THE TEGULA FUNEBRALIS SHORE-LEVEL SIZE GRADIENT:
IMPLICATIONS OF SUSCEPTIBILITY TO PREDATION, AND SETTLEMENT
R. Eben Polk
June 7, 1998 Advisor: Jim Watanabe
Biology 175H Hopkins Marine Station
Permanent address
Permission is granted to Stanford University to use the citation and
abstract of this paper
Abstract: Past studies have found that the black turban snail, Tegula funebralis, exhibits
a shore-level size gradient, with mean shell size either increasing or decreasing in a down-
shore direction, at different sites. Predation has been implicated as a driving factor. A
shore-level size gradient was found at Monterey Bay, CA. Snails » 15 mm were more
abundant than other size classes in the Low intertidal zone, while snails 9 -15 mm were
the most abundant class in the Mid zone, and snails £9 mm were the most abundant class
in the High zone. Mean size increased in a down-shore direction. The study site had
virtually no Pisaster ochraceus, a known predator.
Laboratory experiments revealed that small snails are significantly less susceptible
to predation by Pisaster, compared with both medium and large snails. Experimental
responses to direct contact with tube feet show some size-related differences in short-term
behavior. However, these would tend to make small snails more susceptible than larger T.
funebralis; mechanisms other than defensive behavior must keep Pisaster from preying on
small snails.
A qualitative search for juvenile T. funebralis « 2 mm was undertaken at two sites
in Monterey Bay. Juveniles were abundant in the Mid zone at both sites, and rare or not
present in the High and Low zone, indicating that settlement occurs in the Mid zone, with
migration of T. funebralis becoming possible at greater sizes.
Results suggest that other factors besides predation, such as settlement, are
responsible for some components of a size gradient at a protected rocky shore in south
Monterey Bay, including the general absence of small T. funebralis from the Low
intertidal zone.
INTRODUCTION
Tegula funebralis, the black turban snail, is a common herbivorous intertidal
gastropod that reaches a maximum size of approximately 22 to 24 mm in the Monterey
Bay area. Its predators include Cancer crabs, octopus, and most notably, the seastar
Pisaster ochraceus. It is one of several intertidal invertebrate species that exhibit shore-
level size gradients (Doering et al. 1983), with mean size increasing or decreasing in a
vertical direction along the intertidal zone, at a given geographic site.
There are several biotic and abiotic factors which may create and maintain shore-
level size gradients in T. funebralis. One such factor is environmental stress, including
temperature extremes, desiccation (Marchetti and Geller 1987), and wave action. Another
is habitat choice, on both the scale of microclimates and across the intertidal zone. Food
availability and differential preferences could also contribute to a size gradient. Other
factors include responses to light and gravity (Doering et al. 1983), settlement (Fawcett
1979), and potentially competition.
The most well-studied factor that may affect Tegula size gradients is predation.
Paine (1969) concluded that a size gradient in which mean shell size increases in a down-
shore direction is caused by the presence of Pisaster ochraceus in the low zone. From
observations made in Washington, Paine claimed that juvenile T. funebralis settled high in
the intertidal. In this scheme, with increasing size over their life span, T. funebralis would
migrate down into the low zone to take advantage of the greater food resources found in
the form of macroalgae. Although 25-28% of adult snails would be eaten each year by
Pisaster in areas of overlap, those that survived there would grow faster. Pisaster would
maintain levels of predation in this zone that would continue to leave enough available
resources to maintain a selective advantage to downward migration. In the absence of
Pisaster, Paine felt that there would no longer be a strong advantage to migrating
downward, due to increased intraspecific competition. Consequently, any size gradient
would be less marked, or non-existent.
Fawcett (1984) concluded that a size gradient with mean shell size increasing over
decreasing intertidal heights would actually be observed in an areas with very little or no
predation, including that of crabs and octopus. This model for size gradients operated
under the assumption that T. funebralis settled as larvae in the mid-zone, the direction of
migration of large snails would then determine the size gradient. Large snails, as they are
more mobile, would either move down to take advantage of more abundant food
resources or, in the case of intense predation, up beyond smaller snails. In areas with high
predation, size gradients in which mean shell size increased in the upward direction were
observed at several sites, while the opposite gradient was observed in areas with low
predation intensities.
Despite the prevalence of theories for as a mechanism for establishing and
maintaining a size gradient, data on Tegula's size-specific susceptibility to predation is
lacking. Differences in susceptibility to predation, among snail sizes, when combined with
intertidal height-specific predation, might possibly be an important component of a size
gradient. In the present report, I present evidence for a size gradient at Hopkins Marine
Station at Monterey Bay, CA, and for size-related differential susceptibility to predation
by Pisaster. Evidence is presented for the nature of this size-related difference, gathered
in short-term behavioral experiments. Because results pointed to the importance of the
specific nature of settlement, and because the predation-based size gradient models depend
implicitly on the location of juvenile settlement, I also made qualitative observations of
juvenile distributions; these findings are also presented here.
MATERIALS AND METHODS
Vertical Size Gradient
Over 28 days, 10 transects 10 m in length were selected by placing a measuring
tape parallel to the water line (horizontal across the shore) in an area within the protected
rocky habitat at Hopkins Marine Station, Monterey Bay, CA (36°36' N 121°54' W). I
collected T. funebralis from transects during low tides, and ascertained quadrat heights
relative to the predicted low-tide sea level with a surveyor's level and a stadia rod. I
randomly selected five quadrats 25 cm in size within each transect. Each random distance
along a 10 m length of measuring tape was the center of a quadrat. Surface characteristics
and habitat type of each quadrat were also noted. I collected all Tegula spp. easily visible
to the unaided eye, and measured their sizes (maximum basal diameter) in the lab.
I partitioned the snails into groups based on the following intertidal height zones
(Ricketts et al 1985): a High Zone (between 5 and 2.5 ft, or 1.52-.76 m, above MLLW),
a Mid Zone (between 2.5 and 0 ft, or .76-0 m, above MLLW), and a Low Zone (between
0 and 1.8 ft, or 0-.55 m, below MLLW). Snails were also grouped into size classes: small
Tegula (S9 mm), medium (9 - 15 mm), and large ( 15 mm). Because most snails
collected were between 3 and 21 mm, arbitrary size classes spanning roughly 6 mm were
appropriate.
Mean snail sizes in each quadrat with 6 or more snails were tested for correlation
with quadrat height (as per Fawcett 1984). Abundances of T. funebralis (per 25 cm'
quadrat) were compared by a two-factor ANOVA with size class and height zone as
orthogonal fixed factors. Prior to analysis I tested for heterogeneity of variances, using
Cochran's test.
Susceptibility to Predation by Pisaster
In five outdoor tanks, measuring 49 cm x 52 cm x 32 cm deep, I conducted
replicate susceptibility experiments. In each tank, I placed two Pisaster ochraceus, a
known predator of Tegula spp. Three non-contiguous size classes of snails were used:
small (S9 mm), medium (12 - 14 mm), and large Q 17 mm).
Ratios of the numbers of snails in the three size classes were chosen with the goal
of equalizing the number of random encounters between a seastar and snails of a size
class. A simple 1:1:1 ratio of snails in each class would lead to relatively more seastar
encounters with large snails, as small snails on an individual basis are less likely to be
encountered. On the other extreme, equalizing the substrate area covered by snails in each
size class (a 1:2:5 ratio, between Large, Medium, and Small snails, respectively) would
lead to relatively more encounters with small snails, assuming they were not completely
clustered. Consequently, an intermediate ratio of 1:2:3 was used.
Sea stars were collected from the field and placed in holding tanks. I did not begin
a given trial until seastars began to feed on Tegula, to ensure that all were capable of
capturing and digesting prey. Aster one to three days, all of the seastars had successfully
preyed on a few snails. The preliminary snails were removed, and 120 recently-collected
snails were placed in each tank (20 Large, 40 Medium, and 60 Small). At least once a
day, empty shells were collected and measured. I maintained a constant ratio of size
classes in each tank by replacing snails that had been preyed upon. These replacements
came directly from the field, when collection was possible; when replacing snails during
high tides, I used Tegula that had been kept in an aquarium for one or two days. Each
replicate experiment continued for five days.
Because the natural reaction of T. funebralis to Pisaster is to climb vertical
surfaces to heights above the water line, barriers to prevent this behavior from leading to
escape were necessary. Lengths of thin galvanized steel edging, that had a right angle
with one inch sides, were cut and placed along the edges of the tanks, about an inch above
the water line. I then placed 1.5 inch strips of galvanized mesh over the top of the edging
Although snails could crawl an inch or less out of the water, they were forced to stop, or
crawl upside-down on the underside of the edging, where the Pisaster could reach. I
observed several snails moving up, away from the tank wall under the edging, and then
falling to the bottom after reaching the edge; when snails were found that could not right
themselves after falling, they were turned over with a clear length of plastic.
In addition to the susceptibility experiments with three sizes classes, I conducted
three more replicates using only 120 small snails. After five days, all snails were removed
and counted. Single-factor fixed effects ANOVAs were used to determine differences in
the susceptibility of the three size classes, based on the different proportions of each class
that was preyed upon.
Behavioral Responses to Physical Stimuli
To ascertain if differential susceptibility is the result of behavioral differences
among the size classes, I conducted the following experiments. In the laboratory, twelve
snails of each size class (total n = 36) were subjected to three treatments: no stimulus, a
tweezer point stimulus, and a tube foot stimulus. These experiments were carried out on a
submerged horizontal glass surface in the lab. Underneath a glass pane, I placed a sheet of
waterproof paper marked with concentric circles of radii 2 cm, 4 cm, and 6 cm. Radii
were also marked at 30° intervals. Snails were collected immediately prior to the
experiment to ensure that none had time to acclimate to laboratory conditions. I started
each replicate run by placing a snail in the center of the circles. After the snail had
reattached its foot to the substrate, oriented itself upright, and prepared to crawl, by
feeling the surface with its cephalic tentacles, the stimulus (if any) was delivered to a single
cephalic tentacle. The length of time taken for the snail to cross the 4 em circle was
measured. The initial direction that the snail was facing when the stimulus was delivered
was also noted. As the snail moved, I traced its path on a map of the circular "grid,
noting the angle between the direction the snail faced initially and the direction in which it
crossed the 4 cm circle. Each snail was given all three treatments consecutively, with no
stimulus given first, followed by tweezers and then the tube foot. Following a snail's final
run, the glass pane was cleaned to remove mucus and other traces, and the water in the
tub was replaced. Two-factor ANOVAs were used to test for differences in both
displacement time and the number of degrees between the initial direction and final
location of a snail at 4 cm displacement. Stimulus type and size class were fixed factors.
Distribution of juveniles
To define the vertical distribution of recently-settled T. funebralis, I conducted a
field search for juveniles. In Oregon, Moran (1997) found that most newly settled
juveniles (£3 mm) were on the underside of cobbles. As cobble-sized rocks are found in
all height zones, this micro-habitat was one of a few relevant to a determination of height-
specific distribution and settlement. Juveniles raised in the lab grew to 2 mm in
approximately 160 days (Moran 1997), so I assumed that snails under 2 mm were 5
months or less in age. I searched for juvenile T. funebralis less than 2 mm in maximum
diameter at two sites: the protected area at Hopkins Marine Station (hereafter H.M.S.)
described above, and a very protected area behind the Monterey Breakwater. The
Breakwater site has a more even slope, and many cobble-size rocks, whereas most rocks
at H.M.S. are large and fixed. I took two samples in each of the three height zones at each
site; for each, a thorough inspection of a group of 8-12 cobbles lying adjacent to each
other was made with a hand lens. Presence or absence of juvenile snails was noted. In
each sample where snails were found, several were returned to the lab for identification
under a microscope, using characteristics outlined by Moran (1997 and personal
communication).
RESULTS
A Vertical Size Gradient
Average snail size increased with decreasing intertidal height (Fig. 1), and the two
were significantly correlated (p +.001). In sum, 1902 snails from 39 quadrats were
included in the correlation analysis.
A Cochran’s test revealed a heterogeneity of variances (Test value =.254, Critical
value = 235), so data underwent a square root (x+1) transformation before further
analysis. There was a significant interaction between intertidal height zone and size class,
for T. funebralis abundances (Table 2). Small snails were the most prevalent size class in
the High zone. In the Mid zone, medium snails were the most abundant, and Large snails
dominated the Low zone (Fig. 2) While large and medium T. funebralis snails were
found in all zones, small snails were significantly lower in number than large snails, in the
Low zone (Fig. 3, Table 2). In this zone, many small and juvenile Tegula brunnea were
collected. Large snails increased in abundance with decreasing intertidal height, while
small and medium snails reached peak numbers in the Mid zone. There were significantly
more small and medium snails in the Mid zone, compared to the Low zone. Overall, there
were twice as many snails in the Mid zone than either of the other height zones (Table 1).
Additional post-hoc comparisons showed there was also a significantly higher average
total number of snails in each quadrat in the Mid zone, when compared with both the High
and Low zones (Table 2).
Predation by Pisaster
The seastars ate significantly more large snails than medium snails, and more
medium snails than small, despite the 1:2:3 ratio in favor of the small size class. In
addition, proportions eaten (Fig. 4) were significantly different (Table 5). While roughly
4% of the small snails were taken, over one quarter of the medium snails and over one half
of the large snails were taken (Table 4). Variances were not heterogenous (Cochran's
Test).
In experimental tanks with only small snails, an average of 8% were consumed
over 5 days (Fig. 5).
Behavioral Responses to Physical Stimuli
Times for each snail to move 4 cm from the center point were compared across
size classes and stimulus types (Fig. 6). Displacement times were shorter for all size
classes when the stimulus was a tube foot, compared to the treatment without a stimulus.
Displacement times in response to a tube foot were also lower than those in response to
the tweezer-point stimulus. Larger snails were, on average, faster than small snails, for
each treatment. Both stimulus type and size class had a significant effect on displacement
time, but there was not a significant interaction between the two (Table 6). Although
times decrease across stimuli, the relationship between size classes remains roughly the
same (Fig. 6); thus, stimuli and size class effects seem to operate independently. Within
terms were pooled with Interaction terms, and planned comparisons were made. Average
displacement times of small snails were significantly greater than both large and medium
snails.
The second behavioral measure was absolute degrees difference between initial
direction of the snail and the final direction at 4 cm displacement, again compared across
size classes and stimulus types (Fig. 7). The interaction between size class and stimulus
type was significant (Table 6). The relationship between size classes changes with
stimulus type. Mean degrees difference increased for both medium and large snails, from
the no-stimulus treatments to the tube foot treatments. In contrast, mean degrees
difference was less for the small snails when stimulated by tube feet, compared with the
difference when no stimulus was given. Planned comparisons showed that within the
group of behavioral responses to a tube foot, small snails displaced themselves
significantly fewer degrees, in comparison with medium snails. Other comparisons
10
between size classes for the tube foot stimulus were not significant. Representative snail
paths are shown in Fig. 8.
Intertidal Distribution of Juvenile T. funebralis
At H.M.S., I found no juveniles under 2 mm in the High zone. While juveniles
were abundant in the Mid zone, I found only a small number in the Low zone. At the
Monterey Breakwater site, I was able to find a small number of juvenile funebralis in the
High zone. Juveniles again were abundant in the Mid zone. I failed to find any in the Low
zone. See Table 3.
DISCUSSION
The results of extensive intertidal sampling indicate the presence of a size gradient
in the protected rocky habitat at H.M.S.: both mean size and relative abundances of
different size classes change significantly among intertidal zones. Because snails increased
in mean size in a down-shore direction and Pisaster seemed to be almost completely
absent from this site, my findings are consistent with Fawcett's (1984) explanation of how
predation (or its absence) can maintain a size gradient.
Although the correlation of mean size with intertidal height explained only 11% of
the variation in snail sizes, the slope was significantly negative. However, a U-shaped
curve might better fit these data, as many of the average sizes in the highest quadrats
increased over those in high mid-zone quadrats (Fig. 1). This possibility is intriguing,
especially when considered in the context of the possible size-gradient factor of
environmental stress. Marchetti and Geller (1987) demonstrated that small T. funebralis
desiccated faster than large snails in laboratory conditions. If large snails, in addition to
being more mobile, can better withstand the higher desiccation rates of the high intertidal,
one might expect to see a slight increase in mean size within a quadrat in the uppermost
limits of Tegula habitat, despite the fact that food availability would tend to drive the
majority of large snails down in intertidal height.
The predation experiments show clearly that small snails (£9 mm) are in much less
danger of being preyed upon by Pisaster, in comparison with other size classes. If the
decreased susceptibility of small snails in tanks with all three size classes was due simply
and only to a preference of Pisaster for larger snails, then one would expect to see an
increase in the proportion of small snails eaten, in tanks with only the small class.
However, proportions and numbers of small snails eaten increased very little in tanks with
only small snails. Together, these two sets of experiments indicate that the lower
susceptibility of small snails may be due to an inability of seastars to detect, capture, or
handle small snails.
The next set of experiments were intended to examine at the nature of the
differential susceptibility that was observed. Because large snails displaced themselves
away from a tube foot stimulus faster than smaller snails, and because smaller snails did
not seem to turn farther away from their initial direction when touched with a tube foot,
small snails would in fact seem to be more susceptible to predation. Combined with the
results of the predation experiments, this suggests that the decreased susceptibility is not
due solely or perhaps much at all to predator choices based on preference. I conclude that
some sort of physical limitation prevents small Tegula from being preyed upon. Perhaps
Pisaster are less able to recognize small T. funebralis as prey, or perhaps they are too hard
to handle.
That small snails are in much less danger from predation by Pisaster suggests that
a factor different than predation is responsible for their general absence from the Low
zone of the intertidal. Settlement seems to be an obvious choice: Fawcett (1979) found
that larvae settle mainly in the Mid intertidal region. The results of my search for juveniles
under 2 mm, which are likely to be only about 150 days old (Moran 1997), support this.
Because snails this small are virtually immobile relative to the whole intertidal zone, the
height at which they are found is likely to be the height at which settlement occurred. It is
unknown whether larvae actually settle in only the Mid zone, or whether they settle
indiscriminately but suffer from high mortality in other height zones
High mortality in other height zones, following indiscriminate settlement, would
likely also be due to factors other than predation. Jensen (1981) found juvenile T.
funebralis under 4 mm primarily in sandy holdfasts and on the undersides of rocks in
pools, at H.M.S. If these microhabitats are the most favorable for small snails, and they
are found primarily in the Mid zone, then microhabitat may play a role. In addition, Jensen
found that 70% of the stomach contents (and hence of diet) of juvenile T. funebralis under
4 mm was detritus. How this apparently non-selective feeding behavior might influence
distribution has not been determined.
Interspecific competition may affect size classes differently, as well. Almost no
small T. funebralis were found in the low zone, where juvenile T. brunnea were abundant
in this zone. This zonation is quite clear, suggesting that T. brunnea may competitively
displace juvenile T. funebralis in the low zone, although such an interspecific interaction
does not seem to occur between adults of the species at this intertidal height (the transition
from adult T. funebralis to adult T. brunnea occurs in the high subtidal). These other
possible causative factors involving the Tegula funebralis size gradient invite and await
future research.
ACKNOWLEDGEMENTS
I would like to thank my advisor, Jim Watanabe, for helping me in all stages of this
research, from conception to completion, and for his generosity in giving time and sharing
knowledge. I would also like to thank Isaac Kaplan, Natalie Lu, Sarah Girshick, and
Tamara Jaron for making life in our house a ridiculously enjoyable experience. Thanks
also to Amy Moran for providing helpful information on the location and identification of
juveniles.
LITERATURE CITED
Doering, P.H., and D.W. Phillips. 1983. Maintenance of the shore-level size gradient in
the marine snail Tegula funebralis (A. Adams): Importance of behavioral
responses to light and sea star predators. J. Exp. Mar. Biol. Ecol. 67(2): 159-173.
Fawcett, M.H. 1979. The consequences of latitudinal variation in predation for some
marine intertidal herbivores. Dissertation. University of California, Santa Barbara,
California, U.S.A.
Fawcett, M.H. 1984. Local and latitudinal variation in predation on an herbivorous
marine snail. Ecology. 65(4):1214-1230.
Jensen, J.T. 1981. Distribution, activity, and food habits of juvenile Tegula funebralis
and Littorina scutulata as they relate to resource partitioning. Veliger.
23(4):333-338.
Marchetti, K.E., and J.B. Geller. 1987. The effects of aggregation and microhabitat on
desiccation and body temperature of the black turban snail, Tegula funebralis (A.
Adams, 1855). Veliger. 30(2):127-133.
Moran, A.L. 1997. Spawning and larval development of the black turban snail Tegula
funebralis (Prosobranchia: Trochidae). Mar. Bio. 128:107-114.
Paine, R.T. 1969. The Pisaster-Tegula interaction: Prey patches, predator food
preference, and intertidal community structure. Ecology. 50:950-961.
Ricketts E.F., J. Calvin, J. Hedgpeth, and D.W. Phillips. 1985. Between Pacific Tides
Stanford University Press, Stanford, CA.
Table 1. Size gradient data for abundance of each size class at each intertidal height zone,
including total abundance and average abundance per quadrat.
Small
Total
Medium
Large
179
High Intertidal
Abundance
403
132
92
(14 Quadrats)
Average Abundances
12.79
9.43
6.57
9.60
Per Quadrat
Abundance
869
Mid Intertidal
301
374
194
(14 Quadrats)
26.71
Average Abundances
21.50
13.86
20.69
Per Quadrat
Low intertidal
Abundance
118
338
464
(14 Quadrats)
Average Abundances
0.57
8.43
24.14
11.05
Per Quadrat
Table 2. Size gradient ANOVA and post-hoc comparison data, for the two-factor
ANOVA comparing abundances of size classes in different height zones.
Source of
MS
P-value
Variation
Height Zones
0.001
27.70
7.60
3.23
Size Class
0.89
0.414
5.001
Iinteraction
20.47
5.62
117
364
Within
Fotal
125
Significant Tukeys Tests
IP value
Total 4 Mid Zone » Total &4 Low Zone
p-.002
Total ≈ Mid Zone » Total 4 High Zone
1p5.004
Bonferroni Adjustment (18/36 comparisons)
Pvalue
Small Class, Low « Mid
p-.001
Medium Class, Low « Mid
P=.041
Low Zone, Large » Smal
1p5.001
ble 3. Qualitative search results for juveniles under 2 mm, at two sites.
Table 4. Relative susceptibility: total numbers eaten and mean proportions.
Relative
Total Numbers
Average
Susceptibility
Proportion Eaten
Eaten (5 Tanks
ISmall Class
0.04
Medium Class
0.27
Large Class
93
0.53
Table 5. Single factor ANOVA and planned comparisons for susceptibility experiments.
Source of
F oritical value
MS
P-value
Variation
Between Size
0.2956
62.50
4.52E-07
3.89
Classes
Within Groups
0.0047
Total
Significant Planned Comparisons
E value
F(025, 1. 12) Crtical value
Proportion Small Eaten « Medium Eaten
27.48
6.55
Proportion Medium Eaten « Large Eaten
35.18
Table 6. ANOVA and planned comparisons for analysis of 4 cm displacement times.
Source of
MS
P-value
F ontical
Variation
value
78966
5.94E-06
Stimulus
13.62
3.09
Size Class
41993
7.24
0.001
3.09
Interaction
1875
0.86
2.46
0.32
99
5797
Within
107
Total
Significant Planned Comparisons for Displacement Times
F1o25 1 103) critical value
5.17
Small Class « Large Class
11.03
Small Class « Medium Class
11.29
Table 7. ANOVA and planned comparisons for analysis of degrees difference between
initial direction a snail faced and its final degree location.
Source of
P-value
MS
Foritical
Variation
value
3.09
Stimulus
13277
5.74
0.004
3.09
Size Class
0.39
0.94
2176
7478
3.23
0.015
2.46
interaction
Within
2312
Total
107
Significant Planned Comparison for Degrees Difference
Etou. 1. 9) crftical value
Small Class « Medium Class
5.89
8.46
FIGURE LEGEND
Figure 1. Plot of mean snail size in each quadrat vs Intertidal height, fit with a
correlation line. Standard error (+-SE) bars.
Figure 2. Graph of relative proportions of size classes in each intertidal height zone.
Figure 3. Graph of abundances of each size class in each intertidal height zone.
Figure 4. Relative susceptibility: mean proportions of each size class eaten in
susceptibility experiments, (+/- SE).
Figure 5. Proportions of small size class eaten, with and without other sizes present in
experimental tanks (+/- SE).
Figure 6. Mean displacement times for each size class at the three stimuli (+/- SE).
Figure 7. Mean degrees difference for each size class at the three stimuli (+/- SE).
Figure 8. Representative snail paths. Shown are three snails, one in each size class. For
each snail, no stimulus treatment and tube foot treatment are shown. Note times
of displacement, snail paths, and initial direction marked as e
9
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0.5
0.4
0.3
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Small
E Medium
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High Intertidal Mid Intertidal Low Intertidal
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Figure 2
400
350
300
250
200
150
100
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Medium (9 «X « or - 15)
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Low
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Experimental Type
Figure 5
250
200
None
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Stimulus Type
Figure 6
Tube Foot
ESmall
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160
140
120
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Figure 7
Tube Foot
Small
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