Keith Bauer
Behavioral response to light in S. heathiana
INTRODUCTION
Many species of chitons have aesthetes, unique
photoreceptors whose structure has been examined (e.g.
Omelich, 1967), but whose role in behavior has received
little attention. The most common light-mediated
behaviors noted among chitons are: 1) phototaxis
(usually found to be negative); 2) diurnal activity
patterns; and 3) shadow responses (Heath, 1899; and Boyle,
1972). The first two behaviors require only photoreceptors
that are able to detect the presence of light and of light
intensity gradients. The shadow response, on the other
hand, requires the ability to detect sudden decrements in
light intensity. Since the approach of possible predators
could very well be signalled by a shadow, the ability to
detect a sudden drop in light intesity may be of selective
value. Obviously, a shadow response can only occur
when animals are illuminated, on the other hand, since
the sensitivity of most photoreceptors is attenuated by
long exposures to light, due to bleaching of the visual
pigment, it might be expected, that the shadow response
of chitons would weaken during the day. However, in
another mollusk, Lima seabra (Born), the strength of
the sensory response to shadowing increases during light
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Keith Bauer
Behavioral response to light in S. heathiana
adaption, and a photoreceptor mechanism for this phenomenon
has been proposed (Mpitsos, 1973). The present investi¬
gation, on the chiton Stenoplax heathiana (Berry , 1946),
obtained the first measurements of an increase in a
behavioral response to shadowing during light adaption.
MATERIALS AND GENERAL METHODS
Specimens of Stenoplax heathiana were collected
intertidally off Mussel Point, Pacific Grove, California,
in May, 1974, and were used in experiments not more than
three weeks after collection. The animals were maintained
unfed in tanks of running seawater at a temperature of
about 14°c.
The response to shadowing in this animal consists
primarily of a movement of the dorsal surface towards
the substrate, hereafter termed a "clamp-down". In
order to measure this clamp-down response, the dorsal
surfaces of the animals were first carefully dried, and
a pin, fashioned into a hook, was glued to their plates
at about the middle of the body. After allowing the
animals to adjust to this condition for at least 24 hours,
a specimen was selected and placed in a wax-bottomed pan
filled with sea-water kept at about 14°C by a Peltier cell.
A string from a force transducer was hooked around the pin
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Keith Bauer
Behavioral response to light in S. heathiana
on the specimen's dorsal surface and pulled taut. The out¬
put of the transducer was than coupled to a polygraph.
giving a permanent record of relative changes in string
tension. In order to eliminate all lateral movement.
a tight enclosure was built around the animals using
pins inserted in the wax. With this enclosure in place,
string tensions could be changed only by vertical movement
of the chiton's plates.
Two light sources were used during the course of the
experiment. Monochromatic test flashes were supplied by
a high intensity diffraction grating using a 100-watt
tungsten-halide lamp; the intensity of this source was
controlled by alinear neutral density wedge; a camera
shutter was used to control the duration of the test
flashes. Background adaptation lights were provided by
a heat-filtered 18-watt tungsten lamp equipped with a
Corning 5-58 filter, with a transmission maximum at about
450 nm, and a Wratten 22 filter which cut off wavelengths
of light below 560 nm.
EXPERIMENTS AND RESULTS
When presented with a 20 second test stimulus from
the monochromatic source (A = 500 nm), Stenoplax displayed
a behavioral clamping down to both the presentation of
page 4
page 5
ith Bauer
Behavioral response to light in S. heathiana
the flash ("on-response") and to its removal ("off-response").
The behavior as measured was the same for each stimulus;
a rapid lowering of the dorsal surface began 1 to 3 seconds
after the onset of the stimulus, and continued for about
five seconds. The chiton then returned to its original
position, a process which was highly variable in duration
and lasted up to several minutes (see Figure 1). The
actual displacement of the animal's plates during this
process was only about one mm; the tension of the string
from the strain gauge may have decreased the normal
displacement slightly.
The clamp-down response could be evoked by other
stimuli as well. Stroking the dorsal surface of the
animals with a pin, disturbing the water in the tray, or
any sharp movement which might affect the tray, such as
a tap on the table-top close to the animal, resulted in
a clamp-down. Some animals clamped down in the absence
of stimuli perceivable by the investigator. The clamp¬
down responses elicited by all types of stimuli were
indistinguishable from one another. Only clamp-downs that
occurred from 1 to 3 seconds after the presentation of a
light stimulus were counted as responses to light.
In order to determine the relative effects of light
and dark adaptation on the on- and off-responses, specimens
were first allowed to dark adapt for at least one hour.
Ketih Bauer
Behavior response to light in S. heathiana
Then test flashes, each 20 seconds in duration and
given at least six minutes apart, were presented from the
monochromator. The test flashes used within any one
sequence were identical in intensity, and had a wavelength
of 500 nm. After dark adaptation, the on-response was
of much greater magnitude than the off-response (Figure
2). The animals were then light-adapted for about 30
minutes. The monochromatic test flashes were again
used to test the magnitude of the responses. During
and immediately after the light adaptation, the on¬
response was considerably weakened, while the off-response
usually increased during presentation of the adaptation
light, and was always stronger just subsequent to its
removal (Figure 2).
The magnitude of the effect of light adaptation was
found to be dependent upon the intensity of the adaptation
light: the stronger the light used to adapt the animal
prior to testing, the more the off-response was increased
(Figure 3). It does not appear to matter whether the
on-stimulus is given before the off-stimulus or vice-versa,
and furthermore, the rates of change of the two responses
are not necessarily correlated (Figure 4). In Lima scabra
the eame effect has been noted and shown to be highly
dependent on the wavelength of the adaptation light
(Mpitsos, 1973). The effect of using three different
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Keith Bauer
Behavioral response to light in S. heathiana
ranges of wavelengths of the adaptation light on the
magnitude of the off-response was examined in
Stenoplax as well; however, no significant wavelength
dependence could be discerned (Figure 5).
DISCUSSION
While Stenoplax is predominately a nocturnal animal
which seeks the undersides of rocks during the day, it
can be found exposed early in the morning (Heath, 1899).
In addition, presumably the moon can sometimes provide
sufficient light for some predators to see Stenoplax when
it is out grazing. Consequently Stanoplax's ability to
detect and to react to a sudden change in light
intensity may reflect its ability to detect and react
to changes in lighting produced by an approaching
predator. Since chitons are unable to outrun many
potential predators, a clamp-down to impede removal from
the substrate is probably their best defense.
From a behavioral viewpoint, at least, Stenoplax
seems to rely on its "on-receptors" during periods of
low illumination and on its "off-receptors"during periods
of high illumination for its primary source of information
about sudden changes in intensity levels. This may give
the animal a wider range of ambient illumination in which
a potential predator can be detected.
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Keith Bauer
Behavioral response to light in S.heathiana j
page 8
Physiologically, the weakening of the clamp-down in
response to the on-stimulus during light adaptation
can be accounted for by bleaching of the visual pigment.
The increase in the off-behavior during light adaptation y m bnbed.
e te oly t pl in phet re genjon ott-Lespage hay hee svonn jo nelene dedins
is the file clam Lima scabra. 'A wavelength dependence of
Vere
this effect led to a proposed mechanism involving two
visual pigments. The present study has turned up no
such wavelength dependence in Stenoplax, suggesting that
this mechanism is probably not functioning here. However,
it can not be conclusively ruled out, solely on the
basis of this study. In order to more fully investigate
this problem, a neurophysiological approach would be
ideal; unfortunately preliminary attempts to record
from the lateral nerve cord, which is known to innervate,
the aesthetes, were unsuccessful.
SUMMARY
The chiton Stenoplax heathinan was shown to respond
to both the onset (on-response/ and the removal (off-response)
of a light stimulus. In both cases the response consisted
of a slight clamp-down to the substrate, presumably to
help maintain the animals grip on the substrate. If
the animal is light adapted, the magnitude of the off¬
response increases while that of the on-response decreases.
Keith Bauer
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Behavioral response
light in S. heathiana
These responses were shown to be independent of the wavelength
of the light used to light adapt the animal, which
indicates that the differences did not result from
stimulating two separate visual pigments. Thus, while
this effect has clear adaptive significance, its
physiological mechanism has not been determined.
ACKNOWLEDGMENT!
I would like to thank Dr. George J. Mpitsos for
his advice and encouragement throughout this project,
and especially for his critical reading of this paper.
I would also like to thank the staff of Hopkins Marine
Station for their kind assistance.
Keith Bauer
Behavioral Response to Light in S. heathiana
LITERATURE CITED
Boyle, Peter R.
1972. The aesthetes of chitons. Mar. Behav. Physiol.
1(2) : 171-184; 6 plts.
Heath, Harold.
1899. The development of Ischnochiton. Zool.
Jahrb. Abt. Anat. 12: 1-90; 5 figs.; 5 plts.
(9 May 1898)
Mpitsos, George J.
1973. Physiology of vision in the mollusk Lima scabra.
Journ. Neurophysiol. 35(2): 371-383; 15 figs.
(March 1973)
Omelich, Paul
1967. The behavioral role and the structure of
the aesthetes of chitons. The Veliger 10(1):
77-82; 2 plts.
(1 July 1967)
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Keith Bauer
Page 11
A Behavioral Response to Light in S. heathiana
Figure Captions.
Figure 1:
A. Response of Stenoplax to test flashes of light during dark
adaptation (A of the test flash - 500 nm). B. Response to the
same test flash just subsequent to approximately 30 minutes of
light adaptation. (Vertical axis indicates response magnitude in
upper traces. The lower traces indicate the duration of test flashes:
upward deflection indicates the onset of the stimulus and down-
ward deflection indicates the cessation of the stimulus.
Stimulus duration is about 20 seconds.)
Figure 2:
Average of five runs showing magnitudes of the on-response
and off-response during light and dark adaptation. Animals were
dark adapted for one hour prior to the start of the experiment.
The striped bar shows the period of light adaptation (about
30 minutes). Response magnitude, in this and in all figures, is
by the polygraph
in arbitrary units which are in proportion to uncalibrated values recorded
Figure 3:
Magnitude of the off-response after 30 minutes light adapta-
tion under different intensity conditioning lights. Different
intensities were achieved using neutral density filters. The
adaptation light
was removed at time = 0.
Keith Bauer
Page 12
A Behavioral Response to Light in S. heathiana
Figure 4:
Magnitude of the on- and off-responses during various
stimulus regimes. Animals were light adapted for 20 minutes
prior to the start of the experiment using the monochromator, which
was used throughout to give test flashes. During Part I, the
light was interrupted intermittently for about 20 seconds to give
off-on" stimuli. For Fart II, the light was turned off and the
responses of the animal were followed during dark adaptation using
using short flashes of "on-off" stimuli. In Part III, a return
to the first stimulus regime was used to follow the responses during
light adaptation. Note that the rate of change of the off-response
is greater than that of the on-response during dark adaptation.
while during light adaptation the reverse is true.Lower trace same as Fig. 1.
Figure 5:
Nagnitude of the on- and off-responses under three different
colored conditioning lights:. A. White light; B. Blue light (Corning
5-58 filter); C. Red light (Wratten 22 filter). Dashed lines indic-
ate onset and removal of the adaptation light.
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