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

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 also excellent for herbivorous molluscs, such as limpets due to its abundance of macro-algae (brown algae, red algae, green algae). Brown algae falls into a heap when the tide goes out to limit evaporation to the upper front. Red algae regenerate their pigments rapidly to prevent it from bleaching and unable to perform photosynthesis when exposed to sunlight. The peak abundance of limpets is the middle shore, and then it decreases.

The top of the lower shore gets covered for about 80% of the year, and the bottom of it has 99% cover by seawater. Desiccation is much less of a problem and the temperatures are more stable with lower light levels. This is theoretically the best zone for inter tidal organisms. The macro algae present in this zone have low tolerance levels to desiccation e.g. sugar kelp [Laminaria saccharina] (25% tolerance to desiccation) and Serrated Wrack [Fucus serratus] (40% water loss tolerance). The only problems that need to be overcome to survive on the lower shore are low light levels for plants, high level of competition for resources and predation of marine animals.

The vertical range for the common limpet Patella vulgata is the part of the rocky shore environmental gradient in which it can survive in. common limpets have a broad vertical range as they are not specialised to abiotic conditions and can survive changes in salinity, desiccation and temperature changes. They do not have a very specific niche. Limpets are most abundant in the middle of their vertical range, where conditions are thought to be optimum. Below is the theoretical vertical range of limpets and their abundance within that range.

The abiotic factors which control the upper limit of a limpet’s vertical range are tidal cycle (affects when and how long a limpet is immersed in water so that it can breathe), desiccation, wind (this creates waves, therefore affects wave action and sea spray, affecting the amount of water the limpet is submerged in. it also aids the dispersal of eggs, therefore affecting the number of young born. It circulates oxygen, carbon dioxide and water vapour, affecting photosynthesis and respiration of macro-algae, which limpets feed on.), light (affects how much photosynthesis macro-algae are able to carry out), temperature (affects desiccation rate), oxygen and carbon dioxide concentrations (for the life processes carried out by macro-algae), and salinity (this affects the osmotic potential).

The factors which control the lower limit of a limpet’s vertical range are interspecific and biotic competition for water, food, mates, space and predation by carnivorous organisms (common shore crab). Barnacles may harm limpets they live on by weighing it down, or they could also help it by camouflaging it, so it is difficult to decide whether this is a parasitic of symbiotic relationship.

The tidal levels are used to split the seashore into the four zones. The moon is responsible for creating waves. The gravity from the moon is responsible for the tidal bulges (oceans bulging out at the same time which result in high tides). Due to the earth rotating on its axis, the tidal bulges move back and forth causing high and low tides. There are approximately two high tides and two low tides every 25 hours, and about 6 hours between high and low tide. This is the daily pattern.

All organisms on the rocky shore are either autotrophic (photosynthetic) primary producers or heterotrophic (digest organic molecules into smaller products then absorb them into their own bodies) consumers which rely on the autotrophs for their sustenance. For animals in a marine environment there are many feeding types, including grazers, browsers, suspension feeders, deposit feeders, carnivores and omnivores which form a food web. Limpets are grazers and constitute the second trophic level as they are heterotrophic and rely on the autotrophic algae for their food.

Figure 8. Energy pyramid with four trophic levels in a marine environment

At the First Trophic Level in the sea, algae absorb nutrients through their fronds from the surrounding water and create energy by the fixation of carbon in the process of photosynthesis. Algae do not take in nutrients through their holdfasts, like flowering plants take in nutrients through their root systems.

At the Second Trophic Level are algae grazers and filter feeders. Some animals eat macroalgae fronds, while others eat microalgae, scraped off moist rock surfaces, such as, limpets.

At the Third Tropic Level are many animals that are First-Level Carnivores, capturing and eating other organisms. E.g. predatory molluscs. The Second-Level Carnivores include flesh-eating fish, mollusc-eating birds and humans.

Preliminary Work: -

Before carrying out the actual investigation, I did some preliminary work on the zonation of plants and animals on a sheltered rocky shore to determine which species was easily identifiable, collectible and abundant. I used 25cm2 quadrats and investigated up to a height of 6m. I laid the 30m measuring tape from the tide to a height of 6m above the tide in a vertical line and counted the number of organisms at each regular interval along the transect line. After drawing a graph to make my results clearer, and choosing Patella vulgata as my chosen species to study, I used that graph to find out where common limpets are most abundant, and that chart datum was actually correct. The middle shore was found to be highest in abundance of limpets. (See Figure 9, 10, 11.)

Variables: -

Independent: height above chart datum (3.5-4m. This is where limpets are most abundant according to chart datum; therefore it is the optimum niche.)
Dependant: number of limpets/limpet density.
Control: temperature could not necessarily be kept the same due to weather fluctuations, but the experimental procedure was carried out on the same day, at the same time to ensure temperature stayed as constant as possible. The size of the quadrat was kept 25cm2 to ensure the same area was studied for limpet density at each coordinate. The same optical level was used to determine the height above sea level on both shores, and the same person looked through it to cancel out any errors in the measuring technique. i.e. it is fixed error as the same error is present on both rocky shores. Wind can’t be controlled as the different shores experience different wind speeds and directions. However, as the experiment was carried out on the same day at the same time, the wind speed and direction remained constant on each rocky shore, so the two sets of results were subjected to the same wind conditions. The same technique/procedure was used on both shores and I counted the limpets in the same way so that any errors were minimal and my method of counting cancelled out any errors as the same person performed the counting technique, so it becomes almost fixed error. I couldn’t keep the measuring tape 4m all the way along the shore above the tide because the rocks were jagged and rocky so I tried to keep it as constant as possible. The quadrat also didn’t lie flat due to the uneven surfaces, so I tried to count the number of limpets as accurately as possible by envisioning the location of the quadrat had the surface been flat. I used the same measuring tape on both shores as the width of the measuring tape could cover a limpet and therefore affect the limpet density on each shore, but if the same measuring tape is used on both shores, as the same error is present throughout the experimental approach, it is fixed error and is cancelled out. All measuring devices and methods contain fixed error, as well as human error. Using coordinates generated from a calculator on both shores ensures the same area is studied on both shores, so a precise comparison is carried out.

Apparatus: -

25cm2 quadrat because this is the best size relative to the organism’s size.
Tape measure to lay out at 4m above sea level to take random samples along it.
Optical level to accurately measure 4m above chart datum.
Meter rule to place the optical level on.
Exposed rocky shore – Castle Beach.
Grid reference: SM819050

Location: Follow the coast path

Nearest Telephone: Dale Fort

Nearest Vehicle Access point: Through vehicle gate by point house

Escape Route: Coast Path

Sheltered Rocky Shore – Frenchman’s Steps.
Grid reference: SM819052

Location: Through the field of Point house and down the steps.

Nearest Telephone: Dale fort Field centre

Nearest Vehicle access point: Road at the bottom of Point House.

Escape Route: At low tide, along the beach to the dale fort Jetty beach.

Animal key to ensure I correctly identify the common limpet, not any other type, or even a different species.
Calculator to form random coordinates to ensure the method of random sampling is random, therefore removing bias.

Method: -

Use the following tide timetable to choose an appropriate time to carry out the experiment.

Go to Castle Beach (exposed rocky shore) and note the time you arrive. E.g. 10.59. Subtract the height above chart datum from 4m as this is the optimum niche where the limpets are most abundant. E.g. 4m – 2.74m = 1.26m. To investigate limpet density 4m above chart datum, I would have to place my measuring tape 1.26m precisely above the tide level at that moment in time.

Use an optical level to lay the measuring tape accurately at this value (which is 1.26m above sea level). Another person will be needed to assist with this. An optical level is a block of wood that has a smooth grain on the top and in the middle has a weight that dangles loosely to show the user that the optical level is not at an angle. Place it against a meter rule and go down to eye level to look through the grain until you reach a spot on the shore. (Ensure the dangling weight is vertical.) Ask someone to walk to that spot and mark it. This is 1m above sea level. Now put the optical level at 26cm against the meter rule and do the same.

Lay out the tape measure here horizontally here at 4m above chart datum and try to keep the height as constant as possible despite the uneven rock surface. This is the best height because it is the optimum niche where limpets are most abundant.

Now take your quadrat and place it randomly along the measuring tape using previously generated coordinates from a calculator, and count the number of limpets present in the quadrat. Repeat this 20 times. I am using random sampling because this removes bias and leads to more accurate results. Systematic sampling is more biased as there are regular intervals rather than random coordinates.

Now go to Frenchman’s Steps (sheltered rocky shore) and repeat the procedure.

Safety: -

Check the tides so that you do not get cut off by incoming tides. Preferably work on a falling tide.
Ensure the supervisor is carrying a first aid kit
Wear warm, waterproof clothing to avoid hypothermia.
Limit the damage to the environment by putting the quadrat down sensibly. Do not throw it with force onto the rocks. Try not to move any limpets, and if done so, return it to its prior position.
Ensure that footwear has good grip due to slippery, uneven surfaces.

Justification of experimental approach: -

I used random sampling because it removes bias from the experiment and leads to more accurate results. The positioning of the quadrat is not subject to human decision, whereas systematic sampling is, therefore it is biased.
The 25cm2 quadrat has been used as the size of the limpets is relatively small, so a larger quadrat would make counting them difficult, and a smaller one would not provide a good sample size. This size is the best size relative to the organism’s size.
A 30m measuring tape has been used to provide a large set of results across a broader area of study along the optimum niche.
The measuring tape was laid at 4m above chart datum because, according to my preliminary work, the limpet density is highest in the middle shore (see Figure 11). The same height above chart datum has to be used on both shores to ensure an accurate comparison is being carried out, and no abiotic factors are affecting the limpet density.
An optical level is used to accurately measure 4m above chart datum because it is the most accurate instrument. Repeating the procedure of measuring the height above chart datum will ensure the position on the rocky shore is accurate.
A meter rule has been used as it is the largest ruler available to place the optical level against.
A calculator is used to generate random coordinates as it is faster than picking numbers out of a hat. It is a more efficient method.

Hypothesis: -

There will be a statistically significant difference in limpet density on the sheltered and exposed rocky shores.

Null Hypothesis: -

There will be no statistically significant difference in limpet density on the sheltered and exposed rocky shores.

Prediction: -

I believe limpet density will be higher on the exposed rocky shore due to the fact that it is a north facing beach receiving more sunlight than the sheltered rocky shore. Although this causes the problem of desiccation for limpets, it also allows the macro-algae to photosynthesise, which are the main source of food for limpets. Exposed shores also have a lot more wave action as it is facing out into the open sea and prevailing winds causing larger waves. Limpets can survive the harsher conditions on exposed rocky shores, such as changes in salinity, changes in temperature, desiccation, wind etc. limpets are more abundant on exposed rocky shores because they have less competition for food, space, water, and mates than on the sheltered rocky shore which has less harsh abiotic conditions.

Implementing: -

Analysing Evidence and Drawing Conclusions: -

Null Hypothesis: There is no significant difference in the density of limpets on each shore.

Sum of the ranks along the rows:

∑R1 = 590

∑R2 = 230

U1 = (n1 x n2) + (0.5n2) (n2 + 1) - ∑R2 U1 = (20 x 20) + (0.5 x 20) (20 + 1) – 230

U2 = (n1 x n2) + (0.5n1) (n1 + 1) - ∑R1 = 400 + (10 x 21) - 230

n1 = number of samples in site 1 = 380
∑R2 = sum of the ranks for site 2 U2 = (20 x 20) + (0.5 x 20)(20 + 1) - 590
U1 = test statistics of exposed shore = 400 + (10 x 21) - 590

= 20

To check if my calculations are correct, I have to use the following formula:
U1 + U2 = n1 x n2

380 + 20 = 20 x 20

400 = 400 ☺

The smaller of the two values is taken as the test statistic, so in this case, it is 20.

I now have to refer to the tables of critical values, which in this case is 127.

The calculated value is lower than the critical value; therefore we must reject the null hypothesis that there is no significant difference in limpet numbers between the two shores at the 5% significance level and accept the ultimate hypothesis that there is a significant difference in limpet density on each shore at the 5% significance level.

From looking at the table, the results of the Mann-Whitney U test and the graph (figure 12); the obvious trend in the data to notice is that the limpet density is much higher on the exposed rocky shore than the sheltered rocky shore. On the graph, the mean number of limpets on the exposed rocky shore is approximately three and a half times higher than the amount on the sheltered rocky shore.

This might seem strange as the sheltered rocky shore has less harsh abiotic conditions for inter tidal organisms, as there are more stable temperatures, less desiccation, more and lower light levels. However, the competition for food, space, and mates on the sheltered rocky shore may have resulted in a decline in limpet density. Organisms which compete with limpets are sea cucumbers, sea urchins, lobsters, chitons, abalones and snails. On the exposed rocky shore there is hardly any competition to affect survival. The harsh abiotic factors do not decrease limpet numbers as they are adapted to cope with high saline conditions, variable temperatures, desiccation, wave action and other abiotic factors.

Another reason limpets exist in such high numbers on the exposed rocky shores is because there is more light available on exposed rocky shores as they are south facing beaches. The macro-algae on which limpets feed, require light to photosynthesise. The more light that is available, the more abundant the macro-algae, in turn, leading to an increase in limpet density. There are also stronger prevailing winds on exposed rocky shores, which mean the circulation of gases, such as, oxygen and carbon dioxide is better, so the macro-algae can carry out photosynthesis more efficiently.

There is a real danger of desiccation on exposed rocky shores due to wind (which speeds up evaporation of water), light (also speeds up evaporation of water), salinity (causes water to move out of the organism by osmosis), and the tidal cycle which affects the organism’s immersion time. When there is no tide, the limpet clamps down onto the rock using its strong muscular foot and secretion of a chemical, which prevents water loss. The shell prevents evaporation that might take place from the soft tissue inside.

The limpets were being investigated in the optimum niche (3.5-4m according to chart datum), where they are supposedly the most abundant. It shows that hardly any limpets exist on the sheltered rocky shore, as the highest numbers of limpets recorded in the optimum niche were 24 in a quadrat. Compared to 83 being recorded in just one 25cm2 quadrat, this shows that the increased biodiversity on sheltered rocky shores must promote competition tremendously, for limpets to prefer the harsher conditions of exposed rocky shores to escape interspecific and intraspecific competition for biota.

Limpets are most abundant in that particular area of the shore (middle shore to be specific) because it has the optimum conditions for limpet growth. The splash zone has extremely harsh conditions in which limpets cannot survive, although algae e.g. Gutweed exist there. Lichens are better adapted to those harsh conditions and they form a symbiotic relationship with the algae (the algae provides food for the lichens during photosynthesis, and the lichens provide a home for the algae). Limpets do exist in the upper shore, but are not very abundant because there is poor seaweed growth on the upper shore as there is hardly any water. As they are primitive forms of life which carry out all life processes underwater, this poses a problem. These abiotic factors determine the upper range of a limpet’s vertical range. Biotic factors, such as, interspecific and intraspecific competition for resources determine the lower limit of a limpet’s vertical range.

There are no real trends in the data from each shore as I used random sampling, so I cannot tell if there were more limpets towards the east side of the optimum niche or the west. Random sampling is better than systematic sampling as it removes bias. It covers a much broader range on the optimum niche without any bias. There were no anomalous results in this investigation as there was no human error; nature placed the limpets wherever they happened to be. The only strange thing was on the exposed rocky shore, there was an area of sparsely populated limpets, and the number of limpets turned out to be 10, which would be expected of the sheltered rocky shore. This may have been due to the fact the population hadn’t reproduced yet and intraspecific competition forced limpets to populate empty areas.

Evaluating Evidence: -

The main source of error in the procedure was the counting method. The quadrat didn’t lie flat on the surface due to the uneven surfaces, and some limpets were at the edge of the quadrats so it was difficult to determine whether I should count it or not. I may not have counted some of the limpets, or maybe even recounted the same ones by mistake. Human error had a very adverse effect in this experiment. To improve the procedure, I think I should have marked the limpets with washable ink or paint, so that I knew for certain which limpets I had counted already, and which limpets remained uncounted. This may have a detrimental effect on limpets as they are easier to see for predators, so recounting the limpets may be a safer option.

The optical level has a certain amount of fixed error in it, which cancels itself out as it was present on both shores. The measuring tape also didn’t lie flat at 4m, so I wasn’t taking samples always exactly in the optimum niche which may explain why some limpet numbers were low. People walking and stepping on limpets may have caused a few of them to fall off, resulting in lower limpet numbers. I may have misidentified the limpet which is highly unlikely, but still a factor to be considered. I used chart datum to determine the optimum niche, however, this may not be entirely accurate and a reliable source of information.

Limitations of the procedure were the number of samples, duration of the project, and no repetition of readings. The number of samples was too low to estimate the total size of limpet population in the optimum niche. The project only lasted half a day, at only one time of the year, so it did not allow for seasonal changes such as mating, fertilisation, predation etc. I also only looked at one type of exposed rocky shore and one type of sheltered rocky shore, which limits our evidence of higher amounts of limpets on exposed rocky shores than sheltered rocky shores.

There weren’t many anomalies, apart from on the exposed rocky shore, the numbers of limpets got a bit low. This could have been due to the fact the tape measure wasn’t lying exactly 4m above sea level, so the samples weren’t always taken on the optimum niche. Some samples may have been removed from the rocks due to people walking on the limpets and accidentally knocking them off. There may also have been to much intraspecific competition for biota, thus forcing the limpets to repopulate elsewhere.

I think the anomalies did have a serious effect on the conclusion of the statistics, as the calculated result (20) was a lot lower than the critical value (127). This proves the experimental procedure was fairly inaccurate and to improve it the sources of error must be tackled.

Bibliography

Internet: -