An Investigation Into the Effect of Wave Exposure on the Volume of Limpets

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The aim of this investigation was to explore the effect of wave exposure on the volumes of intertidal limpets. Samples of the Patella spp. were measured at 2 different sites at West Angle Bay. A 0. 25m2 quadrat was used to measure along a continuous horizontal belt transect at a fixed vertical height and the base diameter and vertical height of each limpet was measured using callipers. These measurements were then used to calculate the volumes of each limpet. The results obtained showed a measurable difference in the volumes obtained at the exposed shore site compared with the sheltered shore site.

The results showed that there was a higher frequency of smaller limpets and a lower frequency of larger limpets at the wave-exposed site than at the sheltered site. Therefore wave exposure has a profound effect on the size distribution of this intertidal limpet species. The reasons for this are that the effects of wave action are more at the exposed shore site. As a result of the force of the water’s acceleration increasing at a faster rate than the organisms ability to hold on as an organism grows, wave exposure prevents the distribution of larger limpets.

When limpets are mobile their adhesive tenacity is much less than when they are stationary. This suggests that due to their reduced tenacity with movement, the Patella spp. may be limited in size by waves at an exposed shore. Introduction: Limpets are small, herbivorous gastropods that are abundant in many intertidal environments, especially temperate rocky coasts (Branch 1986). Limpet shells are conical shaped and are strong to resist attack from waves and predators. Each limpet has a foot that adheres powerfully to the rock surface using suction and adhesion.

The majority of limpets clamp themselves tightly against the substratum during low tide, and become mobile again once they are submerged by the flowing tide. Limpets are known to be voracious feeders and feed on a wide range of micoralgae such as diatoms, macroalgal sporelings and a variety of other items (Hill and Hawkins, 1991). As limpets tend to feed on the young plants they often prevent seaweed from growing. Limpets feed by the use of a radular, which is a row of hard teeth used to scrape the algae off the rock surface.

These teeth are continually replaced throughout the life of a limpet and are so strong that they ware away the surface of the rock leaving a visible trail. The most common limpet found in northwest Europe is the Patella spp. Patella vulgata is found wherever there is substratum firm enough for its attachment on rocks, stones and in rock pools. This species can be found on a rocky shore from the high shore to the sublittoral fringe. It is abundant on all rocky shores of all degrees of wave exposure. The abundance of limpets on wave-swept shores makes it ideal for examining the effects of wave exposure on them.

Project Hypotheses: Alternative Hypothesis: There will be a statistically significant change in the volumes of limpets found on an exposed shore compared with a sheltered shore. Null Hypothesis: There will be no statistically significant change in the volumes of limpets found on an exposed shore compared with a sheltered shore. Method: The location of this investigation was at a wave exposed rocky shore and a wave sheltered rocky shore at West Angle Bay SM 852 032 on the Atlantic coast of Pembrokeshire in Wales.

The exposed shore has steep, sloping rock faces and facing south, the shore has a large fetch. The sheltered shore faces north and the geology of both sites is Carboniferous limestone. In order to carry out this investigation it was necessary to choose a suitable location on both shores. The middle shore was chosen on both shores because preliminary work carried out on rocky shores showed that a greater number of limpets were found on the middle shore and therefore enough data could be collected to provide as accurate and reliable results as possible.

The middle shore was chosen on both shores to make sure the investigation was as fair as possible and so the middle shore was not compared with the upper or lower shore. After choosing the location of the investigation, a 0. 25m2 quadrat was placed on a rock stratum on the middle shore. Each limpet inside the quadrat was measured using callipers. Callipers were chosen to measure the limpets instead of a ruler because they are capable of measuring lengths to the nearest tenth of a millimetre, which is far more accurate and therefore reliable.

The diameter of the base of each limpet was measured along with the maximum vertical height. These two measurements were needed to calculate the volume of each limpet using the formula ‘r2h’ which is the formula to work out the volume of a cone. After the volume of each limpet in the quadrat was worked out, the quadrat was flipped horizontally in order that the investigation took place along a continuous horizontal belt transect at a fixed vertical height. The quadrat also ran along the same rock stratum.

This was done because different species of limpet could be found on different types of rock and these species could be bigger or smaller than the Patella spp. This would affect any trends and patterns found in the volumes of the limpets. The volume of each limpet was worked out in this quadrat and the quadrat was continuously flipped until a running mean was established. The running mean was calculated after every volume was worked out until half of the results obtained were 2. 5% either side of the mean. The same apparatus and the same method were used on both shores.

It was important to use the same quadrat size on both shores because a bigger quadrat would mean the horizontal belt transect would be larger and therefore a larger section of the shore would be investigated on one shore compared with the other. This could provide a greater variation in the limpets and also different species may be included in the results. This investigation was carried out on two consecutive days at the same time in the day, with similar weather conditions observed on both days. This was important as limpets feed at different times, with some feeding out of the water and some in the water.

The Patella spp. is especially variable in its behaviour. If the investigation was carried out at different times or even different seasons then the limpets could have moved far away from where they originally were and therefore the results would be unreliable. A key was also used in this investigation to ensure that the correct specie of limpet was measured and that it was in fact a limpet that was being measured and not a periwinkle or topshell. The method used here had no further modifications than that stated in the plan for this investigation.

The graph on the following page shows the running means of both sets of data in order to establish when enough data had been collected. The 2. 5% boundary lines show that half the data collected at each site was within 5% of the mean. A similar number of samples needed to be taken at both sites in order for this to happen. The graph clearly shows a difference in the volumes of limpets found at the exposed site compared with the sheltered site.

The mean volume of a limpet found on the sheltered shore is just over 3 times greater than the mean volume found at the exposed site. The standard deviations of both sites show that the volumes found at the exposed site are more closely clustered around the mean. This means that there is a smaller range of volumes and less variance at the exposed site. The tables in appendices 1 and 2 show that there are some anomalies in the data collected.

The sheltered site shows one extremely large limpet that is 2500mm3 bigger than any other limpet recorded (sample number 47). On the other hand, limpets with a much smaller volume than the mean were found at the exposed site (sample numbers 4, 29). The results obtained here seem to be very conclusive, however a statistical test must be carried out in order to make sure.

Where xa and xb stand for the means of each set of data, sa and sb stand for the standard deviations of each set of data and na and nb stand for the number of measurements of each set of data. Using this formula and the data obtained in the investigation the t value was 14. 848. In order to convert this figure into something meaningful, it is necessary to know the degrees of freedom for this set of data. For the t-test, this is simply two less than the total number of measurements in the two samples.

In this case the degrees of freedom is 125. In order to see if the results obtained are statistically significant, the value of t must be compared with the table of t values (Appendix 3). The p values at the bottom of this table show the probability that chance alone could produce the difference between the two sets of data. The p-value used here is 0. 05. The degrees of freedom for this investigation lies between 120 and, which give the critical values of 1. 80 and 1. 960 respectively. The t value obtained was 14. 8 and is clearly bigger than the critical value for the degrees of freedom. This means that I can be 95% confident that the difference between the means is significant and therefore the results are statistically significant. This strong probability shows that the null hypothesis has been falsified by the data. Therefore the null hypothesis can be rejected and the alternative hypothesis accepted.

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