"Comparing the mean ratios of shell height/diameter of the Patella spp on an exposed and sheltered shore"
A2 Biology Investigation
"Comparing the mean ratios of shell height/diameter of the Patella spp on an exposed and sheltered shore"
Rebekah Riley
Candidate number: 5037
Centre number: 28384
Background Information
Limpets are slow moving molluscs, characteristic of rocky shores and are very well adapted to life on the seashore. They have a hard shell to protect themselves against predators and damage from moving rocks, and a large muscular foot that enables them to clamp firmly onto rocks to conserve water and maintain their position during rough weather. The most common limpet is Patella vulgata. The conical shell of Patella vulgata can be up to 6 cm long with radiating ridges and the apex central or slightly anterior. Orton (1928) has shown that individuals that inhabit the upper shore generally have a taller shell and smaller shell length when compared to juveniles and lower shore organisms. The thickness of shells is believed to be concerned with heat insulation and water conservation.
Patella vulgata is found wherever there is a substratum firm enough for its attachment e.g. on rocks, stones and in rock pools, from the upper shore to the sublittoral fringe. It is abundant on all rocky shores of all degrees of wave exposure although the highest densities of Patella vulgata coincide with wave-exposed conditions (Fretter and Graham 1994). The species is not usually abundant on shores with a dense growth of seaweed or on some sheltered shores where silt and algal turfs are dense as it cannot compete for space. Loss of the substratum would result in loss of the population. Unattached individuals are very vulnerable to desiccation and to predation by birds and crabs.
Under conditions of very high wave exposure Patella vulgata may be limited to the upper region of the shore, its place being taken below mean tide level by Patella aspera (Blackmore, 1969). Wave action causes shell muscles to contract vigorously, clamping the animal to the rock. The full strength of pull of the pedal muscles has been estimated as 3.5kg/cm² (Fischer, 1948). This force, together with the fact that the conical shell offers little resistance to waves, secures the animal against the action of the waves in the most exposed situations. A decrease in wave exposure may reduce the Patella vulgata abundance because the species does not favour thick algal cover that is often present on very sheltered shores. In summation, as wave exposure decreases so does the Patella spp diversity.
Patella vulgata is typically intertidal and in ideal conditions may be found up to the high tide level and is therefore, relatively tolerant of desiccation. During exposure to the air, feeding and locomotion are halted unless conditions are very damp. The species creates a 'home-scar' where it clamps down tightly to the rock to reduce water loss during periods of emersion. The species is tolerant of long periods of up to several hours of exposure to the air and can survive up to a 65% water loss (Davies, 1969) although tolerance to desiccation in lower shore organisms is poorer. This is because lower shore organisms tend to be smaller limpets and are more vulnerable because of their high surface area to volume ratio. Shell morphology is also important. Higher shore individuals have taller shells which reduces the circumference to body size ratio and hence water loss from the shell margin. They are also inclined to have thicker shells which is beneficial when exposed to the air as desiccation is reduced and they are better insulated than low level limpets. As a mobile species Patella vulgata has the ability to determine its position on the shore relative to the preferred zone and can orient itself to its desired direction and move towards more favourable conditions.
As Patella vulgata inhabits a wide range of tidal conditions they are likely to be able to tolerate a change in water flow rate. The streamlined profile of limpet shells is of importance in increasing their tolerance of water movement, and this is undoubtedly one factor in determining the different shape of limpets at different exposures. With increasing exposure to wave action the shell develops into a low profile reducing the risk of being swept away. The strong muscular foot and a thin film of mucus between the foot and the rock enable Patella vulgata to grip very strongly to the substratum (Fretter & Graham, 1994).
Adult species have tough shells that offer protection from abrading factors and any near vibration causes the shell muscles to contract vigorously, clamping the organism to the substratum. A short, sharp knock may dislodge an organism leaving it vulnerable to predation and individuals walking on the shore may crush small limpets. Small limpets however have an affinity to occupy depressions, crevices or pools that would provide protection from such devastation.
Patella vulgata is a resilient species and can tolerate long periods of emersion and consequently wide variations in temperature. Stronger, adult species are not affected by temperature changes as Fretter & Graham (1994) showed that adults could survive temperatures of up to 42 °C and a 60% water loss. Temperatures in the British Isles do not generally reach this level and so this factor does not affect them. Adults are also largely unaffected by short periods of extreme cold. Ekaratne & Crisp (1984) found that adult limpets continued to grow over winter when temperatures had fallen to -6°C, and stopped only by more severe weather. However, loss of adhesion after exposure to -13°C has been observed with limpets falling off rocks and therefore becoming easy prey to crabs or birds (Fretter & Graham, 1994).
Title
Comparing the mean ratios of shell height/length of the Patella spp on an exposed shore and on a sheltered shore.
Hypothesis
The mean ratio of height/length of limpets will be higher on the more sheltered beach than on the more exposed beach.
Statistical Hypothesis
. Experimental: There will be a significant difference between the mean ratios of the limpets on the sheltered shore and on the exposed shore.
2. Null: There will be no significant difference between the mean ratios of the limpets on the sheltered shore and on the ...
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Title
Comparing the mean ratios of shell height/length of the Patella spp on an exposed shore and on a sheltered shore.
Hypothesis
The mean ratio of height/length of limpets will be higher on the more sheltered beach than on the more exposed beach.
Statistical Hypothesis
. Experimental: There will be a significant difference between the mean ratios of the limpets on the sheltered shore and on the exposed shore.
2. Null: There will be no significant difference between the mean ratios of the limpets on the sheltered shore and on the exposed shore. Any difference will be due to chance.
Introduction
The higher the ratio between height and length of the shells of the Patella spp, the more pointed i.e. taller and thinner the shell. For example, if a shell were 4cm high and 2cm long, the ratio would be 4/2 = 2. If a shell were 2cm high but 4cm long, the ratio would be 2/4 = 0.5. I predict that the limpets will have lower, wider shells on the more exposed beach than on the more sheltered, and the shells on the sheltered beach will be taller and thinner. This means that the mean height/diameter ratio of the shells on the exposed shore should be lower than on the sheltered. This is because the two shores have varying degrees of wave action, which is an important factor in determining the shape of limpets. With increasing exposure to wave action the shell develops into a low profile reducing the risk of becoming unattached to the substratum. The streamlined profile of limpets shells is therefore of importance in increasing their tolerance of water movement. Wave action causes shell muscles to contract vigorously, clamping the organism to the rock. This force, together with the fact that the conical shell offers little resistance to waves, secures the limpets in the most exposed situations. However, their habit of clamping firmly down on the rocks when exposed to the air has an indirect effect on growth of the shell. Also, limpets living high up the shore are exposed to the air for most of the time so that their shells have a tendency to be secreted when they are in a contracted state. As a result, they are more likely to develop relatively tall and thin. On the other hand, the shells of limpets from near the sublittoral zone tend to be secreted when the organisms are relaxed due to being covered with water for most of the time, and so their shells develop a flatter shape (Heinemann investigations in biology). The experiment will be conducted on Castle Rock and on Constitution Rock in Aberystwyth, Wales. These sites were chosen for several reasons. There is no management or human intervention present on either shore making the environment a completely natural one. There are also no problems with altitude or longitude with this particular site which makes it easily accessible. The shores contain a rich diverse environment with which there is a wide distribution of organisms and visible distribution patterns. The size and mobility of the organisms also means that they can be easily sampled. To determine the degree of exposure a ballentine was constructed. See appendix 1.
Apparatus
A quadrat will provide the area for sampling and a pallet knife will be used to detach limpets from the substrate. The height and diameter of the limpet shell will be measured to the nearest mm using a pair of callipers. A datasheet will be used to record the measurements onto held flat on a clipboard. A polythene bag will be taken to protect the datasheets during periods of precipitation, and a bucket will be taken to carry all the equipment in.
Set variables
My set variables are the two shores of varying exposure, one more sheltered and one more exposed that are in close proximity of each other. The reason for this was to control as many variables as possible.
Measured variables
My measured variables are the height and length of ten randomly sampled Patella spp on the middle shore of the beaches. I chose the middle shore because it is situated between the extremes of the upper shore and the extremes of the lower, and should provide me with a median of them both.
Controlled variables
There are several variables in the experiment that need to be controlled in order for any variation in the results to be due to the different shores and not any other factors. Some of the factors will be unable to be controlled as they are on the rocky shore and cannot be changed. Some abiotic variables are already controlled such as the substrate and topography of the area. One that is, is the geology of the shores. All limpets either live on hard or soft rock and both shores consist of hard rock. All the limpets sampled will be taken from bare rock and not from rock pools. Limpets found in rock pools could have a different shell structure to limpets on the bare rock and could affect the results. The topography of the shores is roughly similar and so is not likely to significant difference. With the two shores situated in close proximity, variables such as the salinity level, temperature, wind direction, pH, light intensity and humidity are all within a constant range of each other and should not significantly affect the results either. As the experiments will be carried out one after another the time of day is controlled as much as possible. A factor that cannot be controlled is oxygen availability. This could be different on each shore due to water movement but it would only affect the abundance of limpets and so it does not affect the experiment, as the same number of limpets will be taken from each shore. Another factor that cannot be controlled is the age of the limpets sampled. Young limpets have flatter shells than adults but the experiment is comparing the ratio of height/diameter and so the age of the limpet should not matter. The experiment could take place on one beach as one side of the shore is less exposed than the other but due to a pier situated adjacent to the more exposed shore it would invalidate the results. It also makes it more difficult to control abiotic factors such as light intensity and temperature; therefore the experiment will be conducted on two different beaches.
Risk assessment
One of the first things that has to be ensured is that suitable clothing and footwear are worn on the rocky shores to avoid any possible injury, as it is a very uneven surface and could be dangerous if not careful. In the case of an accident a first-aid kit must be carried by the leader along with a rescue rope. If any rocks are removed for examination, they must be replaced carefully in the correct position to minimise damage to fragile ecosystems. Care will need to be taken when working on uneven terrain and the slippery surfaces produced by macro algae and diatom films.
Pilot study
Method
Determine where the upper shore and lower shore lie by identifying the type of seaweed present (see appendix 2 kite diagram). Walk 20 paces down the middle of the upper and lower shore and place the quadrat directly in front of where standing. Sample ten Patella spp from each of the shores, making sure that all limpets are returned to their home-scar to minimise damage, and record the height and diameter of the limpet shells onto a datasheet. Calculate the ratios of the limpets by dividing the height by the diameter and calculate the mean ratio of the ten samples from both the upper and the lower shore. Place the mean ratio from each shore into the t-test to test the significance of the results. If there is a significance difference between the two mean ratios then on the different beaches, then the investigation will be carried out by sampling from the upper and lower shore, excluding the middle, on both beaches. If there is not a significant difference between the means then the investigation will be carried out on the middle shore alone of both the sheltered and exposed beach.
Results
Shore
Mean ratio of height/diameter (cm)
Standard deviation
T-test result
Upper
0.53
0.07
5.65
Lower
0.34
0.08
A graph was drawn to show the difference in mean ratio between the upper and lower shore.
(See appendix 3 for raw data for calculations of ratios and appendix 4 for t-test calculations).
For the t-test the degrees of freedom must be calculated to determine the critical value at 5% error level. This is done by the equation (N-1) + (N-1) where N=sample size. For the pilot study the degrees of freedom = 18. The critical value for 18 degrees of freedom at 5% error level = 2.10 as shown in the table in appendix 5. As the value obtained from the t-test is higher than the critical value I can accept the experimental hypothesis and reject the null so there is a significant difference between the height/diameter ratios of the limpets on the upper and lower shore.
Modifications to plan/method
When sampling on the two shores it was found that some of the limpets were too difficult to detach, due to either the limpets clamping down too strongly on the rock to be able to 'pull' off or impracticalities such as being situated in inaccessible areas underneath rocks or on rock unable to reach. When obtaining ten samples from each area of the shores in some of the quadrats there were less than ten Patella spp present, so another sample was taken two paces away. If this occurs in the main experiment then another sample will need to be taken in order to acquire the full amount of data. Due to the significant difference between the upper and lower shore, the experiment will be conducted on the upper and lower shores of the two beaches and the significance will be tested between both shores on the exposed beach and both shores on the sheltered beach. Collecting ten samples took approximately twenty minutes and so forty samples will be taken from each shore, twenty on the upper and twenty on the lower, which should take approximately two hours and forty minutes, leaving enough time to get from one shore to the other. This allows me to complete the implementing within the time limit.
I now predict that not only will the limpet shells on the more exposed shore have a flatter, wider appearance than the limpet shells on the more sheltered shore, but also there will be significant differences between the mean ratios on each of the areas of the shores. I anticipate there will be a significant difference in the ratios on the upper and lower shore of the sheltered beach and in the ratios on the upper and lower shore of the exposed beach. I also believe there will be a significant difference between the total mean ratios for the sheltered shore and the total mean ratios for the exposed shore. I do, however, predict that there will be no significant difference between the ratios on both upper shores or the ratios on both of the lower shores. This is because the limpets on the sheltered upper shore are the least exposed of all the samples taken and therefore should have the most pointed shells. The flattest shells should be of those on the exposed lower shore because they are situated in the most extreme area and with most exposure to wave action.
Method
The upper shore and lower shore were determined by noting the species of seaweed present (see kite diagram) as particular species are more commonly found on certain areas of the shore. Once established the experimenter walked ten equal paces down the middle of the upper shore and sampled five Patella spp. directly in front of them using a 50cm x 50cm quadrat, if it was safe and practical to do so. The forty samples from each beach covers 10m² which will be taken as a representation of the whole beach. If it wasn't safe or practical then another sample was taken two paces in front. If five species were not present or five samples were unable to be taken then another sample was taken at a 90° angle to the right and two paces away until five sets of data was recorded. The pallet knife was used to get underneath the shell and detach the limpet from the substratum. The callipers were used to measure the height and diameter of the shell to the nearest mm and the information was recorded on the data sheet. The limpet was afterwards restored to its home scar to minimise any damage caused. The experimenter then returned to the original position and walked another ten paces and sampled again. This was done four times to collect twenty samples. The same method was also used on the lower shore. Immediately after the samples were obtained from the sheltered beach the experimenter went to the more exposed beach and carried out the same steps on the upper and lower shore. This was done to try and control as many factors as possible, such as time of day, temperature and light intensity.
Results
Table 1
Shore
Mean ratio of height/diameter (cm)
Sheltered beach
Exposed beach
Upper
0.49
0.46
Lower
0.44
0.37
Both
0.46
0.42
Table 2
Beach
T-test
Degrees of freedom
Critical value
T-test result
Difference
Sheltered
Upper shore vs. lower shore
38
2.02
.26
Not significant
Exposed
Upper shore vs. lower shore
38
2.02
4.03
Significant
Both
Upper shore vs. upper shore
38
2.02
0.93
Not significant
Both
Lower shore vs. lower shore
38
2.02
2.19
Significant
Both
Upper and lower vs. upper and lower
38
2.02
.60
Not significant
See appendix 6 and 7 for raw data and appendix 8 for t-test calculations.
Conclusion
The higher the ratio between height and diameter of the limpet shell the more pointed it is. Table 1 shows the mean ratios of the limpets on each of the shores. The results show that the Patella spp. shells on the upper shore of the sheltered beach are the most pointed and the shells on the lower shore of the exposed beach have the flattest shells. It also shows that the limpet shells on the sheltered beach are taller and thinner than the shells on the exposed shore as the ratio of height/diameter is higher. Table 2 presents the results from the statistical t-tests for each of the shores. It shows that although there is a difference between the mean ratios of the limpet shells on the upper and lower shore of the sheltered beach, and on the upper shores of the sheltered and exposed beaches, the differences weren't big enough to be significant. This means that the null hypothesis is accepted and the experimental is rejected for these results. The difference in mean ratios is due to chance. The results also show that, however, there is a significant difference between the mean ratios on the upper and lower shore of the exposed beach and on the lower shores of the sheltered and exposed beaches. For these results I can accept the experimental hypothesis and reject the null.
Explanation
There are numerous reasons why the experiment produced these results. Firstly, although there was a difference between the mean ratios on the sheltered upper and lower shore the difference wasn't significant. This could be due to the fact that a couple of the samples were taken from the middle shore instead of the lower because the tide was coming in and so it was unsafe to sample down on the lower shore. This could be why the results were not significantly different, as limpets on the middle shore will have a more alike shell shape to the limpets on the upper shore. The results showed a significant difference between the two lower shores which was not as predicted. This could be because of the difference in exposure and large variation in abiotic factors between the two shores. A couple of samples though, on the exposed lower shore were taken off rock sheltered from the sea. Depending upon how long they had been there could result in a difference in shell height and width. This would increase the mean ratio and could explain why although the result was proven significant from the t-test it is only slightly above the critical value. The t-test result was much higher than the other results for the test between the exposed upper shore and lower shore, which suggests there is a relatively large significant difference between the two. This is probably because of the wide variation in abiotic factors from the upper to the lower shore. Conditions such as the salinity of the water, temperature and water availability remain more constant on the lower shore than on the upper shore because the time spent emersed is less. Organisms on the lower shore are therefore subjected to fewer oscillations in conditions as the lower shore is immersed for longer periods of time and has to endure to the action of the waves much more than the upper shore. The t-test result proved there to be no significant difference between the ratios of the limpet shells on the upper shores of both beaches. This was predicted, as although the beaches are of different exposure, the proximity of the shore to the sublittoral zone and the action of the waves is approximately the same so the difference in mean ratios should therefore be minimal which in this case is. When comparing both the upper and lower shore of the sheltered beach with the upper and lower shore of the exposed beach there was no significant difference found. This could be because only forty samples were taken from each beach and only from the upper and lower shore, not from the middle. This resulted in a wide range of ratios leading to greater variation. The more varied the results the lower the t-test result, which makes the difference insignificant.
Evaluation
As can be seen from the graphs the results from the exposed beach were much more consistent than the results from the sheltered beach, seeing as the standard deviations are lower. This means there was a higher degree of variability in the results from the sheltered beach. This could be due to several reasons. Some of the quadrats did not give five samples of the Patella spp. and other samples were taken horizontally across the beach. This meant that samples were taken from the middle and right hand side of the shore and excluded the left side, which could have introduced bias in the results. As the data from the sheltered beach was more varied than the data from the exposed beach, it resulted in overlapping of the standard deviation ranges. This suggests that the results are not very reliable. There were several limitations which could have caused the variation in the results. Firstly, the limpets were positioned all over the substratum which could have resulted in the shell shape on the exposed beach developing similarly to the shell shape on the sheltered beach. This would affect the results by reducing the significance of the difference. Another limitation was the different species of the Patella spp. The most common is Patella vulgata but it was difficult to distinguish between the other species and so all the species sampled were counted. Different species might grow at different rates which would alter the height/diameter ratio, and could ultimately invalidate the experiment. The total area sampled on each of the beaches was 10m² out of a possible AREA which questions the fact whether the samples were good enough representations of the beaches. More samples could have been taken to increase the accuracy of the experiment but it was too time consuming to conduct. When comparing the two beaches there was no significant difference between the two, which was not as predicted. To extend the investigation the sample size could be increased and samples could be taken from all three shores of the beaches and not just the upper and lower shore. Using the mean ratio of height/diameter a comparison could then be made between the two. Another possibility of extending the investigation would be to take a large sample from the sheltered upper shore and a large sample from the exposed lower shore and compare the difference between the two. As the experiment was conducted on two shores of varying exposure, one way of furthering the experiment would be to collect samples from both shores and place them together on one substratum and monitor the growth over a period of time and compare the shell measurements.
Resources
http://www.marlin.ac.uk/species/Patvul.htm
http://www.medinavalleycentre.org.uk/limpet_grazing.htm
http://www.personal.dundee.ac.uk/~amjones/rockzon.htm
http://www.marlin.ac.uk/species/Adult_senexp_Patvul.htm
http://www.mrothery.co.uk/module5/handout%20rocky%20shore%20info.doc
http://www.biologymad.com/Ecology/ecology.htm