact worms on each
of the four test days came to rest at a site opposite the direct
light source. Worms on 36 of the 40 total test trials showed
an immediate orientation and movement away from the light
(Fig. 4 A). Their trajectory was relatively direct, deviating
no more than _ 15° to their ultimate destination (mean time
26.0 sec). On the four remaining trials, the worms started in
the direction of the horizontal light for a distance no longer
than 15 mm, then abruptly turned and moved away from the
stimulus (Fig. 4B). The mean time here was 30.2 sec.
Under direct horizontal light exposure, lesioned planaria
on only 28 of 120 total trials reached the circumference. This
is detailed in Table 1. On the remaining 92 trials, the planaria
stopped before reaching the circumference of the test dish.
The mean distance travelled was 24.0 mm and 2 min after a
worm stopped moving the light stimulus was turned off. On
FIG. 5. Photograph of three intact planaria moving away from the
horizontal light stimulus. Two side lights were turned on just prior to
taking the picture, hence the double shadows. Magnification 10×.
37 of these trials worms were found, 24 hours later, in approximately the same location at which they initially came to
rest. In all trials, the direction of movement was independent
of the location of the light source and resembled that taken
by lesioned worms which weren't directly stimulated
(Fig. 3). The number of trials on Days 1 and 2 on which
worms moved to the circumference was compared for the
three experimental groups with and without light stimulation. Three chi square tests showed that the movement of
worms to the circumference was independent of the light
conditions for each experimental group. In 92 of 120 total
trials for the three lesioned groups, the planaria, while moving and being stimulated by horizontal light, stopped before
reaching the circumference of the test dish.
When stimulated with horizontal light the control stationary planaria elicited three distinct responses. Over the 25
total trials, 63% of the worms moved to the side opposite the
448
AREt.IS
light source in an approximate straight-line manner (Fig. 5)
similar to the control planaria in the previous experiment.
Other planaria (27%) remained relatively still but bent the
upper segment of their bodies so that the ocelli were oriented
away from the light. The remaining 10% actually moved
towards the light, pausing several times to orient their ocelli
away from the stimulus. At the end of each trial most
planaria had gathered on the side opposite the horizontal
light source, thus necessitating the rotation of the dish 180°
during the intertrial interval. With no obvious exceptions,
there was a movement response to the horizontal light
stimulus by each intact planaria on each test trial. Under the
concentrated, overhead light stimulus the six randomlyselected intact worms responded by moving continuously in
the water during the l-rain exposure period. Their movements continued for about 15 sec after the stimulus ended.
With two exceptions on the first test day, the 80 worms in
the three experimental groups responded in uniform manner.
Under horizontal light, no movement responses were made
by any worms on any of the 25 test trials. Under overhead
light, all but two planaria did not respond on any test trial.
Fifteen seconds after the stimulus onset, one worm from
Group 2 moved 20 ram, almost in a straight line, then
stopped. Thirteen seconds after the stimulus was presented
one worm from Group 3 moved in a circular pattern for 30
sec. Neither planarian showed any further movement to the
stimulus.
Heat from the 60 W incandescent bulb did not measurably increase the water temperature during the test trials
while the concentrated beam raised the temperature near the
planaria being tested approximately 0.25°C. There were no
perceptible movement responses made by any worms to the
2°C increase in water temperature made by a household
clothes iron, nor for at least 1 min after this temperature
increase was reached.
Lesioned planaria which were transferred to other petri
dishes by a plastic dropped on Days 1, 3, and 5, and repeatedly stimulated by a glass rod, moved from the center of
the dish where they were released to the circumference, then
positioned themselves as the intact worms did. Those
lesioned planaria which weren't stimulated after their release
almost always stopped moving before reaching the circumference of the dish; their behavior very similar to those
worms in the previous experiment. When rotated on their
dorsal surface with a glass rod, the control planaria always
righted themselves by twisting in a corkscrew manner. The
lesioned worms righted themselves in one of three ways.
About half of the total righting responses made over the
three days was done in similar manner with the controls.
Almost all other righting responses were done by planaria
bending the anterior segment of their bodies out of the water,
similar to a human doing a sit-up but continuing until the
worm was oriented with its ventral side down. On occasion,
a lesioned planarian would not right itself at the spot where it
was rotated but moved very slowly, on the surface of the
water, dorsal side down, until it reached the circumference
of the petri dish. it then positioned itself like the other
planaria.
DISCUSSION
Results of several studies have shown that planaria, al-
ready in motion, will alter their pattern of movement when
stimulated by light [2-5, 71. This change in behavior occurs
in both intact and eyeless worms and is independent of the
direction from which the light stimulus is presented. These
results have led to the conclusion that, in addition to ocelli.
there are other photoreceptors on the bodies of planaria.
although their locations or mechanisms of action arc not
known.
In our experiments, intact worms either moved away
from the light or oriented their ocelli away from it. However.
the movements made by lesioned worms, which were already in motion when exposed to horizontal light:, were very
limited and did not correlate with the location of the light
source. They were made in random directions away from the
center of the test dish. Also, movements made by such
worms lacked consistency and uniformity, were quite sluggish, and did not display the smooth, rhythmic manner in
gliding through the water as did the intact worms. Only
movements by the bilaterally enucleated planaria resembled
the undulate patterns of intact worms, yet these planaria also
did not move in a direction away from the light source.
These results are in sharp contrast to those previously
reported [5,7], yet widely accepted. Since physical properties of the sources of light between studies might be quite
different, e.g., a Welsbach burner [5] and a 60 W incandescent bulb, these studies may not be directly comparable. Yet
it is difficult to attribute all the discrepancies in results only to
the physical differences in the experimental conditions, especially since the movement patterns of most lesioned worms in
our stud~ were so dramatically different from those made by
similarly-lesioned planaria in earlier experiments [5,7].
In addition to determining the effects of light stimulation
on an ongoing movement response, we also examined its
effects on stationary planaria. Here, the results appear unequivocal. A light stimulus presented to stationary, intact
planaria consistently elicited one of several movement responses, but always so that the ocelli were directed away
from the light source. Identical stimulation to eyeless
planaria did not generate any movement responses. In addition, intact and lesioned planaria do not respond to a 2°C
elevation of water temperature, a significantly greater increase than that generated by the two incandescent sources.
This suggests that all movements made by the control worms
following light stimulation are due to light rather than to the
detection of any increases in either water or body temperature. Finally, the surgical procedures done on planaria do not
dramatically alter a worm's ability to make movement responses, following mechanical stimulation.
From an evolutionary position, since the flatworm is
credited to be the first organism to have its eyes located
bilaterally in its anterior end [6], it might also be the first
organism to have eliminated receptors to visible light from
the rest of its body. However, our results do not exclude the
possibilities that light receptors still exist in the body surface
of planaria which might elicit or alter motor responses following stimulation to specific wavelengths other than those
used in our experiment. Also, if such receptors still exist,
they might elicit responses other than changes in motor
activity--such as an alteration of electrical activity, as occurs on the body surface of the hydra following its exposure
to light [6].
NO LIGHT RESPONSE IN EYELESS PLANARIA
449
ACKNOWLEDGEMENT
This research was supported in part by Grant No. R R 07143,
Department of Health and Human Services.
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