Rocky shoes are common and easy to study, providing excellent environment for ecological studies. Patella vulgata is a keystone species of grazer inhabiting the rocky shore and occurring across the wave exposure gradient. Therefore it can be used as indicator of morphological changes occurring due to environmental stress. Study was carried out at Cemlyin Bay in Anglesey, Wales. Individuals of Patella were selected over horizontal gradient on each intertidal zone (High, Mid and Low) on two different shores (exposed and sheltered). Population density and individuals’ size were estimated. There was a significant difference in the density between the two shores as on exposed shore the density was higher. Size as well varies with shore, with larger individuals on sheltered shore. There was a significant relationship between size and zonation gradient on sheltered shore as well. The higher the zone, the larger the shell size is. The number of juveniles within zones on the different shores was also estimated, with slight difference in results indicating significance. However, more long-term research needs to be done, to gain useful results.
Rocky shores are common and relatively easy to examine. Therefore, they are a perfect study field for ecological observations (Connell, 1972; Paine and Kinne, 1994). The ecology of animals and plants on intertidal rocky shores has been a topic of interest since the early 30s of the last century (Hatton, 1932; Fischer-Piette, 1936) and is a main research location to this day (Morelissen et al., 2016; Kennedy et al., 2017).
On rocky shores, physical and biological factors such as wave action, exposure and predation create different habitats in a small spatial area on the intertidal zone (Menge, 2000). Therefore, inhabiting species have developed different adaptations to endure this variety of conditions, resulting in some species outcompeting others in a certain area (Crisp, 1965). This leads to a clear “zonation” pattern in which dominant species replace one another along the vertical gradient of emersion time (Harley at al., 2003). In the early studies of zonation, it was commonplace to name different zones after the dominant species (Audouin al., 1832; Forbs, 1846) but today the intertidal zone is simply divided per High, Mid and Low zone (Fig.1).
On High shore, the dominant algal species is Pelvetia canaliculata, followed by Fucus spp. on Mid and Ascophyllum nodosum on Low shore (Fido, 1980). When it comes to fauna, the most abundant animals are grazers (Geller, 1991). One of the common grazers inhabiting the shore are limpets, thus, they have been briefly studied (Snaith et al., 1970; Mercurio et al., 1985; Coleman et al., 2006). The term ‘grazer’ is often synonymous with the common limpet Patella vulgata (Coleman et al. 2006). Understanding this keystone species ecological pattern will lead to understanding ecology itself. Therefore the aim of this study was to examine the variation of species density (0.25m^2) and the population dynamics of Patella vulgata within shore height and exposure. Although there are few species of Patella (P. vulgata, P. ulyssiponensis and P. aspera) found on the rocky shores of the British Isles, individuals were not identified to the species level in this particular study. As one of the most abundant species of limpets around the UK coast (Bowman, 1981), Patella has an important role in habitat structure. On exposed shores the limpet has an extremely important community structuring role regulating the recruitment of algae (Hawkins et al., 1992; Jenkins et al., 1999). As one moves into a shelter shore where Ascophyllum nodosum is the dominating algal species, the density of Patella vulgata (Crisp, 1965; Thompson, 1980), and its role in controlling community structure (Jenkins et al., 1999) declines. As a result, it was suggested that the population density of Patella vulgate would vary with shore exposure and height. Plus, Patella size varies with shores with larger individuals found on sheltered shore (Lewis at al., 1975; Evans, 1998) hence it was expected that on exposed shores, individuals will be far smaller in size than those found on the sheltered shores. Growth rate of Patella is also favoured by wet conditions. Juvenile mortality is great in wet sites, but less in the presence of fucoids (Thompson, 1980). As a result it was suggested that the population dynamic of Patella (number of juveniles vs. adults) will vary with shore exposure and latitude zones (High, Mid and Low).
The study was carried out on 18/10/17 at Cemlyn Bay in Anglesey, Wales. This is a good place for the research as its geographical location creates both sheltered and exposed shores not far away from each other (Fig.2).
A 0.5mx0.5m quadrate was used per transect (exposed and sheltered). The data was randomly collected over a horizontal gradient of the shore, with samples taken from different locations throughout the zone in an attempt to cover maximum spatial areas. Between 10-15 samples were collected per site (High, Mid and Low) on each shore. Limpets under stands of macroalgae were also taken into account. The number of limpets was counted to estimate the species density. The size of limpets was also taken into account, assured to the nearest 0.1mm shell length using vernier callipers, in order to observe the population dynamics. Any individual below 20mm in length was considered a juvenile.
For estimating the population density, the data was plotted in Excel using histograms to achieve a rough estimation of the probability distribution that was made per shore exposure and latitude zone. Then, to support the hypothesis, a statistical analysis (SPSS) was run. A Two-way ANOVA was used, showing the interaction between the two factors (density vs. exposure). Finally, the data per quadrate was plotted in Excel to compare the number of juveniles on different shores.
Species density varies with shore and exposure. Statistical analysis supports these results, showing where the significant differences lie. On High shore there is a significant difference in population density varying with exposure, whereas at Mid shore there is no difference between the two shores (Fig.3). On Low shore, there is a slight difference – on exposed the population density is higher (Fig.3).
Individuals on exposed shore are smaller than on sheltered (Fig 4). There was no significant variation in size within different zones of the exposed shore, with slight decrease in size on High shore (Fig.4). The highest number within a range was between 20-30 mm on Low and 22-38mm on Mid and High zone (Fig.4). On the other hand, on sheltered shore, there was a significant shift in shells length per zone (Fig.4). There is a clear pattern of shell growth with increasing exposure on sheltered shore, whereas there is no significant pattern observed on exposed (Fig.4). Furthermore a statistical method (Two-way ANOVA) supports the hypothesis (Table 1). There were statistically significant differences between Exposure (p= 0.027) and shore height (p=0.056). In spite of p>0.05 for shore height, the null hypothesis cannot be accepted, as the p value is close to 0.05, approaching an acceptable significance level. Plus, there is a significance between Height and Exposure (p=0.045), therefore the hypothesis is accepted.
Finally, population structure of Patella varies with shore type and zonation (Fig.5). On sheltered high shore, juveniles are absent, whereas on Mid shore the number is slightly higher than on Low (Fig.5.A). On exposed shore there is no significant difference in the number of juveniles within shore exposure (Fig.5.B). However, on Low shore, the number of juveniles is slightly higher (Fig.5.B). Results indicate a shift in population density within the two shores.
This study shows that the population characteristics of Patella vulgata vary with types of shore and the vertical gradient exposure. Results indicate that the population density of Patella is slightly higher on exposed shores. It is known that the density of the limpet declines with increasing shelter (Crisp, 1965). This is due to the limpet being vulnerable to turf algae (Jenkinks et al., 2001) and sheltered shore being known for high algal abundance (Kaehler, 1997). Plus, it could also indicate an increased presence of the predator: Nucella lapillus. This is due to the fact that the dog whelk population is known to grow faster on sheltered shore over a wave exposed one (Crothers, 1985). However, Patella individuals under canopy were also counted for this study, which might explain the low difference in density on both Mid and Low shores. On sheltered shore, Patella individuals are likely to be found in high numbers under stands of algae (Ballantine, 1961). Due to variability in the cover of understory species, the effective density of limpets per unit area over the wave exposure gradient varies at different locations (Jenkins et al., 1999). Food supply is essential for determining the population density in different locations, but it was not measured for this study. However, overall it can be concluded that the density of Patella vulgata is higher on exposed shore.
Not only do algae affect population density, the microalgae abundance across the wave exposure gradient has a direct impact on the growth rate of Patella vulgata (Jenkinks, 2001) and decreases the effect of exposure. As limpets are attached to the rocks, the environmental conditions modify the shell shape (Denny et al., 2000). On the wave exposed shore, the repeated muscular foot contraction of Patella vulgata required to maintain the position of the limpet results in the mantle being drawn inwards (Brehaut, 1982). Thus the shell becomes tall and narrow, or more conical (Evans, 1998). Relaxed limpets produce flatter shells than stressed limpets (Branch, 1981), which increase the length size. Therefore on sheltered shore, the shell size of Patella vulgata is larger than on exposed shore (Fig.4). There is a difference in size within a single shore as well. On exposed shore, there is no significant difference in size between the zones (Fig.4) as the harsh wave action forces the limpets to strongly attach to the rock surface (Crisp, 1965) . On the other hand, on the sheltered transect, there is a clear growth pattern. The upper part of the shore is associated with harsh conditions (Menge, 2000) as desiccation, low salinity and access to oxygen (Davis, 1966). In response to these conditions, the shell size varies within the zones (Crisp, 1965). With an increased shell length, more nutrients and water could be retained, allowing the limpet to endure long periods of desiccation (Crisp, 1965).
Growth rate is also a good indicator of the population dynamic. Local variations in recruitment success, growth rate, and mortality produce a diverse pattern of population structures (Thompson, 1980). In this study, on sheltered high shore, there were no juveniles observed. However, this does not necessarily indicate high mortality level in the zone. At higher tidal levels the growth rate of P. vulgata is less variable, as the species is known to withstand and outcompete other species of limpets on high shore (Thompson, 1980; Fletcher, 1984). With increasing exposure to wave action, P. vulgata is almost fully replaced by P. aspera (Fletcher, 1984). On exposed shores, the two species grow at similar rates and P. aspera becomes dominant because it suffers less mortality among juveniles (Cravo et al.,
2002). At low tidal levels, the growth rate of P. aspera does not change with increasing shelter, but P. vulgata grows faster and becomes the dominant limpet species (Thompson, 1980). As identification to species level was not done for this report, this might explain why there is no significant change in juveniles’ number among the shore heights. Also, the study was carried out in mid-October, while P. vulgata reproduction occurs in January (Crisp, 1965). Therefore, migration of juveniles might also be the reason for an absence of juveniles in high sheltered shore. The main difficulty in assessing mortality is that individuals migrate away from the site but do not necessarily die (Thompson, 1980). The month of data collection, plus the migration factor could explain our results.
In conclusion, the difference in exposure modifies the inhabiting species. In order to trace the ecological patterns of a dominant species such as Patella vulgata, a frequent data needs to be collected over a period of time. Species need to be identified, recorded and observed as this type of data will give much clearer results.