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 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 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 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 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 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:

Figure 1: Average length (cm) of individuals by site1.

Figure 2: Density by size (length x width) with trendlines for wave exposure categories.

Table 1: Analysis of variance of the average length of individuals.

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.

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.

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