1) The study examined populations of the pear limpet Scutellastra cochlear along the coast of Betty's Bay, South Africa to determine the physical and biological factors influencing their distribution. 2) The results showed higher densities of S. cochlear in areas with greater wave exposure. Limpet size also varied between sites, influenced by external factors like wave action. 3) Analysis found the distributions of S. cochlear were uniformly spaced at each site, supporting the hypothesis that they exhibit territorial behavior. Individuals also did not migrate from their home territories over the study period.

1. The Physical and Ecological Effects on Distribution of the Pear Limpet,
Scutellastra cochlear
Cory Pickering, Evan Firl, Camille Baettig
Abstract: Intra-specific competition plays a huge role in mediating species populations.
We observed populations of Scutellastra cochlear along the coast of Betty’s Bay,
Western Cape, South Africa. S. cochlear was monitored for several weeks at three
separate locations, each of which was subject to the same criteria of testing. Length,
width and density were measured through NIH’s ImageJ and other statistical analyses,
which allowed for the determination of territoriality and uniformity of S. cochlear
distribution. We found that they are distributed uniformly and do not move away from
home territories. However, the third criteria for competition: interaction with other
individuals was not seen in our study. We concluded that there is a correlation between
population density and wave action and also a similar correlation between size and wave
action. It was found that higher levels of wave action are strongly related to levels of
uniformity of the studied population at specific sites.
Introduction:
Background: Competition in its broadest sense is when multiple individuals compete for
a resource that could potentially become or already is limiting such as food or space
(Branch 1975). For sessile animals such as the pear limpets, Scutellastra cochlear, the
limiting resource is usually space on rock surfaces (Menge and Sutherland 1976). Factors
such as size, growth, and distribution can be affected by these limitations. For example it
has been proven that as S. cochlear density increases the size of individuals decreases
(Branch 1975). S. cochlear is found in the lower intertidal in areas where high wave
action is observed. It has been shown on multiple occasions that in environments with
severe conditions, intra-specific competition plays an important role shaping populations
and moderating communities (Menge and Sutherland 1987; Bertness 1989). Additionally,
once high levels of densities are reached intra-specific competition can regulate the
population size leveling it out at carrying capacity. Branch 1975, explained that S.
cochlear densities can be extremely variable ranging from 90 individuals to 1700
individuals per square meter, the highest densities of limpets observed across the genus.
Although natural disturbances play a role in maintaining the density of S. cochlear, there
is most likely a competitive process that maintains each population at its given carrying
capacity.
The pear limpet, Scutellastra cochlear, is one of eleven limpets from the genus
Scutellastra found in South Africa (Branch 1976). As stated previously they are found in
extremely dense populations in comparison to other species of limpets. Each individual
carves a scar into the rock it settles on and remains there for the rest of its life.
Additionally, adults with established scars maintain a small algal garden of encrusting
coralline algae, Spongites yendoi, around themselves. For the most part they feed on the
garden found within their surrounding area to avoid competition between other
individuals (Branch 1975; Bertness 1987; Sterner 1986; Plaganyi and Branch 2000). The
high population density is in part due to the fact that juveniles settle on the shells of

2. adults until they are large enough to make their own scars in the rock. This works
because the adult shells are covered with lithothamnion that the juveniles can consume
(Branch 1976; Branch 1975). Therefore, juveniles are somewhat density independent.
Because of their highly dense population, S. cochlear excludes other animals from
settling near them and most likely drastically decreases chances of inter-specific
competition. Additionally, the number of individuals in a given area has the ability to
alter the habitat by dictating which species of algae can grow and has led to the area pear
limpets are located being called the ‘cochlear zone’ (Branch 1976).
We were interested in studying S. cochlear because they appeared to exhibit a
uniform distribution on rock surfaces they resided on. Because they were the only
invertebrate species seen on the rocks they occupied we wondered if these limpets could
be completely dominant of that particular zone in the intertidal. Additionally, it is the
only known intertidal zone that we observed to have a single animal species dominating
an entire zone. The red ring of coralline algae that surrounded nearly every individual
also intrigued us. We later discovered that this is a garden that S. cochlear maintains and
grazes. This behavior led us to believe that S. cochlear was territorial and that size of the
individual could possibly be linked to its distance to other individuals.
We aimed to determine whether there are physical and/or biological factors that
influenced S. cochlear distribution within the cochlear zone. We examined the effect of
wave exposure as a physical factor and the competition for space among S. cochlear
individuals as a biological factor. We believed that differing levels of wave exposure
affect the density of S. cochlear from site to site. We expected to find denser populations
in wave exposed areas and less dense populations in areas of low wave action.
Additionally, we hypothesized that S. cochlear size would vary and even correlate to the
level of wave action it was exposed to meaning smaller S. cochlear would inhabit areas
of higher wave action while the larger individuals would inhabit areas of lower wave
action.
Hypotheses:
Density and Wave Action: Initial observations of S.cochlear suggest the necessity
of high levels of wave action and/or water flow in determining population distribution
patterns in the intertidal. It has been suggested that physical pressures, namely wave
action, play a profound role in spatial distributions of intertidal and subtidal species of
sedentary and sessile organism due in large part to delivery of food-related resources and
larvae (Christofoletti 2011). To determine if wave exposure affects limpet distributions,
we formulated and tested the following hypothesis: wave action is a physical factor that
positively correlates with S. cochlear densities. This hypothesis guided our research
toward the use of suitable space by the species, rather than directly determining what
wave action's role is in allowing or preventing occupation of intertidal space, as it is very
closely linked to our second proposition: that S. cochlear exhibits territorial behavior.
Territoriality: In order to understand biological pressures affecting the
distributions of S. cochlear we set out to test for intra-specific competition for space.
For the purposes of this study we are using the term territoriality as defined by Heidt
2013: “the behavior of an organism in defining and defending its territory.” To determine
if territorial behavior is present in these animals three criteria are presented to define and
test for territoriality: (1) distributions are uniform, (2) individuals show no migration

3. from a defined home territory, and (3) interactions between individuals occur to
determine boundaries of territories. Our data collection methods aimed to determine if
each criterion was met in the subject populations, and if so, confirm at least one process
of biological origin shaping S. cochlear distributions.
Materials and Methods:
Site Description: The area of our study included three locations along a six
kilometer stretch of coastline North of Betty's Bay, Western Cape, South Africa during
the month of April 2013. Locations were chosen to represent different exposure
conditions in the region accessible to the team by foot. Each location, labeled on the map
A, B, and C, was composed of three to seven sites located in the lower intertidal almost
exclusively on the furthest rock faces from shore that received the most direct wave
contact from the open ocean. Sites
were chosen in varying conditions
of protection by the orientation of
the land (i.e. bay or point) and
shelter from local rock formations.
S. cochlear occupies the lowest
range of the intertidal zone and
many areas occupied by the
species are only exposed by the
lowest tides. This fact, combined
with limited safety equipment,
prevented us from choosing sites
that were the most exposed to
uninhibited wave action and
lowest in the intertidal.
Data Collection: Our field study employed the use of photo analysis techniques
and the imaging software ImageJ (NIH, Washington, DC, USA). Each site chosen was
photographed keeping the software analysis in mind. More specifically, each photo
contained (1) a meter tape for setting scale of measurements done in the software, and (2)
a relatively flat surface mostly free (or cleared) of algal cover. Portions of photos were
not included in data analysis if the angle of the substrate or unavoidable cover prevented
proper measuring techniques. In each photo, a scale was set and using this scale
measurements were made of a variety of aspects for each limpet present.
Experimental Design:
Density and Wave Action: To address the hypothesis that sites with increased
wave action have higher densities of S. cochlear, we designed the following test: Relative
density for each site was determined by measuring the average distance between each
individual (focal individual) and its nearest neighbors (satellite individuals). Smaller
average distances between individuals represent higher densities (individuals per unit
area). We also predicted as part of our hypothesis that sites exposed to higher wave action
would not only contain denser limpet populations but also be made up of smaller

4. individuals. Measuring the length and width at the longest and widest points,
respectively, of each individual will test this. To determine if these data correlate with
wave action, each site was assigned a qualitative category, determined in the field, of
wave action levels from high, medium, and low.
Territoriality: To test the criteria of territoriality as described above we designed
two tests. The first test addresses our first criterion: uniformity in distribution and uses
the same measurements and methods described above to measure density. To test for
uniformity, however, average distances between focal and satellite individuals was not
used, and instead a statistical test using variance to mean ratios (VM ratios) of each
individual was compared to established theoretical standards. An average VM Ratio for a
site equal to one indicates random distribution. VM Ratios closer to zero indicate
uniformity (Zar 1996). Calculated VM ratios for each site were compared to the standard
to confirm the presence of uniformity at each site.
The second test addressing the second criterion of territoriality is a simple
marking test. Individuals were tagged along with the substrate they are found on,
matching each individual to a home territory. Tracking movements over multiple days
determined if individuals at low tide are in fact staying in their home territories and are
not migrating elsewhere over a multiple-tide sequence. If individuals are shown not to
move, the second criterion of territoriality is confirmed.
Results:
Variability in Length: Measurements of average lengths of individuals at each site
show a wide variety of growth with little overlap in variance. Values fell between 2.5cm
and 6.5cm (Figure 1, P<0.001). These results indicate conditions are different enough at
each site to affect overall limpet growth, suggesting the influence of external factors.
Density and Wave Action: Density, as measured by the average distance from
each individual to each of its closest neighbors, was found to show a positive relationship
with average limpet size in every wave action category. As size increases, sites
categorized as low wave action have disproportionately lower densities (i.e. larger
average distances between individuals) than sites marked as high and medium wave
action (Figure 2, P<0.008). High and medium wave actions sites show nearly identical
trend lines for density versus size.
Uniformity: Variance to mean ratios for each site were all significantly lower than
one, strongly indicating uniformity (Figure 3). All values fell between 0.25 and 0.05.
When VM Ratios are averaged by assigned wave action category, ratios decrease as
exposure increases in a clear trend (Figure 4). This unexpected, but significant, result will
be discussed in the context of our initial density hypothesis below.
Migration: The tagging of individuals used to test for territoriality and
migrational movement showed no movement of individuals over a period of four days.
This was checked daily at low tide.
Discussion:
Density and Wave Action: The positive relationship between S. cochlear densities
and wave action agree with our predictions and confirm wave action as an important

5. physical process affecting their distributions. The nearly identical densities at high and
medium wave action sites could be interpreted in two ways: The first is that our
categorization of wave action was not consistent or specific enough to detect differences
in the two categories. A more quantitative method would rectify this problem. The
second possibility is that our categorization was not the problem, and the data is accurate
in this area. Such a case would imply some sort of minimum threshold for supporting
higher densities of limpets of larger sizes. This could be due, in part, by the delivery of
food or other chemical resources to the limpet or to the algae it grazes.
The differences in densities between wave action levels gets larger as the average
size of limpets in any site also increases. Overall, for every wave action level density
decreases with average limpet size. We suggest here that this is due to the nutritional
demands of larger limpets. A larger individual would require more grazable algae, which
would require more space. If nutrient levels are low due to poor delivery from low wave
action, algal cover may suffer further increasing the minimum grazing area required by
an individual. This may explain both observed trends in Figure 2.
Territoriality: Our study found that every site photographed displayed clearly
uniform distribution. We also found that uniformity increased with wave action. These
results match our predictions for territorial behavior and confirm our first criterion of
territoriality. The uniformity trend paralleling wave action connects our first hypothesis
concerning physical processes and densities. Following the same logic as above, we
suggest that higher wave action allows for the delivery of more nutrients (for the limpet
directly or its food source algae) allowing an individual to survive on a smaller, but more
sustaining, grazing area, here suggested to be a defended territory.
Concerning our second territoriality criterion, migration from home territory, our
results suggest once a home territory is established, individuals do not leave its
boundaries. We cannot confirm this with the data collected, as duration, season, weather,
or other biological cues may be necessary to induce greater movement. It is generally
assumed in the literature, and supported by our data and unpublished experiments, that S.
cochlear do not move beyond its home territory.
The third criterion presented above, “ interactions between individuals occur to
determine boundaries of territories”, was not formally addressed in our study. Conditions
and equipment did not permit the long-term observation of individuals during their most
active periods. We suggest, however, the there is evidence that the species Scutellastra
cochlear exhibits territorial behavior, but further study would properly support this
hypothesis.
Suggestions for future research: In future studies, there should be a longer time period in
which to study the individuals to ascertain possible recruitment and settlement patterns.
Also, further research should be done on territorial habits especially pertaining to
relocation of individuals away from their respective home scars. This should be done
with a better way of removing the limpets than a knife or credit card, which harms the
individuals somewhat regularly. This test should be done over the course of several
weeks where individuals are removed and replaced as well as removed and placed in a
separate area to test homing behavior. These displacement tests would further lend
evidence towards the conclusion that S. cochlear is in fact homing. Developing better
ways of tagging and marking individuals and locations is also essential. By using types
of non-harmful water proof paint, the marking of individuals would be much easier. A

6. way of better removing the crust on the S. cochlear shells and rocks beside them is vital if
tagging consistency and accuracy is to improve. The ability to return to specific locations
at different tides was difficult due to the change in landscape with different tidal heights.
Better tools or tags for marking locations of low intertidal areas that won't be destroyed
by high wave action are necessary if there is to be improved marking ability of sites. The
ability to properly weight of S. cochlear is also necessary. The use of an accurate scale
down to 1g would be optimal. Future research could identify whether there is a
correlation between individual biomass and area per individual. Gathering information on
when and how juveniles settle on adults and when their optimal time/stage to relocate and
form their own scar. Discovering how limpets protect their “territory” is also a key factor
that has not yet been studied. Discerning whether wave action correlates to higher
resource availability should be studied to see if there is a relationship between density of
individuals and wave action.
Conclusion:
Our study confirmed that physical and biological processes are affecting S.
cochlear distributions. We discerned that wave action has a strong relationship to the
density of S. cochlear populations and that there is significant evidence that the species
exhibits territorial behavior. High variability in length from site to site was observed
indicating extraneous natural forces that contribute to the size and density of populations
of varying sites and locations. We also discovered that S. cochlear exhibits higher levels
of uniform distribution with increasing levels of wave action. Our study found wave
action to be a physical factor influencing S. cochlear densities and distributions, and that
there is evidence to suggest the species exhibits territorial behavior.
Figures and Tables:
Table 1: Analysis of variance of the average
length of individuals.
Figure 1: Average length (cm) of
individuals by site1.
Figure 2: Density by size (length x width) with
trendlines for wave exposure categories.

7. References:
Bertness, M. D. 1989. INTRASPECIFIC COMPETITION AND FACILITATION IN A
NORTHERN ACORN BARNACLE POPULATION. Ecology 70:257-268.
Branch, G. M. 1975a. INTRASPECIFIC COMPETITION IN PATELLA-COCHLEAR
BORN. Journal of Animal Ecology 44:263-&.
Branch, G. M. 1975b. MECHANISMS REDUCING INTRASPECIFIC COMPETITION
IN PATELLA SPP - MIGRATION, DIFFERENTIATION AND TERRITORIAL
BEHAVIOR. Journal of Animal Ecology 44:575-600.
Branch, G. M. 1976. INTERSPECIFIC COMPETITION EXPERIENCED BY SOUTH-
AFRICAN PATELLA SPECIES. Journal of Animal Ecology 45:507-&.
Christofoletti, R. A., C. K. Takahashi, D. N. Oliveira, and A. A. V. Flores. 2011.
ABUNDANCE OF SEDENTARY CONSUMERS AND SESSILE ORGANISMS
ALONG THE WAVE EXPOSURE GRADIENT OF SUBTROPICAL ROCKY
SHORES OF THE SOUTH-WEST ATLANTIC. Journal of the Marine Biological
Association of the United Kingdom 91:961-967.
Figure 3: Graphical
representation of the
variance to mean ratio. A
value equal to 1 is random
distribution, <<1 =
uniform, >>1 = clumped.
Figure 4: Increased uniformity by wave
exposure category.

8. Menge, B. A., and J. P. Sutherland. 1976. SPECIES-DIVERSITY GRADIENTS -
SYNTHESIS OF ROLES OF PREDATION, COMPETITION, AND TEMPORAL
HETEROGENEITY. American Naturalist 110:351-369.
Menge, B. A., and J. P. Sutherland. 1987. COMMUNITY REGULATION -
VARIATION IN DISTURBANCE, COMPETITION, AND PREDATION IN
RELATION TO ENVIRONMENTAL-STRESS AND RECRUITMENT. American
Naturalist 130:730-757.
Zar, J. H. BIOSTATISTICAL ANALYSIS: THIRD EDITION. Prentice Hall, Upper
Saddle River, NJ 1996,84,74 Chapter 24.3 Testing for Randomness pg 574-5