A comparative study of the density of patella vulgata (common limpet) across a sheltered shore and an exposed shore.

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A comparative study of the density of patella vulgata (common limpet) across a sheltered shore and an exposed shore

Aim: -

        To investigate the limpet density of patella vulgaris (common limpet) across a sheltered rock shore and an exposed rocky shore at the optimum niche level at both shores.

Introduction: -

        Limpets are distinctive animals that are best known for their ability to cling onto rocks. Patella vulgata (common limpet) can be found wherever there is a rock layer firm enough for it to attach to the rocks or stones, this can be from the high shore down to the lowest part of the tide. It is abundant on all rocky shores of all wave exposure. The limpet is usually not abundant on shores where there are large growths of seaweed.

        The conical shell of Patella vulgata is up to 6 cm long with radiating ridges and the top central or slightly forward. Individuals from the high shore generally have a taller shell and smaller shell length when compared to juveniles and low shore animals. The outer surface of the shell is greyish white, sometimes with a yellow tint, and has crude radiating ridges and well-marked growth lines. The inner surface is smooth and greenish-grey in colour. The sole of the foot is yellowish, dull orange or brown with a grey or greenish tinge. The mantle skirt is fringed with transparent tentacles arranged in three series of different lengths, internal to which lays a complete circlet of gills.        

Patella vulgata on rock.

A mixture of biotic and abiotic conditions will determine the distribution of patella vulgata across an exposed shore and a sheltered shore. Biotic factors involve interactions between different organisms. Abiotic factors are the non-living factors that will affect the limpet density across two shores.

Desiccation: -                

The species is naturally inter-tidal and in ideal conditions maybe found up to the high tide level and is therefore fairly tolerant to desiccation. The common limpet creates a home scar on the rock to which it attaches itself to, this allows it to clamp tightly to the rock, which reduces water loss during periods of emersion. The species is tolerant to long periods of exposure to air and can survive up to 65% water loss. Limpets that live lower down the shore have lower tolerance to desiccation, also smaller limpets are more exposed to desiccation than larger ones. Limpets are mobile species so patella vulgata has the ability to determine its position on the shore relative to its preferred zone, it can adjust the direction it wants to go and can move into more suitable conditions.

Wave action: -

Patella vulgata is found on rocky shores from the most sheltered parts of the shore to the most exposed. Under conditions of very high exposure the common limpet can be limited to the upper region of the shore. Wave action causes the shell muscles to contract strongly, clamping the limpet to the rock. The force to pull the limpet of the rock is quite high, so this force along with the conical shell offers little resistance to waves. This secures the limpet against even the strongest wave actions on the most exposed shores. However a decrease in the wave action could reduce the abundance of patella vulgata because it does not favour thick algal cover that is often present of sheltered shores.

Temperature: -

        A change in temperature does not have an affect on the preferred zone for patella vulgata. It is an inter-tidal species that can tolerate long periods of exposure to air and also variations to temperature. Experiments show that limpets can survive temperatures up to 42ºC and 60% water loss, and temperatures in the UK normally do not reach this level. Also limpets are unaffected by short periods of extreme cold temperatures.

Nutrient levels: -

Higher  nutrient levels increase the growth of algae, if there is an increase in algae then there is more food available for the patella vulgata. So limpet density would obviously be more abundant in the regions of the shore where nutrient levels are high.

To get a very accurate count for the population of limpets across the shores we would have to count all the number of limpets on the shore. This is not a realistic thing to do, so we use samples. The best method of sampling to use will be random sampling. This way the results are not biased and I can get quite an accurate number of the density of limpets on the shores. I will not choose a set amount of readings because if the sample numbers I take are too small, then I might not have enough information to draw up a conclusion. If I take too many samples, it could be too hard too study and analyse all the data in the given time. I will have to strike a balance, and will take as many samples as I can within one hour on both shores.    

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Preliminary experiment: -

Before I decided to do my investigation I had carried out a preliminary experiment of limpet density at two different heights across one shore. One height was 4 meters from chart datum and the other height was 6 meters from chart datum. There were transect lines placed across the shore at both these heights. I then picked random coordinates and placed a 50x50 cm² quadrat at these coordinates and counted the amount of limpets within the given area at each coordinate. The main point of this preliminary experiment was to see if there is a ...

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**** This is a detailed account of preliminary and main investigations. Data is clearly presented and analysed. The discussion of the effects of biotic and abiotic factors might have been developed further.