Zonation of Tegula
Monique Rocca
Abstract
The two intertidal species of Tegula, T. funebralis and T. brunnea show distinct
vertical zonation patterns. T. funebralis is found throughout the mid intertidal zone anc
into the low zone, while T. brunnea is found in the low intertidal and shallow subtidal
zones. Experiments were conducted to test whether desiccation, predation, or
microhabitat preferences were affecting the distributions of the two species. Results
indicate that the inability to tolerate prolonged exposure to heat and desiccation keeps T.
brunnea low in the intertidal zone. The predators Pisaster ochraceous and Cancer
antenarius, which live low intertidally, were not shown to preferentially eat T. funebrali
in the lab or the field. Predation pressures on both species were shown to be low at
Hopkins Marine Station, suggesting that predation was not a major force maintaining
the lower limit of T. funebralis. Microhabitat preferences of the two species differ,
indicating that microhabitat choice may play a role in maintaining zonation patterns.
The role of other factors such as competition, behavior, and larval settlement are
discussed.
Zonation of Tegula
Monique Rocca
Introduction
One of the goals of ecology is to explain what causes organisms to live where they
do. The rocky intertidal is an ideal habitat to study factors affecting species composition
because of the steep gradient of abiotic stresses to organisms caused by the diurnal
fluctuation in tidal height. Classic studies such as those of Connell (1961), Dayton (1971),
and Paine (1974), have lead to the commonly accepted notion that the upper limits of
intertidal organisms' range tend to be caused by abiotic factors, such as the ability of the
organism to tolerate desiccation, and that the lower limits are caused by biotic
interactions such as predation and competition. The two intertidal species of turban
snail, Tegula funebralis and T. brunnea, are abundant and vertically zoned. While many
aspects of their ecology have been studied (Paine 1969, Markowitz 1980, Geller 1982,
Watanabe 1983, 1984a, 1984b, Fawcett 1984), the factors causing the zonation of Tegula are
still unknown.
T. brunnea is found in the low intertidal zone and is abundant in shallow subtidal
waters of 0-6 m depth (Abbott and Haderlie 1980, Watanabe 1983). T. funebralis is
abundant throughout the intertidal zone, especially between .6 and 1.2 m above MLLW
(Abbot and Haderlie 1980, Wara and Wright 1963). Despite their differing ranges, T.
funebralis and T. brunnea are evolutionarily closely related. Their main respiratory
pigments, hemocyanins, are very similar, and both species excrete uric acid (Abbot and
Haderlie 1980). It is quite probable, then, that the two species diverged relatively recently
from a common ancestor and should therefore have similar physical and behavioral
characteristics.
A series of experiments was conducted to examine what differences between these
species could be causing their ecological separation. These included experiments testing
how long each species can survive desiccation and heat stress, predation experiments in
the lab and the field, and an experiment testing the preferred microhabitats of each
species.
Zonation of Tegula
Monique Rocca
Materials and Methods
Vertical Distributions
In order to determine the relative distributions and abundances of Tegula
funebralis and T. brunnea at Hopkins Marine Station in Pacific Grove, California, two
vertical transects were set up perpendicular to the shoreline during minus tides. Twelve
distances along each transect were chosen from a list of random numbers and a .25 m2
quadrat was sampled for both species at each selected point. Heights above mean lower
low water were visually estimated, then selected points were measured with a surveyor's
level and the estimated heights adjusted accordingly.
Desiccation Experiments
To test how T. brunnea is able to withstand desiccation pressures compared to T.
funebralis, snails of both species were exposed to air for various lengths of time, and
mortality rates compared. First, snails of each species were divided into three size classes
Small snails were approximately 1.0-1.6 cm in maximum shell diameter, while medium
and large snails were -1.7-1.9 cm and -2.0-2.3 cm respectively. Snails were then placed in
shallow, 10 cm diameter, glass bowls according to species and size (48 bowls total, 24
bowls for each species, 8 for each size class within species). In order to keep the overall
biomass in each bowl constant, the bowls with large snails contained fewer individuals
(4-5) than the bowls with medium snails (5-6) and the bowls with small snails (7). All the
bowls were placed in direct sunlight outdoors on a partly cloudy day. Due to rain, the
bowls were moved inside after 5.5 hours and placed in front of a fan for the remainder of
the experiment. The room stayed at approximately 17°C while the fan provided wind to
mimic the outside environment.
At 6, 12, 24, and 48 hours after exposure began, two bowls for each species and size
class were covered with a shade cloth (to prevent snails from crawling out of their bowls'
Monique Rocca
Zonation of Tegula
and submerged in running sea water. At least 24 hours after submersion, percent
mortality was calculated by counting the number of dead snails and dividing by the total
number in the bowl. Snails were considered dead if they were partially open and did not
respond to tactile stimulation, or if they were still completely closed after 24 hours. The
results were analyzed using a fully-orthogonal three-way ANOVA with species, size, and
time as fixed factors.
A second desiccation experiment was undertaken on Tegula brunnea to explore
the unexpected mortality of all the T. brunnea in the first desiccation experiment. The
experiment was conducted as described above, with the following changes: only the smal
and large size classes were tested (7 snails / bowl for large samples, 10 snails / bowl for
small samples), bowls were placed indoors in front of a fan for the entire experiment,
two bowls from each size class were submerged after 3, 6, 12, and 24 hours, and the snails
were given approximately 48 hours to emerge before the percent mortality was
calculated.
Predation Experiments
If benthic predation causes zonation of Tegula, each species of snail should be
more susceptible to predators found outside its own zone (Watanabe 1984b). Pisaster
ochraceous, an intertidal sea star, lives in the middle and low intertidal zones (Feder
1980), extending slightly into the zone of T. funebralis. Cancer antenarius, à rock crab, is
found only in the low intertidal zone (Garth and Abbott 1980). To test whether 1.
funebralis is more susceptible to Pisaster or Cancer predation than is T. brunnea, four
Pisaster or two Cancer were placed in outdoor fiberglass tanks with 30 snails of each
species. Tanks were 1 m2, 20 cm deep, and supplied with running sea water. The only
exception was trial 2 of the Cancer experiments, in which the tank was 25 m2 and half
the number of organisms was used. All the organisms except Cancer were collected at
Hopkins Marine Station. Crabs were obtained from the Monterey Bay Aquarium. An
Zonation of Tegula
Monique Rocca
effort was made to find Pisaster that were submerged at the time of collection so that they
could be removed easily from the rock with minimal damage to the star’s tube feet.
Snails measured 1.8-2.3 cm in maximum basal diameter, Pisaster ranged from 10-20 cm
(ray tip to opposite side of oral disk), and Cancer had carapace widths of 8-12 cm.
At least once daily, the number of remaining snails was counted and the missing
snails replaced. In the Pisaster tanks, the number of empty shells provided the best count
of the number of snails eaten, whereas in the Cancer tanks, all the missing snails were
assumed eaten. Care was taken to prevent snails from escaping the tanks (a rare'
occurrence). As often as possible and at least once daily, snails that had crawled above the
water line were dropped back into the water.
For each trial, the proportion of total consumed snails belonging to each species
was calculated. Then, using the normal approximation to the binomial distribution, the
probability of the observed proportions was calculated assuming an equal probability of
either species being eaten. If this probability was less than .05, then it was assumed that
the predators were selecting one snail species over the other.
Tethering experiment
A field experiment was also conducted to determine whether the results of the
laboratory desiccation and predation experiments were reflected in the field. Snails of
each species were tethered to rocks in the intertidal zone at Hopkins Marine Station at
three tidal heights. If desiccation is important in keeping T. brunnea low, then the T.
brunnea at the high stations should show mortality due to exposure (and vice versa-if T
funebralis suffer from lengthy submersion, they should drown at the low stations). If
predation is affecting the distributions of either species, species should suffer a higher
rate of mortality outside of their own zone than in it.
Medium sized snails (1.8-2.3 cm in maximum basal diameter) were collected from
Hopkins Marine Station and were kept in running sea water. Small holes were drilled ir
Zonation of Tegula
Monique Rocca
the lip of each snail shell and a .5 meter length of monofilament was tied through the
hole such that it did not interfere with snail movement. The other end of the
monofilament was fitted with a slip knot so that it could be secured to a screw in the field
(Fig. 1).
The snails were tethered in a wave-exposed site at Hopkins Marine Station where
both Pisaster and Cancer have been observed. Three tethering stations were set up at
each of three intertidal heights. The three low stations were placed in the zone where
only legula brunnea is found but above mean lower low water (0.5-1.0 ft. above MLLW)
The medium stations were placed in the zone of overlap of the two species (1.0-2.0 ft.
above MLLW). The high stations were located above the range of T. brunnea where
only T. funebralis is found (2.0-5.0 ft. above MLLW). Each station consisted of two bolts
placed less than .25 m away from each other, 5 T. funebralis were secured to one screw
and 5 T. brunnea were secured to the other.
The experiment ran for 17 days. The stations were checked every 2-3 days except
between days 8 and 15 when neap tides made access impossible. Snails that escaped from
their tethers due to an untied knot were replaced on day 4, but subsequent escapees were
excluded from the experiment. Intact, empty shells were assumed to be the result of
Pisaster predation, pieces of broken shell left on the string and knots remaining on empty
tethers were assumed to be the result of crab predation, and dead snails whose bodies
were still present were assumed to be the result of desiccation stress. The results were
analyzed using an orthogonal 2-way ANÖVA, with species and tidal height as fixed
factors.
Microhabitat experiment
This experiment controlled for tidal height while testing whether the two species
of Tegula prefer different microhabitats. Rocks with different algal cover were collected
from the field and placed in an outdoor 25 m2 tank supplied with running sea water.
Zonation of Tegula
Monique Rocca
The types of algal cover were: no cover, low cover (Endocladia, Gigartina leptorhynchos),
encrusting coraline algae, Gastroclonium, dense cover (lridea, Prionitis, Mastocarpus),
and a blade of Gigartina corymbifera). The microhabitats were approximately of equal
area. The spaces between rocks were filled with sand to keep snails off the bottom of the
tank. Twenty-five individuals of each species of snail were placed in the tank. Daily, the
number of snails of each species in each microhabitat was counted. Also counted were
the number of snails on the sand, on the walls of the tank, or missing. (Closer
examination determined that missing snails were hiding on the bare undersides of the
rocks.)
Results
Distributions
The lower limit of Tegula funebralis was approximately 0 ft. above MLLW, while
the upper limit of T. brunnea was approximately 2 ft. above MLLW (Fig. 2). Snail
densities in the zone of overlap were low (1-2 snails of each species per quadrat)
compared to the areas of maximum T. funebralis density (40-60 snails per quadrat).
Desiccation Experiments
In the first experiment, all the T. brunnea died within 6 hours of exposure while
no T. funebralis individuals died before 12 hours of exposure; some T. funebralis
survived 48 hours of exposure (Fig. 3). There were no significant differences between
size classes, but the species X time interaction is significant (Table 1). Further analysis
with a Tukey (HSD) Test reveals that the differences in mortality rates between species
are significant for each time interval. However, the differences between the mortality
rates for different time intervals within each species are not, except for the difference
between 6 and 12 hours, and 6 and 24 hours, for T. funebralis.
Zonation of Tegula
Monique Rocca
For the second desiccation experiment, all the snails survived except for three
large snails, one that was exposed for 12 hours and two that were exposed for 24 hours.
Because of the low mortality rate, these results were not analyzed quantitatively.
Predation experiments
In all trials except for Cancer 2, T. brunnea was eaten more often than T. funebralis
(Table 2). However, the predators were only significantly biased eaters in the Pisaster 2
and Cancer 1 trials.
Tethering experiments
Twenty-seven percent of the total number of snails escaped their tethers as
evidenced by strings with no knot at the end (Table 3). At the high station where no
natural population of Tegula exists, all 10 snails escaped. Of the remaining snails,
mortality was low (Fig. 4). Therefore, the ANOVA was run comparing the number dead
for each height and species, regardless of cause of death. None of the differences were
statistically significant (Table 4).
Microhabitat experiment
As evidenced by the lack of overlap of the 95% confidence intervals, significantly
more 1. funebralis than T. brunneu were found on the rocks with thin or no algal cover.
i.e.. the no cover, low cover, missing, and encrusting coraline algae microhabitats (Fig. 5).
However, the numbers of T. brunnen on rocks with thick algal cover (dense cover,
Gastroclonium) were not significantly different than the numbers of T. funebralis.
Thère were significantly more T. brunnea than T. funebralis on the sand and the walls of
the tank.
Zonation of Tegula
Monique Rocca
Discussion
Desiccation
Tegula brunnea clearly cannot survive exposure to air as well as T. funebralis. The
first experiment indicates that it is a physiological tolerance rather than a behavior that
allows T. funebralis to live in the high intertidal, in contrast to Wolcott's (1973) findings
for intertidal limpets. The differences in mortality of T. brunnea between the two
experiments reveal that a combination of heat stress and water loss probably determine
how long T. brunnea can survive out of water. In the first trial, the snails spent 5.5
hours in direct sunlight. Temperatures in the bowls reached 27°C, 10 degrees higher
than the temperature maintained indoors. Even under cooler conditions when T.
brunnea were not dying, T. funebralis appeared better adapted for survival. It was noted
in preliminary experiments that surviving T. funebralis emerged almost immediately
from their shells upon re-submersion, while some T. brunnea took over 24 hours to re-
emerge. It was also observed that T. brunnea individuals retreated into their shells
within 3-6 hours of exposure while T. funebralis individuals subjected to the same
environmental conditions remained attached to the bowl by the foot for up to 9 hours.
In this amount of time, snails would surely get tossed around in the surf and would be
unable to attempt escape from predators. Desiccation and overheating appear to be
factors which maintain the upper limit of T. brunnea.
Past research has indicated that size may be an important determinant of the
amount of heat absorbed or the amount of moisture lost from a snail (Paine 1971 cited in
Markowitz 1980). However, since all the T. brunnea died, there was no way to elucidate
a size effect for that species. The high variance among replicate bowls containing T.
funebralis may have hidden a size effect. The variance was so high that differences
between the different exposure times were not statistically significant after 6 hours, and
the average mortality went down with exposure time. To determine whether mortality
rates depend on size, further research should be conducted with large numbers of snails.
Monique Rocca
Zonation of Tegula
Predation
Pisaster ochraceous and Cancer antenarius, are the major predators of l’égula at
Hopkins Marine Station, although otters, octopi, and perhaps birds eat Tegula as well
(Paine 1969, Abbott and Haderlie 1980). Therefore, if predation was maintaining the
lower limit of T. funebralis, one would expect to see more T. funebralis consumed by at
least one of these two predators. It was especially expected that Cancer would eat more of
the thinner-shelled T. funebralis than the thick shelled T. brunnea because Cancer is
known to be more successful at cracking the shells of thinner shelled species (Abbott and
Haderlie 1980, Awad 1994). While only one of the Pisaster predation trials and one of
the Cancer trials showed a significant difference between the two species, the data
indicates that T. brunnea were consumed more frequently than T. funebralis. This
unexpected result can be explained, however, by the observation that the escape response
of T. funebralis was more effective than the escape response of T. brunnea : Individual
T. funebralis crawled up the sides of the tank until they were above the water line, and
sometimes substantially higher. In contrast, individual T. brunnea crawled up the
sides of the tank until they reached the surface, and then either moved horizontally
along the water line, or, if trapped by a Pisaster, let go of the surface and tumbled over
the sea star down into the bottom of the tank. These are the same escape responses that
have been well documented by others (Yarnall 1963, Geller 1982, Watanabe 1983, Awad
1984 ). Because neither Pisaster nor Cancer ever foraged above the water line, most of the
T. funebralis were out of reach of the predators. In contrast, individual 1. brunnea
remained in the range of the predators, and, consequently, were captured more often.
This probably occurred even though all snails were pushed below the water line as often
as possible. In the field, T. brunnea probably escape predators by hiding in the dense algal
cover which occurs in the shallow subtidal zone (Watanabe 1984b).
The escape response raises the possibility that a behavioral mechanism is keeping
T. funebralis up high in the intertidal zone. If it is, and if the climbing behavior is
10
Zonation of Tegula
Monique Rocca
observed only in the presence of predator scents, T. funebralis would have to be
surrounded continuously by enough predator scents to keep the snails from descending
into the low intertidal. If the behavior is observed consistently in the absence of predator
scents, it would be of interest to elucidate the selective force that caused the evolution of
the behavior. Unfortunately, past research has failed to demonstrate whether or not
climbing is observed in the absence of predators. It seems that climbing behavior does
occur without predators, but not as far, as frequently, or as fast (La Roe 1963, Geller 1982)
Control Tegula tanks without predators yielded a similar result. The possibility that a
climbing behavior is keeping the T. funebralis up high is a good one that merits further
investigation.
In the tethering experiment, overall mortality in the field was unexpectedly low,
Mortality may have been somewhat exaggerated due to experimental artifacts. For
example, most of the deaths in the middle zone were due to one Pisaster who found a
tangle of strings and snails and took advantage of their inability to run away. Also,
leashes with missing snails and an intact knot were considered Cancer predation, even
though it was never observed directly. Few T. brunnea in the high zone died of
desiccation. However, the T. brunnea survivors appeared unhealthy; they were partially
retreated into their shells and not holding onto the substrate. Because the mortality was
so low, the ANOVA was run by grouping all the sources of mortality together and testing
for species and height effects. The lack of species and height effects indicate that
prédation pressure is not strong on the snails at Hopkins Marine Station.
The finding that predation pressures are not strong contradicts past studies that
have documented the effect of predators on the zonation of T. funebralis. There is
évidence that predation pressures may keep T. funebralis from occurring lower on the
shore. Fawcett (1984) shows that at sites where predator density is high, the lower limit
of the T. funebralis range is higher than at sites where predator density is low.
Markowitz (1980) shows that T. funebralis migrate upwards at the same time of year that
11
Zonation of Tegula
Monique Rocca
sea stars move higher on the shore. Paine (1969) found that 25-28% of the T. funebralis
which overlap with Pisaster get consumed by the stars. One possible explanation for the
difference between this study and others is that, at Hopkins, sea otters are prevalent. Sea
otters are known to have strong effect on the ecology of an area because they eat so many
of the smaller predators such as sea stars and crabs, thus keeping their populations low
(Riedman and Estes 1990). The Cancer crabs found in the Hopkins area are quite small (8
12 cm) compared to those used in past studies (15 cm) (Awad 1994, Watanabe, personal
communication), and smaller Cancer eat more slowly in the lab (personal observation).
In contrast, the past field studies of Paine (1969) and Markowitz (1980) were done
in areas where the sea otters have not repopulated since they were hunted out -100 years
ago (Riedman and Estes 1990). Paine’s study was done in Washington state and
Markowitz’s study was done at Point Reyes, California. These areas probably have higher
densities of invertebrate predators due to the absence of otters. The selective forces that
Tegula would have experienced historically were probably closer to those found at
Hopkins Marine Station today. It seems, therefore, that predation is not a strong force
affecting the distributions of Tegula. Further investigation, however, would be of value.
Microhabitat preference
It is known that, in the field, T. brunnea is found most often in areas of dense
algal cover (Watanabe 1984b) whereas T. funebralis avoids dense algal cover, even when
it is found high in the intertidal zone (Wara and Wright 1963, Paine 1969). The tidal
height where dense algal cover tends to change to sparser cover correlates with the tidal
height where the two Tegulu species overlap. The fact that T. funebralis was found
significantly more often than T. brunnen on rocks with light algal cover indicates that
microhabitat preferences differ between the species. If more counts were taken, T.
brunnea might be shown to occur significantly more often on the rocks with dense algal
12
Zonation of Tegula
Monique Rocca
cover. More research would be of value to determine whether microhabitat choice
affects the zonation of Tegula.
Other possible factors
The fact that densities of both species are low compared to the centers of their
ranges suggests that interspecific competition may not be affecting zonation patterns. It
would be unlikely that snails are experiencing interspecific competition pressures in an
area of such low density when they can tolerate intraspecific densities of up to 50
snails/.25 m2. Furthermore, Underwood (1978) demonstrates that interspecific
competition between mobile species usually does not result in the exclusion of the
competitively inferior. In addition, Watanabe (1984b) has shown that between T.
brunnea and two other subtidal Tegula species, interspecific competition is no higher
than intraspecific competition. It is probable that these results extend to the relationship
between 1. funebralis and T. brunnea. Competition between Tegula and other grazers is
also unlikely because Tegula is much more abundant then other potential competitors
(Watanabe 1984b).
Another factor that might affect the distributions of intertidal Tegula is differential
larval settlement. One might tend to find adults in the same areas that the juveniles
settle. Watanabe (1984b) found that larval recruitment is an important cause of the
spatial segregation of the three subtidal Tegula species. Further, it is known that T.
brunnea larvae settle in the low intertidal/shallow subtidal zone (Watanabe 1984b) while
the larvae of T. funebralis settle high in the intertidal zone (Paine 1969). It therefore
appears that larval settlement is, at least in part, affecting the zonation patterns of adult
Tegula. Why the differences evolved, however, has yet to be elucidated.
Monique Rocca
Zonation of Tegula
Conclusions
Tegula brunnea seems to be limited to low intertidal regions due to its inability to
tolerate heat and desiccation. In contrast, the lack of a simple explanation for 1.
funebralis's lower limit indicates that the snail’s zonation patterns might be the effects of
a complex combination of factors rather than the action of a single strong factor.
Predation does not seem to be a strong force at Hopkins Marine Station, but because this
finding contradicts all previous study, it should be investigated further. Competition is
probably not occurring, as evidenced by the low densities of both snails in the zone of
overlap. The climbing behavior observed in T. funebralis, while seemingly complex,
may be an important reason why T. funebralis does not attempt to live down lower in
the intertidal. This behavior should be investigated to elucidate its origins. Microhabitat
choice and differential larval settlement may prove to be important factors causing the
zonation patterns of the intertidal Tegula species. Finally, it is possible that adaptations
caused by past selective pressures are contributing to the zonation of Tegula, making the
causes of the zonation difficult to elucidate.
T. funebralis demonstrates that the biotic interactions maintaining a species’ lower
limit in the intertidal zone may be more complex than just strong predation or
competition. Life history factors, simple lifestyle preferences, complex behaviors, or à
combination of factors may contribute to the observed distributions of organisms in the
field.
Acknowledgments
I would like to thank my advisor Jim Watanabe for being one of the most
dedicated teachers I have known. Jim is always ready to lend a helping hand, give
encouragement and suggestions, or share his boundless knowledge. I thank him for his
guidance and his support.
14
Monique Rocca
Zonation of Tegula
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La Roe, E. T. 1963. Distribution limitations of the marine Gastropod Tegula funebralis.
(Unpublished MS. on file at Hopkins Marine Station Library.)
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Zonation of Tegula
Markowitz, D. V. 1980. Chemically mediated avoidance response of a Gastropod, l'’égunt
funebralis. Journal of Experimental Marine Biology and Ecology 45: 1-13.
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Wara, W. M. and B. B. Wright. 1963. The distribution and movement of Tegula
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Zonation of Tegula
Monique Rocca
Table 1. Three-way ANOVA determining whether species, time of exposure, or snail size
had an effect on the mortality of snails by desiccation.
Source
Sum-of-
Degrees of
Mean-
F-Ratio
P-Value
Squares
Freedom
Square
Species
6.663
6.633
0.000
355.093
Time
0.109
0.328
5.850
0.004
Size
0.019
0.009
0.609
0.506
Species Time
0.328
0.109
5.850
0.004
0.019
0.009
Species“Size
0.506
0.609
Time Size
0.135
0.023
1.208
0.336
Species Time
0.135
0.023
1.208
0.336
Size
Error
10.448
0019
Tukey HSD Test-Matrix of Pairwise Mean Differences (p-values in parentheses).
Statistically significant comparisons in boldface.
T. fun 6 hrs.
T. fun. 12 hrs.
T. fun. 24 hrs
T. fun. 48 hrs.
T. fun. 6 hrs.
0.000 (1.000)
T. fun. 12 hrs.
0.458 (0.000)
0.000 (1.000)
T. fun. 24 hrs.
0.310 (0.012)
-1.148 (0.582)
0.000 (1.000)
T. fun. 48 hrs.
0.258 (0.055
-0.200 (0.229)
-0.052 (0.997)
0.000 (1.000)
T. brun. 6 hrs.
1.000 (0.000)
0.542 (0.000)
0.690 (0.000)
0.742 (0.000)
T. brun. 12 hrs
1.000 (0.000)
0.542 (0.000)
0.690 (0.000)
0.742 (0.000)
T. brun. 24 hrs.
1.000 (0.000)
0.542 (0.000)
0.690 (0.000)
0.742 (0.000)
T. brun. 48 hrs.
1.000 (0.000)
0.690 (0.000)
0.542 (0.000)
0.742 (0.000)
T. brun 6 hr.
T. brun. 12 hr
T. brun. 12 hr
T. brun. 48 hr.
T. fun. 6 hrs.
T. fun. 12 hrs.
T. fun. 24 hrs.
T. fun. 48 hrs.
T. brun. 6 hrs.
0.000 (1.000)
T. brun. 12 hrs
0.000 (1.000)
0.000 (1.000)
T. brun. 24 hrs.
0.000 (1.000
0.000 (1.000)
0.000 (1.000)
T. brun. 48 hrs.
0.000 (1.000)
0.000 (1.000)
0.000 (1.000
0.000 (1.000)
Zonation of Tegula
Monique Rocca
Table 2. Summary of results from predation experiments.
Predator
No. of
Trial
No. T.
No. T.
Proportion Proportion
p-value of
Predator
difference
Length
brunnea
of T.
of T.
funebrali
(days)
(No. of
eaten
brunnea
funebrali.
eaten
each snai
eaten
eaten
species)
4 (30
41
Pisaster
562
583
2
4 (30
756
244
0001
4 (30)
294
706
102
2 (30
Cancer
303
607
024
1 (15)
364
636
200
Days before predators began eating were not included in trial length.
Zonation of Tegula
Monique Rocca
Table 3. Breakdown of the sources of mortality in the tethering experiment, by species
and tidal height.
Tidal height Result
Tegula
Tegula
Funebrali:
brunnea
No. snails
No. snails
of 15)
ot
Low
Cancer predation
Pisaster predation
Desiccation
Escape-knot undone
Survivors
Medium
Cancer predation
Pisaster predation
Desiccation
Escape-knot undone
Survivors
Cancer predation
igh
Pisaster predation
Desiccation
Escape-knot undone
Survivors
Monique Rocca
Zonation of Tegula
Table 4. Two-way ANOVA determining whether the number of tethered snails
found dead depended on species or intertidal height.
Sum-of-
F-Ratio
p-value
Degrees of
Mean-
Source
Freedom
Square
Squares
0.019
0.894
Species
0.056
0.056
0.208
5.389
796—
10.778
Height
1.056
0.710
0.352
Species-Heigh
2.111
3000
36.000
Error
12
Zonation of Tegula
Monique Rocca
Figure Legends
Figure 1. Diagram showing how the snails were tethered to the rocks. Rubber washer
helped reduce wear on the monofilament.
Figure 2. Abundances of the Tegula funebralis and T. brunnea at Hopkins Marine
Station. Error bars show 95% confidence interval.
Figure 3. Average mortality per bowl due to desiccation. Error bars show 95% confidence
interval.
A. for small snails.
B, for medium snails.
C. for large snails.
Figure 4. Snail mortality by species and tidal height. Error bars show 95% confidence
interval.
Figure 5. The average number of snails found in each microhabitat. Error bars show 959
confidence interval.
Zonation of Tegula
igure 1

Monique Rocca
seret
meta
Washer
monofilament
Sitk
sliekoot
—rotser
I
Sasher
— Wal
onchoc
hole
drilled
Zonation of Tegula
L
2 to6
102
O to 1
-1to0

10
Monique Rocca
2
Figu
T. brunnea
E T. funebralis
20 30 40 50 60 70
Average number of snails per .25 m°2 quadrat
Zonation of Tegula
Figure 3




6 hrs.
12 hrs
48 hrs
24 hrs.
ngth
Exposure



6 hrs.
12 hrs
48 hrs
Length
Exposure


22
2

6 hrs.
12 hrs
24 hrs.
48 hrs.
Length of Exposure
24
Monique Rocca
T. funebralis
T. brunnea
funebralis
F. brunnea
T. funebralis
T. brunnea
Figure 4




Low
Medium
High
Intertidal height of stations
Monique Rocca
□ T. funebralis
T. brunnea
Zonation of Tegula
4
Zonation of Tegula
2
Ha
A


Figure 5


Monique Rocca
E T. funebralis
T. brunnea

4
Microhabitat