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"Comparing the mean ratios of shell height/diameter of the Patella spp on an exposed and sheltered shore"

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Introduction

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. ...read more.

Middle

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. ...read more.

Conclusion

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 10m� 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 ...read more.

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