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PLEASE TYPE ABSTRACT DOUBLE SPACED BELOW
ROSS, THOMAS L. (Hopkins Marine Station of Stanford University,
Pacific Grove, California). Light Responses in the Limpet
Acmaea limatula (Mollusca:Gastropoda: Prosobranchia).
The Veliger
Light responses in six species of the genus Acmaea
(Acmaea scabra, A. digitalis, A.scutum, A. pelta, A. asmi,
and A. limatula) were investigated. Acmaea limatula showed
the strongest responses to light. All responses exhibited
were negative phototaxes. Experiments varying color and
intensity indicate the eyespots of A. limatula are important
as photoreceptors in colors blue, green and red at high andh
low intensities. There is at least one other photoreceptor and
pigment, which is functional only in blue light at higher
intensities. Other observations showed no correlations
between size and response to light.-Author.
PLEASE DO NOT TVPE BELOW THIS LINE
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PONSES IN THE LIMPET ACMAEA LIN
LIGHT RE
(MOLLUSCA: PROSOBRANCHIA)
By
Tom L. Ross
Hopkins Marine Station of Stanford University
Pacific Grove, California
* Fetnot
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Tom L. Ross
Preliminary observations of the phototactic responses
of the limpet Acmaea limatula Carpenter, 1864, led to the
studies of spectral sensitivity reported in this paper.
Mean response times of dark-adapted animals to light of
different colors and intensities were determined, and at¬
tempts made to anatomically localize the photoreceptors.
The results suggest the possibility of at least two photo-
receptors and several visual pigments in this organism.
METHODS AND MATERIALS
The animals used were all collected at Mussel Point,
Pacific Grove, California, in the intertidal zone between
plus 2-6 ft. The observations of responses were made in a
plastic lined, water-tight wooden trough, which could be
continuously supplied with fresh sea water. At each end
of the trough, a 150 watt tungsten lamp, enclosed in a
sheet metal box, supplied the illumination. The boxes.
painted flat black to reduce reflection, had 3/4" holes
drilled in the face at the height of the lamp filaments.
Each light, connected through a rheostat, could be inde¬
pendently operated for illumination from one, or both ends
of the trough.
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Tom L. Ross
Various areas of the spectrum were isolated by
Corning glass filters, nos. 554, 401, and 244, correspond¬
ing respectively to blue, green, and red. Maximum trans¬
mittance with the blue filter was at 425 mu, decreasing to
10% transmittance at 500 mu, and cutting off completely
above 540 mu. The green filter transmitted maximally at
520 mu, with no transmittance below 460 mu or above 600 mu.
The red filter transmitted all wavelengths greater than 600
mu. A standard infrared absorbing filter, composed of acidic
0.5% CuSOg in distilled water, was also used. Light inten¬
sity was varied by adjustment of a rheostat.
In experiments with white light the animals were
placed 32 cm from the light source and the intensity at
each rheostat voltage setting determined with a thermopile.
Where color and intensity were varied, this thermopile
reading was used to approximate equal intensities of the
light at each color at any given voltage. This was accom¬
plished by varying the distance of the animals from the
light source, as dictated by the thermopile readings. The
difference in distance between the placement of animals in
white light and the placement in any other color never ex¬
ceeded 10 cm (in blue), and in some cases was as little as
2 cm (in red). Because of the insensitivity of the galvanom¬
eter, accurate measurements were not possible below 60 volts.
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Tom L. Ross
The equalizations of intensity are, therefore, only
approximate.
Data were obtained on response (yes or no), response
time, and response time as a function of color and intensity,
The response time given is from initial illumination until
a definitive turning response (described below) was ascer¬
tained. All animals used in the experiments were allowed
to dark adapt for a minimum of one hour.
III
SUETS AND DISCUSSION
In the initial work it was necessary to determine a
definitive response of the limpet to illumination, such as
a turning response (Fraenkel and Gunn, 1940), or to changing
illumination, such as in the clam, Ma (Hecht, 1919). Also
it was necessary to determine whether the genus Acmaea would
exhibit sufficiently consistent responses on which to base a
study. Ten animals of the species' Acmaea pelta, Eschscholtz
1833, Acmaea scabra, Gould 1846, Acmaea scutum, Eschscholtz
1833, Acmaea digitalis, Eschscholtz 1833, Acmaea limatula,
Carpenter 1864, and Acmaea asmi, Middendorf 1849, were col¬
lected and tested for light responses.
The general response to light, when exhibited in any
species, was found to be a combination of backward movement,
sidewards movement, and a 180° reversal of orientation.
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Tom L. Ross
During illumination the animal shows a characteristic exten¬
sion of pallial and cephalic tentacles, while the shell is
kept fairly close to the substrate.
Quantitative results (Table I) show that both the
percentage of animals responding to light, and the speed of
this response, is greater in Acmaea limatula than in the
other species. This species, therefore, was chosen for
continued study. It was decided that the reversal response,
if exhibited within four minutes after illumination, would
be scored as the definitive response. Responses after four
minutes were scored as negative.
fty-one Acmaea limatula were collected and tested,
of which fifty showed positive responses with a mean re¬
sponse time of 76 seconds and a standard deviation of 26
seconds. The responses of these animals to illumination
from the opposite direction was also tested. In only 7
out of 50 cases did the animal stop in his initial turning
response when the illumination was changed from one end of
the trough to the other. After an extended time those ani¬
mals which did not initially respond to the second light
would migrate away from it. These observations suggest
some process of bleaching or adaptation to illumination.
All the following studies were done with the fifty animals
which exhibited the initial positive responses, regardless
of the response to the second illumination.
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Tom L. Ross
A control experiment was then done to determine the
animal's natural response to being removed from an aquarium
and being placed, in total darkness, into the testing
apparatus. Of twenty-five animals tested in the dark,
only two showed behavior which might have been cons
rued
as a positive response had the animal been illuminated.
The above findings show that the results obtained in all
experiments are not significantly affected by the animal's
natural response to being moved, but depend only on
illumination.
In the next experiment ten animals were placed in a
dry trough to ascertain the effect on response time and
percentage of responses. Six of the ten animals showed
responses with a mean time of 112 seconds and standard
deviation of 36. This experiment shows that although
the animals do respond to light when out of water, there
is a significant decrease in both pet rate and anumber of
responding animals.
In the next experiment correlations between size
and response time were examined. Thirty animals were
chosen; ten each in size ranges less than nine om, nine
to sixteen om, and greater than sixteen cm. All three
groups showed at least nine out of ten responses, with
respective times and standard deviations of 71122, 68114,
7+0
and //.21 seconds. These results suggest no correlation
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Tom L. Ross
between response to light and size of the animal.
Spectral sensitivity of the response was then in¬
vestigated, using the various color filters. The results
of this experiment, shown in Table 2, indicate equal sensi¬
tivity in the visible spectrum at the maximum intensity of
the lamp.
In the next experiment, attempts were made to
localize the photoreceptors of Acmaea. The presumptive
receptors are the eyespots, which in Acmaea limatula are
greenish in color and are located at the base of the
cephalic tentacles on the back of the head. Several at¬
tempts were made to remove the eyespots by cauterization
with a hot needle. The five out of 18 animals which
recovered completely had lost their cephalic tentacles as
a result of the operation, and hence no indications of
eyespots were apparent. These five animals were then
tested for sensitivity to light of various speotra and
atensity. The second part of Table 2 shows the eyeless
animals react significantly only to white and blue light
at maximum intensity.
The last experiment run in this study determined
the effect of varying intensity of white, blue, green,
and red light on the responses of normal and eyeless Acmaea
limatula. The results of this experiment are shown in
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Tom L. Ross
Table 3 and Figures 1, 2, and 3. In normal animals, the
response time in white, blue, and red light is biphasic.
whereas in eyeless animals responses are only seen i high
intensities in blue and white light.
The results of the last two experiments suggest the
presence of at least two pigments in Acmaea limatula, with
the eyespots probably most important at the lower intensities.
The greenish color of the eyespot might account for the fact
that the animal exhibits poor responses to green light at
lower intensities. The site of the other photoreceptor is
completely undetermined. Two possible candidates are either
the pallial tentacles, which normally are extended from
under the shell edge, or, in A. limatula, the heavily pig-
mented side of the foot. Either of these possibilities
would entail an amazing nerve network to determine the
source of light since both the pallial tentacles and the
foot would be symmetrical sites for the photoreceptors as
opposed to the normal condition of directional asymmetry
of photoreceptors.
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Tom L. Ross
SUMMARY
Light responses in six species' of the genus Acmaea
(Acmaea scabra, Acmaea digitalis, Acmaea scutum, Acmaea
pelta, Acmaea asmi, Acmaea limatula) were investigated.
The limpet, Acmaea limatula, showed the strongest responses
to light. In all cases, responses when exhibited, were of
a negative phototactic nature. Experiments varying color
and intensity indicate the eyespots of Acmaea limatula are
important as photoreceptors in colors blue, green, and red
at high and low intensities. There is at least one other
photoreceptor and pigment, which is functional only in blue
light at higher intensities.
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Tom L. Ross
ACKNOWLEDGEMENT
The author wishes to express his thanks to Drs.
Lawrence Blinks and David Epel of Hopkins Marine Station,
for their ideas and comments throughout this project. This
work was made possible by grant GY806 from the Undergraduate
Research Participation Program of the National Science
Foundation.
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LI
ERATURE CITED
Fraenkel, Gottfried, and Donald L. Gunn
1940. The Orientation of Animals. Oxford, The Clarendon
Press, vi + 352 pp.
echt, Selig
1919. Sensory equilibrium and dark adaptation in
Mya arenaria. Journ. General Physiology 1:545-558.
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Table 1:
Table 2:
Table 3
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Tom L. Ross
TABLE LEGENDS
Responses of 6 species' of Acmaea to white light.
Animals 32 cm distant from 110 volt light source.
Responses of Acmaea limatula, normal and without
eyespots, to light of varying spectra. Intensity
is approximately equal in all colors.
Responses of normal Acmaea limatula to light of
different colors and intensities, and responses
of eyeless animals to varying intensities of blue
light in seconds. The numbers indicate mean re¬
sponse time in seconds, standard deviation, and
(in parentheses) the number of responses versus
animals tested.
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Tom L. Ross
IGURE LEGENDS
Figure 1: Responses of Acmaea limatula to varying inten-
sities of white light.
Responses of Acmaea limatula to blue, green,
Figure 2:
and red light of varying intensities.
Figure 3:
Responses of Acmaea limatula, without eyespots,
to blue light of varying intensities.
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FOOTNOTES
Page 1. * Permanent address:
Tom L. Ross
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TABLE 1
No.
Responses
Mean
response time
(Seconds)
173
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(seaands)
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TABLE 2°
Normal animals
No.
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Animals
Responses
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(seconds
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10
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57
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10
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10
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10
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