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An Investigation into the height to length ratio of Limpets on exposed and sheltered shores.

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                                                                             Zameer Mohamed candidate no: 0471

An Investigation into the height to length ratio of Limpets on exposed and sheltered shores.

Aim: To investigate height to length ratio of Limpets on the middle shore, comparing the difference between an exposed and sheltered shore.

Site:(Sheltered shore) Angle Point (SM 875033)

  • Ballentine’s Exposure Scale  grade 7.
  • Faces North-East.

          (Exposed shore) West Angle Bay (SM 852032)

  • BES: Grade 3
  • Faces South, exposed to the Atlantic Ocean.

Hypothesis: There will be a statistically significant difference in the mean height to length ratio of Limpets between the sheltered and exposed shores.

Independent Variable: The fetch over which the prevailing wind can blow over, which creates a difference in wave action. In each case an exposed or sheltered shore are chosen.

The Ballantine’s Scale was used to show the level of exposure of both shores. This scale allows there to be a method for deciding whether a shore is exposed or sheltered.

The scale ranges from 1 to 8. The higher the grade, the more sheltered the shore is.

Dependent Variable: The height to length ratio of common limpets (Patella vulgate) on the middle shore. The middle shore has been chosen as a result of a preliminary study, which indicated limpets are mostly located in this area.


  • Height: As the height above Chart Datum changes the size of the limpets at these particular heights will also change. The middle shore, which is about 3-5m above Chat Datum, will always be used to carry out the investigation, on both sheltered and exposed shores.
  • Species of limpet: There are many types of limpet species, so it is important to measure the sizes of the same species (as a different species may vary in size). The common limpet, Patella vulgata, will be used in the investigation.  
  • The sampling technique will remain constant. This important as a different means of measuring limpet size may bring in error, causing the data to be unreliable.
  • Layout of shore and rock type: The size of limpets is affected by variations in rock type and layout, therefore the same size of the rock and rock type will be used.
  • Age of the limpets: The age of limpets is hard to measure quantitatively. However it is possible to roughly assume that younger limpets will have a smaller size compared to older limpets.

To measure the size of the limpets, the base diameter and the vertical height of each was measured. With these values the height: base ratio can be calculated. These rations allow the age factor to be accounted for, effectively as it takes into account the age difference. Overleaf is a diagram showing how the limpet size was investigated:

  • Time: The time of day and the month are also factors that have to be kept constant. As the experiment is performed on the same day the “year and month” are kept constant. However, it is not possible to carry out both experiments on each shore on the same day.  
  • Abiotic factors like; temperature, pH, salinity and wind speed will be measured. If, for example, the salinity of the water has a high concentration of salt, the limpets may have a problem with the regulation of water content, causing osmotic rupture and water leaving the limpet. Although these cannot be controlled, if there is a change, it will be taken into account.
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st & Thursday 22nd October 2003 the investigation will be carried out.Low tide on each of the days is different, so the start times at each shore will be different. Low tide on Wednesday is 1.9m at 10.43am and on Thursday it is 1.4m at 11.29am. This means the investigations will commence at this time on each day respectively. On arrival, each of the abiotic factors will be measured and noted. Temperature will be measured in ºC, using a thermometer.Wind Speed will be measured using an anemometer in ms-1.The method being used to sample the limpets is a continuous belt transaction.The sampling will only be carried out on the middle shore. A cross staff will be used to measure, accurately, the site at which the investigation will be carried out. (see diagram below)
  • Looking through the cross staff there is a marker with an air bubble. When this bubble is between the lines the staff is level.
  • A partner is used to help. They are instructed to walk to the point which lines up with the staff.
  • This method is repeated until a suitable height, between 2.5m and 5.2m is reached.
  • This height will be kept constant on both sites.
  • The ¼ m2 quadrat is placed on the front of the rock and all the limpets will be measured in heights and lengths using Vernier Callipers, which are accurate to the nearest 0.1mm.
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When limpets are forced to compete for their space they do have a powerful defence mechanism. Submerged, it lifts itself off the rock and slams its foot back down onto the rock.    

Preliminary Investigation

Prior to the investigation a preliminary study was carried out on both an exposed and sheltered shore.    

Aim: to investigate the distribution of limpets on at different heights on an exposed and sheltered shore.

Site: Manorbier (exposed): SS 055976              

        Sawden Point (sheltered): SM 888033

Hypothesis: There will be a change in the abundance of limpets as we move from low tide up the shore.

Method: Interrupted belt transect. A tape measure was laid from the top of the shore to the bottom. Using the cross staff technique (see above), results were taken every 0.6m. The number of limpets was counted in each 1/4m2 quadrat. The abundance of the limpets was then estimated using the Crapp Abundance scale. This value was noted at each sample. The results are represented on a kite diagram (see overleaf).

Conclusion: From the results obtained, it is apparent that the most densely populated area of the shore for limpet habitation is middle shore. This is diagrammatically represented in the kite diagram. This piece of data is important, as it forms the basis, backed by the scientific knowledge, for choosing the middle shore as the point most suitable for further study.    

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