A Comparative Study of the Density of Patella Vulgata (Common Limpets) in the Optimum Niche on an Exposed and Sheltered Rocky Shore.

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A Comparative Study of the Density of Patella Vulgata (Common Limpets) in the Optimum Niche on an Exposed and Sheltered Rocky Shore

Planning: -

Aim: -
To find out if there is a difference in limpet density on shores receiving different wave action.

Introduction: -
        
Patella vulgata (common limpet) is a dome shaped mollusc with no spiralling of the shell. It has a strong muscular foot and secretes a chemical to adhere to the rocks. As it clamps down and settles onto a particular rock, it rotates, and upon death, leaves a scar on the rock. This is called the home scar rock, to which the limpet must return after feeding or it will die. This is because the limpet will only form a vacuum against that particular rock.

Figure 1. Patella Vulgata situated on rocks                    Figure 2. A limpet’s foot and mantle

        They feed mainly at high tide and at night. Sometimes they can be seen grazing during the daytime at low tide, especially on exposed shores and in humid conditions. Limpets feed at night on microscopic algae, making them very important seashore herbivores. They feed by scraping their tongue (radula) across the algae covered rocks.

Figure 3. Scanning Electron Micrograph of part of a limpet radula showing rows of teeth, some of which are hardened with iron oxide. 100 µm.

When the tide is high, the limpets move about grazing on algae. As the tide drops, they usually return to their resting place. This homing behaviour is evident by looking at distinctive scars. Some limpets have been able to find their "home" when relocated as far as a metre away. They use chemoreceptors to find their way back as they don’t have any eyes. The algae prevent themselves from being eaten by limpets by recolonisation. They start to grow on the backs of limpets, and other limpets do not feed off the back of limpets. Black lichen (Verrucaria mucosa) also grows on the back of the limpet’s shell.

Figure 4. Boulder showing limpet radula marks.

        Clamping down at low tide prevents desiccation (drying out). Limpets clamp down onto rocks to reduce their metabolism, which helps reduce the need for water when the tide goes out. They can adhere to the rock with a force of 75 lbs/sq.in. making them very difficult to move. After four years of growth, limpets can be 5cm across at the base. The shell is waterproof which is extremely helpful with conserving water. The amount of wave action affects the shell shape. In sheltered areas, they tend to be more flattened, and in exposed areas which have more wave action, they tend to be dome shaped. This is because the waves cause more muscles and a stronger shell to develop. This means the limpet clamps down onto the rock with more force preventing any growth on the outer rim making it dome shaped. The shell protects the limpet from predators or damage from moving rocks or sand.  

        Limpets breathe by removing oxygen from the water, and as most of the oxygen will be near the top 10cm of the water, the dome shape of the limpets helps with breathing. Oxygen is drawn into the gills via a hole above their heads.

     

Figure 5. Diagram of body parts of Patella Vulgata       Figure 6. Diagram of Acmaeid Limpet body parts.

They are protrandrous hermaphrodites that shed gametes in to the water where there is external fertilisation. Limpet eggs hatch into a planktonic larvae which live in the plankton for two weeks before settling out onto the shore. This happens during the autumn and winter months. They often begin life as males, but like many molluscs, change into females later on at about two to three years of age. Spawning occurs annually and is induced by rough seas and onshore winds, and the eggs are fertilised externally. The eggs are green in colour due to the presence of the pigment chromoprotein Y. The lifespan of a limpet is about 10-20 years.    

Limpets are very susceptible to oil pollution and their death results in a marked increase in the growth of weed. In the period following a spill of oil there is a "green flush" caused by the unlimited growth of green algae such as Enteromorpha and Ulva.

It is a very abundant mollusc across the entire tidal region of rocky seashore, not being limited to any one zone. This is because it can avoid as well as tolerate most of the problems of the seashore.

Figure 7. Limpets are most abundant on middle shore

        Limpets are most abundant on rocky shores with high wave exposure and low seaweed growth. They are important for shaping the habitat structure as they keep the number of algae down. Their only commercial uses are culinary and for research purposes. Their only real predators are sea stars, birds, humans and the common shore crab, which uses its pincers to prise the limpet off its rock.

        There are two types of rocky shores: sheltered and exposed. Exposed shores have lots of wave action and sheltered shores have little. Sheltered rocky shores face away from the prevailing wind and open sea, which is why they have smaller waves. Exposed shores face out onto the open sea and prevailing winds, so the waves are larger. Sheltered rocky shores are north facing shores so they get less sunlight, therefore the organisms inhabiting these shores are less susceptible to desiccation. Exposed rocky shores on the other hand are south facing beaches which get more sunlight and it is an inhospitable place for inter tidal organisms as they are susceptible to desiccation.

The rocky shores can be divided into four different zones: splash zone, upper shore, middle shore and lower shore. At the top (splash zone) there are no marine species as there is hardly any water, which is essential for the survival of marine species. There is a sparse diversity of life near the top of the rocky shores. However, the biomass increases as you go down the shore as there is richer, thicker seaweed and more crustacean life due to less harsh abiotic factors.

The splash zone is at the top where the water never reaches it and it is only splashed by spray. So any organisms existing here are never covered by the tide, so desiccation is a problem. There is no soil so no plants can grow in the splash zone. The high levels of salt causes dehydration as the water is drawn out of the organism via osmosis. It is extremely dry in this zone so there is a lot of salt due to water evaporation and the crusting of the rocks. The harshest abiotic conditions exist in the splash zone and only lichens survive. No macro-algae exist in this zone (therefore no limpets) as there is no water to carry out their life processes, such as, metabolism, reproduction, growth and photosynthesis. Only green algae (Gutweed) exists here as it has no real zonation because it is not specialised as it has a high tolerance for differences in salinity, extremes of temperature and can also survive freshwater conditions, so there is equal abundance of Gutweed at the top and bottom of the rocky shores. The advantages to any organisms present here, is that there is less competition and they have their very own niche.
        The top of the upper shore is covered by the sea for less than 1% of the year, and the bottom of this zone is covered for 20% of the year. Desiccation is also a problem here for organisms due to the minimal amount of water. The temperatures also vary in this zone as the sea has a variable heat capacity. There is a low absorption of minerals in this zone as most of the organisms are primitive (brown, green and red algae) and absorb nutrients directly across their surface as they have no xylem or phloem. All life processes are carried out underwater. Any animals present in this zone have a shell to help avoid water loss e.g. limpets. Limpets are not very abundant in this zone as there is poor seaweed growth.

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        The top of the middle shore gets covered by seawater for 20% of the year, and the bottom of this zone gets covered for 80% of the year. There is some desiccation and temperatures are variable, but not as much as the upper shore. There are also lower light levels than the upper shore. Limpets are quite abundant in this zone as there is a lot of seaweed (as seaweeds move towards the light) and they have shells to help avoid desiccation. This zone is the best for inter tidal organisms in terms of its abiotic conditions, and it is ...

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