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Study of the height/diameter ratio of limpets inhabiting the middle shore region of exposed and sheltered shores

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Study of the height/diameter ratio of limpets inhabiting the middle shore region of exposed and sheltered shores


In this study it will be possible to assess the effect of shore type on the height/diameter ratio of limpets.  It is plausible that the conditions (exposure to wind, tide, predators etc.)  that limpets have to confront at the different shore levels will be linked to the way their sizes occur.  A higher ratio would tell us that the limpet is comparatively taller than wider as opposed to limpets with a lower ratio.  The study was performed at Port Eynon Bay, Kilvrough in South Wales (grid reference on map 47, 85).  Here it was possible to do measurements on limpets at the middle intertidal level of sheltered, semi-exposed, and exposed shores [1](See picture 4 for reference on how to locate the middle shore region.).

  • In the case of this study one could expect the mean ratio of the limpets on exposed shores to be significantly less than the ratio of the ones inhabiting the sheltered shore.    

Limpets can be found attached to rocky surfaces where they make a home on what is known as a rock scar[2].

They are molluscs (Class: Gastropoda, Family: Patelidae) that can be found on most seashores.  They feed on microscopic algae and have a conical shell (see picture 1) in order to resist wave attack and predators.  When the tide rises and covers these molluscs, they move around and feed on algae before returning to their rock scar.

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  • Group members must be well dressed for the conditions of the shore; cold weather, rain, winds etc.  Wellington boots are highly recommended for protection from water and the sharp rocks.

The following is what each person of the group should do at their corresponding shores.

  1. Once the middle shore region is reached:

With the interval marked string, set up a study area of 10 X 10 meters in the same way as one would draw co-ordinate axes for graphs (perpendicular lines). This is important in order to follow an orderly method of work, and to apply the use of the random co-ordinates.  

  1. Use the co-ordinates produced before to identify the area where the quadrat will be placed (using the markings on the string as values for the co-ordinates) Place one of the corners of the quadrat on this point. (if there is a large rock pool where the co-ordinates indicate, produce new random co-ordinates until an adequate area is found)
  2. Identify the limpets that are closest to the four corners of the quadrat and the one closest to the middle (see picture 3).  Use the vernier callipers to measure their height and diameter.* Remember to be accurate to 0.05 millimetres (Be careful with the metal rod sticking out of the calliper!)
  3. Now take the second set of coordinates and repeat step 5.  Do this for all sets of coordinates.
  4.  Reunite all the data with group members in order to perform a statistical test.

*If time allows measure the orientation of each limpet with the compass.

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Apart from the latter, the results seem to make sense.  The samples don’t produce perfect normal distributions or patterns, but this is just due to the fact that we are not counting the whole population (the trend observed in the whole population would be clearer).  A larger sample will always be an improvement someone can make if they have the time.

All these factors combined can push the results towards inaccuracy.  The confidence limits might change and the difference in means might prove to lose statistical significance. The confidence limits obtained at the exposed shore are well separated from the ones obtained at the other two shores.  I think there is enough space between the two means to be confident that the trends obtained are not down to chance; they do have a statistical and scientific importance.

Picture 4


If we divide the length of the shore in 12 parts, the middle shore region will be found in the shaded area; the area bounded by 5/12 down the shore and 8/12 down the shore.  Limpet measuring should start at mean tide level time, when the water is exactly 6/12 down the shore.


Cremona, Julian. A field Atlas of the Shore

Little, Colin and Kitching. J.A The Biology of Rocky Shores

Hawkins, Stephen J. The Rocky Shores

[1] Sheltered, Semi-Exposed, and Exposed are measures of the exposure of the shore facing the tide

[2] A flat area of rock shaped by the years of presence of the limpet

[3] Mean tide level: Time of day when the tide is covering half the tidal range.  The time at which this happens will vary depending on the day.  In our study mean tide level was at 12:03

...read more.

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