Territoriality in the South African Intertidal Limpet Scutellastra Longicosta

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Territoriality in the South African Intertidal Limpet Scutellastra longicosta Amanda Heidt, Amrita Khalsa, Shannon Myers, Travis Trinh, Victoria Wade Abstract The existence and effects of territoriality are documented in vertebrate systems but have received less attention among invertebrate species. This study attempts to create a clear and concise working definition of territoriality through the experimental confirmation of three tenets in the South African Long-Spined Limpet Scutellastra longicosta, a species suspected of displaying territoriality as a result of its mutualistic relationship with the brown alga Ralfsia verrucosa. We posited three criteria for establishing territoriality: (1) uniform distribution, (2) restricted movement to the immediate territorial space (garden), and (3) the presence of defensive behavior (i.e. active expulsion of intruders). Spatial distribution was found to be uniform (p= 0.035) when accounting for a correlation between limpet size and garden size (p= 0.0002). Movement was restricted to a region well within the mean garden radius (p= 0.0001). Lastly, in situ manipulations induced territorial and homing behaviors among territory holders and transplanted individuals (p= 0.0001), respectively. The statistical confirmation of these tenets has ramifications in standardizing the way territoriality is studied in the field. In addition, it provides a baseline for examining the role of territoriality in maintaining the diversity and stability of localized intertidal ecosystems.

Introduction In community ecology, competition is thought to be one of the most important processes driving the structure and function of natural species assemblages [22]. In particular, competition for space has received a great amount of attention to better understand distribution and abundance and species compositions in coastal marine ecosystems such as the rocky shore [24]. The sheer amount of theory to arise from studies undertaken along the world's coastlines, exemplified in landmark papers such as Paine ‘66[24], Connell ‘77[6], and Sousa ‘79[31], lends credence to a rich source of interactions between the physical environment and those organisms contained therein. Equally as numerous are the strategies that have evolved to combat abiotic factors such as desiccation and temperature fluctuations, as well as biotic interactions such as resource competition and predation. Novel strategies amongst individuals have evolved to maximize fitness in a system largely dominated by limitations, either for space or for other resources. Traditionally, it can be difficult to tease apart these biotic and abiotic effects, as a limitation on an abiotic factor such as

1 space can drive fierce competitive interactions between individuals [2-3]. One manifestation of these interactions is the development of territoriality, a prevalent example of how best to secure resources. A result of territorial behaviors, including competitive exclusion and territory maintenance, is that some species significantly affect their environment in terms of localized habitat modification and species composition. These interactions create a system of habitat and species mosaics on both small and large geographic scales. In turn, habitat mosaics can support a broader level of overall species diversity [9,14-15,18-19,25]. Territorial behaviors have been shown in a variety of intertidal species, from blennoid fishes [10] to birds [5] to limpets [1-4,12-13,28,30,33,35]. Territoriality is a thread that links many limpets together, as several species demonstrate some degree of territorial behavior, primarily Scutellastra longicosta, Scutellastra caerulea, and Lottia gigantea [1-4,12,27-28,30,32]. The forms that these behaviors take are as variable as they are effective. L. gigantea has been studied extensively and has been shown to defend a small territory (< 900 cm2) while moving locally to forage [28]. Contrastingly, Scutellastra cochlear maintains a territory of red algae not much larger than the size of its own body, taking many years to mature and moving very little (if at all) during that time [2]. S. Longicosta is a species of South African limpet suspected of engaging in territoriality as a byproduct of a mutualism with the brown algae Ralfsia verrucosa [8,11,21]. Characterized by long protrusions extending outward from the shell, these spines grant the animal a two-fold advantage: (1) they allow the animal to extend its surface area without a drastic increase in body size, as the visceral mass extends into each spine, allowing for greater suction and (2) the spines act as a tool for the animal to pry off intruders from its territory [2]. Past research has suggested that mutualistic species affect algae via biotic and abiotic factors such as grazing and nutrient supplementation by herbivores [20,32]. By grazing away existing algae, S. longicosta provides a bare substratum required for settlement by Ralfsia. Once established, the limpets cultivate and fertilize their space through feces and mucus [8], promoting growth. In addition, individuals will directly engage any intruders, driving away any would-be competitors or algal predators. Both juvenile and adult S. longicosta prefer to feed upon R. verrucosa, and through differentiation of life cycles have minimized intraspecific competition for this resource [2,26]. These seemingly territorial behavioral patterns have been well-documented [1-4,12,27- 28,30,32], and it may be appealing to brand this an example of territoriality in action, but very little work has been undertaken to develop a concrete working definition of what actually constitutes

2 territoriality. An analysis of the methodology used in studying vertebrate territoriality found that only 12% of the papers actually provide an operational definition of the word, and that within this group 50% used only the single criterion of 'defended area' as an indicator [16]. No such large- scale analysis has yet been undertaken in establishing a working definition of territoriality in invertebrates, although there would certainly be overlapping factors at play. A review of ecological determinants of vertebrate territoriality determined food resources (and several related variables such as quantity, predictability, distribution, etc.) to be the main ecological driver [17], and the mutualistic relationship between S. longicosta and R. verrucosa seems to support this idea. The purpose of this paper is to take the first step in establishing concrete tenets of territoriality, here defined as “the behavior of an organism in defining and defending its territory.” Specifically, we would like to begin the discussion of how best to frame this phenomenon in the context of an invertebrate species. We propose the following questions as indicators of true territoriality: (1) Does S. longicosta exhibit uniform distribution, (2) Is movement restricted to the immediate territorial space (garden), and (3) Do individuals exhibit territorial behavior (i.e. active expulsion of intruders)? In establishing the existence of these three tenets in a population of S. longicosta, we hope to provide a means for defining territoriality in the field.

Materials and Methods

Site Description This study was conducted in the Western Cape of South Africa, in the South-Western Cape marine bioregion, near Betty’s Bay, approximately 0.5 km north of the Stony Point, and just below Mooi Hawens resort (S: 34.37371, E: 018.88407). The shoreline at our site is largely composed of bedrock and various aggregate ranging from large, jagged boulders to small pebbles that together form a substantial and varied rocky intertidal region. Our study site is characterized by smooth bedrock, an incline of less than 15o, minimal topographic variance, extensive R. verrucosa cover, moderate to high densities of adult S. longicosta, and a tidal range of 50-85 cm above the mean lower low water (MLLW). On the rocks and in the various pools surrounding our study site, there is a plethora of other invertebrate, algal species, birds, and fish. Our site is

3 situated in the lower-mid tidal range. Due to seaward reef-like subtidal and exposed rock formations, wave exposure is minimal and the sites experience only gentle tidal flushing each day.

General Methods We tested our general hypothesis that S. longicosta exhibit territoriality by performing two observational studies and one manipulation study in order to test the following three specific hypotheses: individuals are uniformly distributed (distribution), largely remain within home territories (movement), and exhibit some form of defensive and or homing behavior (behavior). Studies were conducted during the month of April 2013, in Betty’s Bay, South Africa at our site, described previously. This site satisfies conditions where S. longicosta have been observed in highest densities. We initially investigated the correlation between several parameters of limpet size and garden area and later applied it to both our distribution and movement studies. We randomly removed 15 individuals of varying sizes from the substrate (using a knife to pry them from the surface) and took the following measurements: length of the largest diameter (spine-to-spine), height, and weight. Additionally, we measured between the farthest points of each garden to estimate the diameter (and thereby determine the area), and used statistical analysis to find the best variable to use for determining the area of an individual’s corresponding garden. (We performed a linear regression on each possible parameter versus garden area and chose the most significant one.) All statistical analyses were performed using a 95% confidence level.

Distribution In order to test the hypothesis that S. longicosta exhibit uniform distribution, we sampled quadrats in areas meeting our substrate criteria and statistically compared the observed distribution of S. longicosta to the two other possible distributions, random and clumped, to either support or reject our expectation of uniformity under the first tenet of territoriality. Quadrats were constructed using 1.5 inch PVC pipe that measured 25 x 25 cm. We sampled our entire site by laying a quadrat down at one boundary (defined by where substrate no longer met our criteria) and rolling it over in all directions until the entire site had been sampled. For each quadrat placed, we recorded the number of S. longicosta individuals with at least half of

4 their shells contained within the quadrat. Distribution was assessed by analyzing the variance to mean ratio (V:M) of the number of S. longicosta individuals across all quadrats and comparing the calculated value to known V:M values associated with either random (V:M = 1), clumped (V:M > 1), or uniform distributions (V:M < 1). Additionally, we conducted a Kolmogorov-Smirnov (KS) test in order to compare the obtained distribution data to that of a Poisson (random) distribution. Although not directly utilized in our testing of the above hypothesis, for a visual representation of distribution, we manually laid out an XY plot using seamstress tape. We measured the X and Y positions and greatest length for each individual in the plot and calculated their garden area using the formula derived from the previously mentioned correlation. We generated a bubble plot to represent the territory size and respective location for each individual.

Movement In order to test the hypothesis that S. longicosta exhibit limited movement outside their home territories, we monitored individuals’ daily movement in our study site. We compared calculated territory size for each individual to observed movement and determined if S. longicosta’s mean movement extended beyond home territories. We randomly tagged and measured 109 individuals at greatest spine-to-spine diameter. We tagged individuals using super glue to attach 3x5 mm rigid, plastic tags at just below the apex of individuals’ shells. We placed corresponding substrate tags directly adjacent to each individual. We marked all tagged individuals with a dot of nail polish as a means to monitor our tag failure rate. Immediately following tagging, we measured each individual from center of shell tag to center of substrate tag for seven consecutive days at low tide. We calculated mean movement across all individuals and compared it to mean radius (as a proxy for territory), derived from the established correlation of shell length to garden size previously mentioned, using a two-sample, one-tailed t-test. We interpreted the results of the test assuming a null hypothesis of movement greater than or equal to mean garden radius.

Behavior In order to test the hypothesis that S. longicosta exhibit defensive/homing behavior, we manipulated the spatial positioning of individuals in relation to both conspecifics and to their

5 home territories. If S. longicosta defend their home territories, then placing conspecific competitors in close proximity to other individuals’ home territories should create a defensive response in residents that will generate a measurably different response by intruders when compared to those placed in vacant territories. Homing behavior was determined by comparing movement between individuals moved to either vacant or occupied territories. We conducted manipulations at the same site used in the previous movement study so as to utilize previously tagged individuals whenever possible. For each trial we used sets of five similarly sized adult individuals residing in close proximity to each other. We located ten such trial groups and tagged any individuals not previously tagged. For each trial, we removed individual 1 and placed it in its original location to serve as a control for the effect of removing individuals from their substrate. We removed individual 2 and discarded it to create a vacant garden. We removed individual 3 and placed it on the scar of individual 2. We removed individual 4 and placed it directly adjacent to unmanipulated individual 5 on its home scar. We conducted a total of ten sets of manipulations. We took measurements from the center of shell tag to the center of substrate tag for individuals 1,3,4, and 5 in each trial group immediately following manipulation, and then again each day at low tide for seven consecutive days. We compared mean movement of individuals 1,3, and 4 across all trials and replicates using an ANOVA test. We expected one of three possible outcomes as the primary driver of movement: if only transplants moved to occupied territories move, we will conclude defensive behavior is responsible; if transplants moved to both occupied and empty territories move equally, we will conclude homing is responsible; if transplants moved to occupied territories move more than transplants moved to empty territories, we will conclude both defensive behavior and homing is responsible.

6 Results After we performed linear regressions on each of the potential parameters of size to garden area, we found length to be the most significant (P < 0.0002) (Figure 2). We used the following formula to calculate garden area: Area of garden = -91.72303 + 29.321444*Length +

5.5994887*(Length-4.594)^2.

Distribution The results of the experiment designed to test for uniform distribution of S. longicosta strongly support the hypothesis that S. longicosta are uniformly distributed based on the variance to mean ratio being less than one (0.632) and the distribution being different from Poisson distribution (P=0.035) (Figure 3). A total of 80-90% of all quadrats contained

between one and two limpets (Figure 4).

7 Movement The results of the experiment designed to test if S. longicosta move farther than the radius of their garden strongly support the hypothesis that S. longicosta limit their movement to within their garden (P<0.0001) (Figure 5). Average movement = 0.67 cm and average garden radius =

3.97 cm.

Defensive Behavior The results of the experiment designed to test for defensive behavior strongly supports the hypothesis that S. longicosta defend their territories. When placed in a territory with an existing resident, on average the transplant would move more than when placed into an empty territory (P = <0.0001) (Figure 6). On average the control moved 1.24cm, the transplants moved into an empty territory moved 12.65cm, and the transplants moved into

occupied territories moved 32.63cm.

Discussion Based on the results, we were able to confirm each of the 3 tenets, and in doing so suggest the presence of territoriality in the limpet species S. longicosta. Territoriality has a variety of implications for the health and maintenance of ecological systems. While many vertebrate studies focus on relatively large territories and even larger home ranges, such models used to examine territories and territorial

8 interactions are limited by the logistics of working on such large scales. Under a finer lens of analysis, small invertebrate species such as S. longicosta can have drastic localized effects in altering the landscape and helping to impose strict distributional regimes. More broadly, this is a species that also paradoxically may help maintain higher levels of diversity through continued disturbance [22] and in so doing be responsible for increasing system stability [24].

Distribution Individuals adhered to a uniform distribution (Figure 3) consistent with relatively equally spaced, non-overlapping gardens. When interpreting these results, it is important to bear in mind a positive correlation between limpet size and garden size (p = 0.0002, Figure 2), as a spatial representation (Figure 4) may appear random at first glance. However, we were able to statistically factor out both random and clumped distributions using a variance: mean ratio (0.632, Figure 3). Furthermore, we were able to show statistical deviations from Poisson using a KS Test (p = 0.035, Figure 3), giving additional statistical power to our final results. A uniform distribution allows for individuals to exist in close proximity without actually coming into contact, maximizing resources that may be patchily distributed in areas of varying density.

Movement Movement was shown to be minimal (Figure 5), as would be anticipated in a species initially described by Branch as “non-migrating [2].” Limpets were expected to move no further than the edges of their garden (as measured by the radius), and this was shown to be true across the duration of the study (p = 0.0001, Figure 5). The average total movement (0.67 cm) was significantly less than the average garden radius (3.97 cm), indicating very little motion (Figure 5). This lack of movement is most likely the result of a need to protect the garden space from any potential intruders. Excluding the need to graze and fertilize Ralfsia, energy is best stored for use in territory defense.

Behavior Manipulations performed in situ (Figure 1) as a means of assessing territoriality yielded a two-fold response: not only was territoriality an inducible phenomenon when intruders were introduced, but individuals that were moved into territories (occupied or not) were shown to

9 demonstrate homing (Figure 6), a concept not initially addressed in the experimental design. When placed in a territory with an existing resident, movement by the transplant was greater on average (32.63 cm) than when placed into an empty territory (12.65cm), indicative of a response to territorial behavior (p= 0.0001, Figure 6). Controls moved very little (1.24cm, Figure 6), behavior supported by movement data (Figure 5). Territoriality has applicability in several ecological paradigms. The idea of intermediate disturbance was first described by Connell [7] and posited that intermediate levels of disturbance in a system would promote the highest levels of diversity. An analogous example would be the way fire carves a terrestrial biome into patchy mosaics of territories in various stages of succession [23]. Part of the mutualistic relationship between S. longicosta and R. verrucosa involves the clearing of space for R. verrucosa to settle in higher densities than would be expected in the absence of territoriality on the part of the limpet [2]. This relationship adds to the mosaic of the local environment, promoting healthy levels of disturbance as well as increased diversity. As an example, grazing by limpets has been shown to inhibit growth of Ulva spp., a standard early successional species [6,21]. Indeed, unpublished algal species counts in this study yielded no less than a dozen different species within the study space. These other species, which contained members of all three algal divisions, took hold in the proverbial “no man’s land” between limpet territories where no active grazing was occurring. Gardens rarely overlap (Figure 4), and a uniform distribution averaging only 1-2 limpets per quadrat (Figure 3) means that oftentimes spaces exist between gardens that are available for colonization by other algal species but too small for S. longicosta to establish territories. Despite being outside of a defended territory, the aggressive behavior exhibited by S. longicosta discourages other individuals (both co- and conspecific) from entering the area[2], lowering grazing pressure. While it seems paradoxical to suggest that S. longicosta promotes greater diversity while simultaneously stimulating the growth of a single algal species and excluding other species from the area, it is again a matter of scale. On the smallest scales, S. longicosta does indeed surround itself with only R. verrucosa. However, if overall diversity of algal species occurs as a result of territoriality, on a larger scale vast configuration of S. longicosta occurring all along the South African coast may contribute to a more healthy and diverse system. Suggestions of increasing diversity leading to a more stable system have been championed by famous ecologists [9,14-15,18-

10 19,25] for decades, and it may well be the case that concrete forces such as territoriality operating in such a dynamic, space-limited system as the intertidal are essential for healthy ecosystem functioning. Diversity exists beyond the species of algae, but also in the limpets themselves. South Africa supports at least two-dozen limpet species [26], and many co-occur in the same areas. As mentioned previously, the life history strategies are as varied as the species themselves. S. longicosta was shown to be largely static in its movement patterns (Figure 5) and has been previously classified as a territorial, non-migratory species [2]. However, other species have adopted extensive ranges of motion, nocturnal foraging behaviors, or strict substrate requirements as a means of co-existing [1-4,34]. In turn, many predators have evolved to specialize in limpet predation. For example, African Black Oystercatchers (Haematopus moquini) have been described as specialized feeders of S. longicosta [26]. Homing was an unanticipated behavior that manifested itself within the manipulation data (Figure 4). While not included in the tenets, homing is a behavior highly congruent with territoriality and has been previously described in other members of Scutellastra [2, 29, 34]. In both manipulations where a limpet was moved into a new territory (either with or without a territory holder in place), individuals were observed moving back to their home scars, sometimes within the scope of a single day. While some work has suggested a mucus trail as the mechanism by which limpets return to home scars [8], this could not have been the case in this study as individuals were pried off and moved without laying down a scent trail. How individuals were able to return is undetermined, but it may well have been an unintended result of manipulation design that allowed some individuals to be close enough after manipulation to pick up on cues from the garden itself. There were a few additional points of contention in our protocol we would like to address in the hopes of guiding future research. To begin, we suggest a more thorough distributional analysis of S. longicosta along the coast of Southern Africa, including any potential affinities for certain topographical and substrate features or biological attributes such as the local algae community. While certain general patterns in distribution exist [1], our field site presented a bit of an anomaly across a relatively small spatial area. Though resolution was high enough to achieve the desired statistical significance, bolstering the sample size would serve to lend additional credence to our conclusions.

11 In addition, our study was constrained to periods of low tide when limpets were exposed. The premise was that because limpets typically do not move out of water, any manipulations and subsequent responses would not take place until the area was again submerged. However, similar attempts by Branch [2] to prompt aggressive behavior failed to do so in 35 out of 38 manipulations when performed out of water. This fact did not appear to hamper our findings, as we found significant results and had a tagging failure rate of only 1%. Though, it might prove more effective when dealing with greater sample sizes. Having addressed these points, future work could take many potential manifestations. Because S. longicosta is not as well studied as other territorial limpet species (e.g. Lottia sp.), any forthcoming information is likely to prove insightful. While it is known that they are broadcast spawners [1,26], less is known about age-based population structure, including how juveniles factor into the territorial scheme. Smaller individuals were observed with corresponding smaller territories, but as of yet very little information exists to describe the cues individuals use in the transition to establishing territories of their own. Furthermore, given the seemingly intensive intraspecific [2], competition taking place as a result of territoriality, a logical progression would be to test for the presence of interspecific competition between other intertidal species, in congruence with work done by Branch [3]. An example interaction founded on field observations occurs between S. longicosta and another non- migrating species of limpet, Cymbula oculus. While C. oculus does not actively maintain a territory [34], it was found living in close proximity to S. longicosta, and in several cases was close enough to elicit a potential response. Shifting the focus from intra- to interspecific competitive forces may yield additional insight into the ways in which S. longicosta helps shape local community structure. Ideally, this study would serve as a baseline from which to launch further enquiry. Additional time, intensive surveying, and a larger working sample size would help to standardize and cement the ideas presented herein. However, the first step of clearly defining three tenets and proving that they have applicability in the field is important if only for its novelty. It is our hope that this discussion on territoriality, especially within the context of the relatively understudied invertebrates, can be carried forward.

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