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

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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).
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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 ...

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