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INTRODUCTION
Tegula funebralis (Adams, 1854) is one of the most abundant of
the larger gastropods in the rocky intertidal along California shores.
Clumped together beneath rocks and in crevices at low tide, the animals
periodically move up and outward, littering the open rock surfaces
by the thousands. Some aspects of the movement of Tegula have been
studied by previous researchers. Wara and Wright (1964) studied the
intertidal distribution of populations at Pacific Grove, California,
and watched snails disperse over a two week period in several
environmentally different areas to gain insight into factors which
might influence movement. They found the animals remained in sheltered
areas during daytime high tides byt moved upward on rocks during high
tides at night. Upward movement was diminished in bright moonlight and
under turbulent conditions. Glynn (1965) observed that small T. funebralis
in the Balanus-Endocladia zone move out of their clusters when wetted
and distribute themselves more evenly over the substrate. The
responses of Tegula funebralis to turbulence (Overholser 1964) and
light (Kosin, 1964) have been studied in laboratory experiments.
Yet what is known about the movements of the snails in the field
with respect to diel and tidal cycles is based on qualitative
observations. Thetheepresentustudy the movements of T. funebralis
he Feld
were monitored,to determine what environmental events and/orrconditions
correlate most closely with those movements.
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FIELD STUDIES
-METHODS-
Studies were carried out on the north side of Mussel Point, Pacific
Grove, California, during the period May 8-31, 1978. After initial
observations three sites were chosen on the basis of (1) accessibility
at high tide, (2) the presence of numerous Tegula funebralis,
(3) the availability of vertical of sloping rock surfaces on which
the snails could climb, and (4) the consistency with which Tegula
moved on and off those surfaces. At each site the Tegula population
was clustered in crevices and under narrow rock overhangs below the
elevatedrrocks at low tide. As conditions changed, T. funebralis
emerged from hiding and spread outward and upward onto exposed surfaces
permitting periodic counts to be made easily.
Site I was a pyramid-shaped rock with a square base and four
vertical faces. It was approximately 30 cm at the base and 38 cm
high. This rock was divided about halfway to the top into upper and
lower zónes. At low tide Tegula were clustered in the crevice
where the base of the rock met the substrate. As the snails dispersed
they climbed the pyramid, moving into the lower, and then upper zone.
Site II was a flat, steeply sloping rock face on the side of a
large boulder. An area 20 cm high and 110 cm wide was marked on
the face near the base of the boulder. This area was divided into
twoee vertical zones, each about 20 cm. high and 110 cm wide.
At low tide Tegula were hidden underneath the boulder in a narrow
space between the boulder and the underlying rock substrate; in dis¬
persing they climbed up the rock face into the marked areas on the rock
face.
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Site III consisted of three rocks close together and of approximately
the same height, with snails clumped at the base of the rocks at low
tide. Each rock had a flat, horizontal top. On each rock,, dispersing
snails climbed up a short vertical face and roamed over the upper
surface. Since the three rocks were subjected to nearly identical
conditions they were condidered together.
Data were taken hourly over a 24 hour period on eight separate
occasions diring the month of May. At each observation the number
of Tegula in each designated area of each site was recorded, as well
as whether that area was dry, washed intermittantly by water, or
submerged. Surge and wave action were rated on a scale of one to
four, ene representing nearly calm conditions and four representing
strong surge. In determining whether an area was to be considered dry,
washed or submerged, a previously dry site was considered washed
when first splashed or wetted by water on a rising tide. An area
was considered submerged only when covered continually by the sea.
More frequent observations had to be made at critical periods of
incoming and outgoing tides when these conditions were rapidly changing.
A plastic tube with a transparent plexiglass bottom was used for
underwater viewing to count smails on areas that were submerged. At
night, a flashlight with an orange filter provided the neccessary light.
-RESULTS-
Figures 1,2, and 3 show the tidal cycle and the number of Tegula
in each of the study sites at each observation. Also shown are the
particular periods when the sites were dry, washed, and submerged, and
an estimate of surge strength. Accurate observations were difficult
or impossible to obtain at high water (see areas marked "no data" on
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Figures 1-3). The results obtained were consistent for all three
study sites and can be considered together.
Certain features of the movements of Tegula funebralis show up
very clearly. First, the animals exhibit a tidal rhythmicity. The
snails were clustered and hidden when exposed to air at low tide.
They dispersed from their clusters when wetted, climbed steep slopes
where these were present, and distributed themsleves in the areas which
were washed and then submerged. The snails moved up along with the
rising water level, but did not move up into dry areas ahead of the
advancing swash. At ebbing tides as the water receded and the study
sites were first washed and then subsequently left exposed to dry,
Tegula moved downward out of those areas, back to the crevices and small
caves below. There are two important exceptions to this general scheme.
(1) On the nights of May 17-18 and May 28-29 (Figures 1,2,3) and
May 30-31 (Figures 1 and 3), significant numbers of snails were counted
in study sites which were neither washed nor submerged. (2) During
periods of high surge at high tide, the number of Tegula seen in the
study sites decreased significantly. This can be seen in data taken
on May 22-23 (Figures 1 and 3), May 28-29 (Figures 2 and 3), and
May 30-31 (Figure 1).
In order to compare the amount of movement of the Tegula populations
at night with that during the day, the data from all study sites
were combined (Figure 4). Only the counts made when the animals were
dry or awash were included since data taken at high tide when the animals
were submerged were too incomplete for valid comparison. It can be
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seen that many more snails were present in the study sites at night
than during the day. Nearly all animals dispersed during the day
were awash (or submerged), whereas many of those dispersed during the
night were exposed to air above the swash. Those animals which
were distributed on rocks above the swash zone at night had not moved
up under dry conditions; they had moved up into the study sites
during the previous high tide, and simply had not moved down as the
waters receded.
In summary the results show that in the field, the movement of
Tegula funebralis is positively correlated with the tide cycle.
The dates on which data were taken were chosen in order to portray
all possible relationships between the tidal and diel cycles. In
Figures 1,2, and 3 it is evident that as the time of a high tide gets
later and later in the diel period, so does the corresponding peak of
movement. The basic pattern of movement clearly followsthe tidal
cycle. During dry conditions, Tegula were clustered beneath rocks
and in cracks and crevices. Upon being wetted, Tegula funebralis
moved out over the rock surfaces. The reverse occurred as the tide went
down, the animals retreating to their protected areas when washed and
subsequently left above water. During high tide there were significantly
fewer Tegula on the rocks when the surge and wave action were at their
strongest. There were also clear differences between day and night.
At night, in contrast to the day, not all the snails moved off the rocks
at low tide, and numerous snails were left on rock surfaces which were
neither washed or submerged. It was also shown that Tegula moved out
in larger numbers on the rocks at high tide during night than during
the day.
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-DISCUSSION¬
The finding that Tegula move up during daylight high tides modifies
observations made by Wara and Wright (1964) that Tegula move up to rock
tops during high tide only at night. The upward movement is shown here
to be related primarily to the tidal cycle. However, the diel cycle
appears to play a modifying role. This is indicated by the fact that
more snails move up at high tides during the night than during daylight
high tides and the fact that some snails are found high on the rocks above
the swash during night low tides. An important observation in assigning
the diel cycle a role secondary to the tidal cycle in influencing
Tegula movement is that Tegula found on rock surfaces above the swash
at low tide had moved up during the previous high tide and had been
left there. Under similar tidal conditions during the day these
snails would havebbeen "high and dry", but the rocks at night tend
to remain wet for a long time after being exposed to air on a falling
tide. Perhaps the residual moisture on the rocks at night was a suffi¬
cient stimulus to keep some snails out on thetrocks. This is consistent
with the general conclusion that Tegula movement is dependent primarily
on moisture and perhaps only secondarily on light or factors whose
variation is correlated with light. Field observations by Glynn (1965)
and Hewatt (1934) show that there is an increase in feeding activity of
Tegula at night. Increased numbers at night during all conditions
would then correlate with an increase in feeding activity.
Although taking counts of Tegula in the field at times of strong
surge and wave action is often impossible, the results that were obtained
agree with laboratory studies by Overholser (1964) and field observations
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by Wara and Wright (1964) that Tegula funebralis move down to sheltered
areas in response to a strong current.
STUDIES CONDUCTED IN OUTDOOR TANKS
-METHODS-
In order to study the movement of Tegula funebralis in relation
to the diel cycle in the absence of tidal fluctuations, snails were
exposed to the natural day-night regime but under constant submersion.
Tegula of various sizes were collected from various intertidal sites
at Mussel Point and placed in two fiberglass tubs provided with fresh
flowing seawater. The large tub (91 cm x 91 cm, with a water depth
of 55 cm) contained 212 snails, the smaller tub  cmxcm, with a
water depth of cm) had 100 snails. Rocks from the natural habitat
were placed in the tubs to provide protected areas where the Tegula
could clump together out of view. Äfter a 48 hour acclimation period,
observations were made hourly during some of the same 24 hour periods
when field data were being collected. In each tub, animals that could
be seen on the rock surface, the tub bottom, and the tub sides were counted.
-RESULTS-
Under conditions of constant submersion, the movement of Tegula
to and from clusters beneath rocks is clearly related to the diel
cycle. Figure 5 shows the number of snails visible in the tub at
each hour. The tidal cycle shown is that of the sea during the
observations; the tanks showed no know tidal effects. During most
of the day Tegula were hidden beneath the rocks. At approximately
1500 hours they began to move outeon the rock and the tub surfaces
until most of the snails had moved from underneath the rock. Starting
at around 0500 hours the number of visible snails decreased as the
rocks.
There is
under
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Tegula began to move back below the rocks. There is no correlation
between movement of Tegula in the tubs under constant submersion and
the events of the tidal cycle present in the field.
-DISCUSSION AND CONCLUSIONS¬
The general finding that Tegula tend to be negatively phototactic
supports the conclusions of Kosin (1964). In Figure 5 it can be seen
that the movement of Tegula from beneath the rocks is more gradual than
the movement back to those protected areas. The tubs were located
on the northeast side of a large building. They received direct sunlight
from sunrise until about 1230 (small tub) or 1430 (large tub). The
animals were thus exposed to a more rapid change in light intensity during
the early morning sunrise than during the late afternoon sunset.
The most rapid change in light intensity occurs at a time when the
animals might be expected to show the greatest dark adaptation.
These findings and the work of Kosin (1964) and Wara and Wright (1964)
support the statement that the diel cycle exerts an important influence
on the movement of Tegula funebralis, although the field data show it
plays a role secondary to that of the tidal rhythm. Surge plays an
important modifying role during periods of strong wave action at high
tide.
No hint of an endogenous rhythm in the movement of Tegula was noted.
No movement correlated with the tide cycle in the animals kept submerged
in the tubs. Kosin (1964) found no evidence of rhythmic behavior in
T. funebralis maintained continually submerged in either continuous
light or continuous darkness. Tegula do show a predictable rhythmic
pattern in their movement, but these data and observations of earlier
researchers suggest that the movementof Tegula occur in response to such
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stimuli as changes in moisture (eg. wetting by swash after a dry period),
changes in light, or marked changes in turbulence. Even relatively
small fluctuations in environmental conditions may have an effect.
SUMMARY
1) Studies in the intertidal zone and in outdoor tanks were carried
out at Pacific Grove, California, to determine the movements of
Tegula funebralis and the environmental events correlated most closely
with them. The snails cluster in sheltered areas at low tide especially
during the day.
2) Movement of Tegula funebralis into the open and upward on rocks
is positively correlated with periods when the animals are washed and
submerged.
3) Tegula move upward when awash and submerged during both day and
night high tides, but more snails move up during the night than during
the day.
4) During the day as the tide recedes, Tegula stay below the water
line and retreat back to area where they cluster and remain inactive,
At night, numerous snails are left exposed to air on the rocks as the
tide recedes. Unwetted Tegula do not move up on dry rocks during either
night or day.
5) During periods of high surge, Tegula submerged or awash move
down rock surfaces to protected areas.
6) Tegula constantly submerged in outdoor aquaria showed no evidence
of tidal rhythmicity, but followed a diel cycle of activity. During
most of the day the snails stayed clustered under the rocks. In late
afternoon and evening the snails moved out and climbed up the rocks
and sides of the tanks.
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7) Tegula movement shows a basic tidal rhythmicity, modified by
the daily cycle of light and darkness and by conditions of high surge.
ACKNOWLEDGMENTS
I would like to thank Dr. Donald P. Abbott for his suggestions,
criticisms and encouragement during my work. His unlimited enthusiasm
and energy helped make this project possible.
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LITERATURE CITED
Glynn, Peter W. 1965 Community composition, structure and interrelation¬
ships in the marineintertidal Endocladia muricata- Balanus glandula
association in Monterey Bay. Beaufortia V.12., No. 148
Hewatt, W.G. 1934 Ecological studies on selected marine intertidal
communities of Monterey Bay. Ph.D. thesis, Dept. Biol. Stanford
Univ.: 118-120
Kosin, Dorothea F. 1964 Light responses of Tegula funebralis
(Mollusca: Gastropoda). Veliger 6 (Suppl.): 30-37
Overholser, J. Alan 1964 Orientation and response of Tegula funebralis
to tidal current and turbulence (Mollusca: Gastropoda). Veliger 6
(Suppl.): 38-41
Wara, William M. and Wright, Benjamin B. 1964 The distribution and
movement of Tegula funebralis in the intertidal region of Monterey
Bay, California. Veliger 6 (Suppl.): 30-37
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FIGURE EXPLANATTONS
olle porban
Figure 1: Number of Tegula funebralis in Study Site 1. The top half
ver
Hack pad
of thebar represents the upper zone of the rock, the bottom half
represents the lower zone. Horizontal bars indicate periods when the
zones are washed (hollow bars) and submerged (black bars).
Figure 2 : Number of Tegula funebralis in Study Site 2. The top half
of the bar represents the upperzzone of the rock, the bottom half
represents the lower zone. Horizontal bars indicate periods when the
zones are washed (hollow bars) and submerged (black bars).
Figure 3: Number of Tegula funebralis in Study Site 3. Horizontal
bars indicate periods when the zone is washed (hollow bars) and
submerged (black bars).
Figure 4: Total number of Tegula funebralis in the Study Sites under
washed (X's) and submerged (black dots) conditions.
Figure 5: Number of Tegula funebralis visible on rock surfaces and
onl the sides and bottom of each tub.
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