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An experiment to see whether limpets are more abundant on sea-facing or land-facing rocks

Research and rationale

Aim: To determine whether limpet height is affected by whether they are situated on land-facing or sea-facing rocks

Hypothesis: there will be significant difference between limpet height on sea-facing and land-facing rocks.

Null Hypothesis: there will be no significant difference between limpet height on sea-facing and land-facing rocks

Thepatella vulgata,more commonly known as the common limpet is a marine organism that is part of the Mollusca

phylum and thepatellagenus1. It is found in the lower and middle eullitoral zone on the shore, however generally they

are more evident in areas with large amounts of algae as this is what the organism feeds on. The limpet feeds by

moving along the surface of the rock and uses its radula to scrape off the algae2. Furthermore, throughout the first year

of the limpets life they are all neuter, however after year 1 roughly 34% become females and the rest become males2.

I chose to study limpets due them being extremely abundant on exposed rocky shores and therefore it would have

been able to collect a sufficient amount of data easily. Being able to collect a lot of data is important as the more you

have the more reliable and representative the data you collect is. Another reason was due to their ecological

importance within their habitat. Limpets play a vital role in stopping large seaweeds such as bladder wrack being able

to establish them within the habitat3. This is because limpets feed on microscopic algae; therefore meaning it is unable

to grow into the larger seaweeds3. The limpet is known as a Keystone Species due to it have a significant role within

the community. Limpets help promote the growth of barnacles as by removing algae they consequently create more

room for the barnacles to grown on 3.

Why did I reach my hypothesis?

The limpet is comprised of a hard outer shell and a soft fleshy body on the inside. At the bottom of the body there is a

large 'foot' which contracts to keep the limpet attached to the rock4. The contraction of the foot causes the shell to

clamp down bringing the lower rim of the shell and the teeth into direct contact with the rocks surface4. Research fromthe University of Portsmouth4has even said these tiny teeth they use to 'lock' onto the rock may well be the strongest

naturally occurring material in the world, which indicates just how difficult they are to remove when in their home

scars. This is important for two reasons; firstly is prevents desiccation (drying out of the limpet) during low tide and

secondly it prevents the limpet being dislodged during harsher tidal conditions4. Furthermore, as the limpet settle

down they rotate their shell resulting in the limpet grinding into the rock causing a 'home scar' to form. The limpet

always returns to its home scar after it feeds during high tide4.

In general limpets which are exposed to more wave action (sea facing) will have greater shell height5. This is because

its 'foot muscle' is contracted more often which results in a domed shell forming, where as more relaxed limpets have

a flatter shell. It is necessary for its foot to be contracted more often as without this the limpet would be easily

dislodged during periods of high wave and sea action5

.

How does my investigation relate to the outer world?

Due to the importance of limpets it is important to maintain their populations. With Global Warming nowadays being

a huge issue, undoubtedly organisms will be under threat due to the changing conditions along the coast. Limpets

usually release their gametes into the sea when sea temperatures drop below 11 degrees Celsius 2 and therefore Global

warming could lead to this time of year changing. Furthermore, NASA has predicted that increasing sea temperature

will also lead to storm frequency and intensity6. This again could have a dramatic effect on the limpet as they usually

release their spawn in October due to rougher weather starting to occur at this time, along with the temperature

generally dropping below 11 degrees. This could again lead to the limpets releasing their spawn at a different time as

increasing sea temperature causing these storms to occur earlier, which could consequently have an affect limpetheight.

Due to the limpets essential position within its ecosystem, I will aim to see how rock aspect affects limpet height. By

knowing how the environment affects limpet height and population, which will gain more of an understanding on the


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species thus allowing us to act quickly to stop the species being too greatly affected by the changing environmental

conditions.

Pilot Studies

Before I carried out my main investigation there were certain aspects of my method that I was unsure about and

therefore before I started to carry it out, I felt it was necessary to complete 2 pilot studies in order to make sure my

method was suitable for achieving accurate results in a safe manner.

Pilot Investigation 1:

Firstly, I was uncertain with what height above sea level was the optimal in order to be able to gather a sufficient

amount of data in order to make my results as representative as possible. Therefore in order to determine this height I

completed the following experiment:

1) Set up a vertical sampling line from sea level to the upper zone of then

shore.

2) From sea level, using a 0.6m cross staff, go up the shore in 0.6m vertical

increases and at each point mark the location using chalk (see Fig 1.1).3) After you have reached the upper shore, go back to each mark and place a

0.5m quadrat over it making sure the mark is in then middle of then quadrat.

4) Then count then number of limpets you see within the quadrat.

5) Repeat until you reach the upper shore.

Results

From my results I saw that 4.2 metres above sea level seemed to be the best height to measure the limpets, as theywere more abundant there. This informed me that 4.2 metres above sea level would be the optimum height for me to

collect a sufficient amount of data in order to make it representative.

Pilot Investigation 2:

Another aspect of my method I was unsure of was the sampling technique I was going to use when choosing which

limpet to measure along my sampling line. I conducted a mini investigation to see whether a quadrat would be more

suitable than simply measuring the closest limpet to each interval.

1) Locate a rock at any height and facing either the sea or the land and set up a 4 m transect using a tape

measure along the top of it.2) Every 40 cm locate the closest limpet to the interval and measure its height using a calliper.

Continue along in regular interval in a systematic manner.

OR

Figure 1.1- showing how I used the

cross staff

Height above Sea level

(m)

Number of limpets present

3 23

3.6 29

4.2 43

4.8 30

5.4 17


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Every 50 cm place the 0.5m quadrat down and measure the height of every limpet using the callipers within

the

Middle

Square

of the

quadrat.

Results:

Selecting limpet closest to

interval method

Quadrat method

Although both of these methods made me obtain consistent and valid data, after the trial method I decided to use the

selecting method due to it being more practical and having less subjectivity. For example, on certain parts of the rocks

it was unable to place then quadrat completely flat and therefore often it was hard to know which limpets fell within

the Middle Square of the quadrat. Furthermore, it was common for some limpets to fall on the boundary of the middle

square and therefore again it was subjective to decide which ones count and which ones do not.

Modifications

Interval spacing-

From my pilot study I decided to use smaller intervals along my sampling line, as when I used 40 cm intervals I felt

then data I would have collected wouldn't have been representative as there could have been major anomalies

Distance along transect (cm) Limpet Height (mm)

40 11.2

80 6

120 15.3

160 9.5

200 7.4

240 7.5280 7.9

320 6.3

360 9.5

400 6.3

Quadrat Average Height of limpet in middle square (mm)

1 7.9

2 7.6

3 6.7

4 6.6


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within each interval. From this I decided to use 10 cm intervals so that firstly I could collect a sufficient amount of

data and also so that it was representative.

Which limpets not to choose-

From both methods I realised that it is common for limpets to be situated in

crevasses (see Fig 1.2). From this I realised that in my main investigation I

must discard these limpets and not measure them even if they were closest tothen intervals, as they would have had a different aspect to what I wanted to

investigate. For example often then limpets would be found on then wall of

then crevasses an therefore were neither sea facing or land facing.

Choosing limpets closest to interval instead of quadrat-

I chose the choosing method instead of using a quadrat due to 3 main reasons.

These were:

1. There was too much subjectivity as sometimes then quadrat could not be laid flat an therefore it was difficult to

know whether or not then limpet fell between the middle square.

2. Quadrat often slid down rock and therefore it was hard to measure all the limpets in one go.

3. Sometimes limpets were half in the middle square and consequently there was subjectivity over which limpets

to measure (see Fig 1.3).

Risk Assessment

The rocky shore is undoubtedly an area, which does have safety risks. These risks can be avoided or minimised if the

appropriate actions are taken.

Risk Actions taken to minimise or avoid risk

Drowning Check tide times before going into the field so you know

how they are going to progress. Also keep checking to

see how the sea is behaving.

Tripping Follow the route that the guide takes, and also wear

sturdy boots with a hard sole.

Hypothermia Check what the weather is predicted to be like and dress

accordingly. Wear waterproof jackets and trousers if it is

predicted to rain.

Getting lost Follow the guide at all times and make sure you can see

him/her.

Cutting hands on limpets Where rubber gloves when handling them.

Fig 1.2

Fig 1.3


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Risk rating

You generate the risk rating by multiplying the worst-case outcome score by the probability score. If the value

generate is greater than 12 then the fieldwork should not be conducted. As you can see with my Risk Assessment table

the highest score is just 6, and therefore I concluded that my investigation was safe to carry out.

Main method

Go to Angle point exposed (grid reference: 51.687758, -5.110952).

Choose two rocks, one sea facing and one land facing. It is pivotalthat the sea facing rock is directly in front of the sea and is not

obstructed by another rock, as if this were the case then you would

be investigating the hypothesis.

Next using a cross staff and known tide heights locate the point on

the rock which is 4.2 metres above sea level (it is critical to

remember the height the sea level was already at as it can never be

0m).

When you have found this height mark it with chalk and place tape

measure along the face of the rock.

Set up a 4.5m sampling line. It doesn't matter on which part of the

rock as long as the height remains constant at 4.5m Next work along the sampling line in intervals of 10cm.

At each interval you should measure the height of the limpet using callipers

(See Fig 1.4).

Where there is doubt oh which limpet is closer to the interval it may be necessary to use a 30cm ruler to

accurately determine which limpet you should measure.

Furthermore discard limpets in crevasses for reasons stated above.

You should collect enough data so that you have a stable running mean ( i.e. each mean is within 2.5% of the

mean collected 5 samples ago, once this has occurred you should double the number of samples you have

collected).

Complete this process on both the sea facing and land facing rock.

Statistical Test

Fig 1.4


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The most suitable Statistical Test for me to use on my data was a Z Test. This test is used when you wish to test for a

statistically significant difference between the

means of data taken from different areas. I had

to use a Z test instead of a T test because my

sample size was greater than 30. With my Z

value obtained, I can compare it to the critical

value to see if there is any statistical difference

between my results. If there is then I can accept

the hypothesis and reject the null hypothesis.

Workings out:

Analysis and Conclusion

Results

Land facing rock Sea facing rock

Temperature 14.4 14.4

Wind Speed 1.4 2.4

Light Intensity 1974 1974

Humidity 97 97

Graph

See attached.

Conclusion

Land Facing rock Average

Height (mm)

Sea Facing rock Average

Height (mm)

4.76 8.4


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As you can see from looking at my Z test workings out, my results generated a Z value of 8.84. Straight away this

high figure suggested that there was a significant difference between the two sets of data, however to be completely

certain it was then necessary to compare this value to the relevant critical value. To find out the critical value in a Z

test you need to work out the degrees of freedom (total number of data -2). Therefore for my investigation this value is

88, and the corresponding critical value was 2.000 at p=0.05. Since the Z value was greater than the critical value, we

can reject the null hypothesis and accept our hypothesis that there is a statistically significant difference between

limpet height on sea-facing and land-facing rocks. Furthermore as the value was greater at p=0.05 it means that we

can be 95% certain that the results did not occur due to chance.

From looking at my results table and my graph there were some clear observations to be made. Just by looking at the

table of averages you can see that limpets on sea facing rocks have a much larger height than limpets on land facing

rocks. Therefore we can conclude that the independent variable (rock aspect) does have an affect on our dependant

variable (limpet height).

These conclusions can be explained not only by the increase to wave exposure but other factors too. As stated in the

research and rationale, I predicted that limpets on sea-facing rocks would have a larger height than those on land-

facing rocks. This was because generally sea-facing rocks are exposed to harsher tidal conditions, thus meaning the

probability of them becoming dislodged during these harsh conditions is increased. In order to combat this risk the

limpet consequently will contract its foot muscle more frequently, which in turn leads to a domed shell forming

rather than a flatter shell. Another explanation, which can be used to explain my conclusion, is the migratory patternsof limpets. Larger limpet tend to migrate further up the shore in order to reduce competition, as often the younger

limpets stay in areas which are immersed for longer periods of time (lower shore) 7. When I conducted this

investigation it was obvious to me that, although on both rocks my sampling line was at the same height, there was

still substantially more limpets on the land-facing rock than the sea-facing rock. Due to only the strongest limpets

being able to survive on sea-facing rocks there is substantially less competition, as limpet abundance is far fewer here.

Consequently due to the bigger limpet beings able to survive on these rocks, if they can, they will relocate there in

order to see off competition. This theory of the limpets migrating elsewhere however is disputed, as it conflicts with

another known aspect of limpet behaviour, of them returning to their home scars after feeding. An experiment

conducted on limpet homing showed that it is 95% successful7,which therefore contradicts the idea of limpet

migrating into space. However, one possible explanation for homing to be the norm up until a certain time, is that as

the limpet grows in its home scar until it becomes too small and there not being enough room for it to expand,meaning the limpet needs to re-locate in order to find a suitable location to form a new one.

Discussion

Once I had gathered enough data to produce a stable running mean, the first thing I checked for was anomalies.

Although at first glance it may seem as if there were various anomalies (14.4mm height 30cm along sea-facing rock),

actually after doing some research I found that limpets can grow from anywhere between 2mm to 25mm in height

depending on external conditions. There could be a number of reasons why results have arisen which didnt fit the

general trend. Firstly, there is a possibility that some of the limpets I measured were not actually the patella vulgata

but that of a different species. This could suggest a reason why the occasional limpet measured was abnormally large.

In the field it was too difficult to distinguish the different limpets, as it is near impossible to do so without looking

inside the shell. However, removing the limpet from their substrate in unethical as by doing so there is a high

possibility that the limpet would be killed, furthermore, sometimes even experts find distinguishing the different

species very difficult as the differences are minimal. This is the reason therefore why throughout my investigation I

have not been specific to which type of limpet I was measuring. Furthermore, another improvement I could have made

to my investigation would have been to control the age of limpet I was measuring. Although, I tried to limit the effect

of the age of the limpets on their age by measuring at constant height on both rocks, it is still a possibility that in fact

some of the limpets that I measured were still juvenile. This would have had a direct effect on my results as it would

be expected that the younger limpets would be smaller in height due to them having had less time to grow.

Another issue I had which could have affected the accuracy of my results and therefore their reliability was that on

both rocks the limpets form their home scars by abrasion. Thus meaning the limpet shell was slightly indented into therock. This made measuring the exact height of the limpet shell very difficult as it was hard to get the bottom of the

calliper into these home scars. We could therefore expect the limpets height to be a couple more mm higher if we

were to include the part of their shell which was actually in their home scar. One last difficulty I had with measuring

their height was that due to the nature of the exposed shore often it was hard to accurately record their heights using


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the callipers and therefore there could have been some inaccuracies due to human error. One way I could improve this

investigation in the future would be by using digital callipers.

Although my results showed a statistical difference between limpet height on sea-facing and land-facing rocks which I

believed was down to the limpets there being exposed to more sea action, there is a possibility that other variables

involved could have had an impact to. When in the field I measured other variables so that when analysing my results

I could use the other data collected in order to explain my findings. The only variable which seemed to be

significantly different between the two rocks was wind speed (2.4m/s on sea facing and 1.4m/s on land-facing). As

these variable werent kept constant there is a possibility that wind speed has a profound effect on limpet shell height.The reason for this could be because as the limpets are exposed to stronger winds they have to keep their must

muscle contracted more often in order to avoid desiccation. Similarly to wave exposure, the increase in time that they

have their foot muscle contracted will results in a more conical shell forming, thus increasing the limpets shell height.

Therefore although I believe wave exposure has the greatest effect on limpet shell height I cannot be certain that it is

the only condition which has an impact as wind speed was not constant between the two rocks.


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Bibliography

1. http://www.marlin.ac.uk/biotic/browse.php?sp=4220

2. http://www.theseashore.org.uk/theseashore/SpeciesPages/Limpets.jpg .html

3. http://www.asnailsodyssey.com/LEARNABOUT/LIMPET/limpComp.php

4. http://britishseafishing.co.uk/limpets/

5. http://www.researchgate.net/publication/236946487_Body-

size_variation_exhibited_by_an_intertidal_limpet_Influence_of_wave_exposure_tidal_height_

and_migratory_behavior

6. http://climate.nasa.gov/effects/

7. http://naturcymru.blogspot.co.uk/2012/03/counting-and-measuring-limpets-in.html
http://www.marlin.ac.uk/biotic/browse.php?sp=4220http://www.theseashore.org.uk/theseashore/SpeciesPages/Limpets.jpg.htmlhttp://www.theseashore.org.uk/theseashore/SpeciesPages/Limpets.jpg.htmlhttp://www.theseashore.org.uk/theseashore/SpeciesPages/Limpets.jpg.htmlhttp://www.asnailsodyssey.com/LEARNABOUT/LIMPET/limpComp.phphttp://britishseafishing.co.uk/limpets/http://www.researchgate.net/publication/236946487_Body-size_variation_exhibited_by_an_intertidal_limpet_Influence_of_wave_exposure_tidal_height_and_migratory_behaviorhttp://www.researchgate.net/publication/236946487_Body-size_variation_exhibited_by_an_intertidal_limpet_Influence_of_wave_exposure_tidal_height_and_migratory_behaviorhttp://www.researchgate.net/publication/236946487_Body-size_variation_exhibited_by_an_intertidal_limpet_Influence_of_wave_exposure_tidal_height_and_migratory_behaviorhttp://climate.nasa.gov/effects/http://www.theseashore.org.uk/theseashore/SpeciesPages/Limpets.jpg.htmlhttp://www.asnailsodyssey.com/LEARNABOUT/LIMPET/limpComp.phphttp://britishseafishing.co.uk/limpets/http://www.researchgate.net/publication/236946487_Body-size_variation_exhibited_by_an_intertidal_limpet_Influence_of_wave_exposure_tidal_height_and_migratory_behaviorhttp://www.researchgate.net/publication/236946487_Body-size_variation_exhibited_by_an_intertidal_limpet_Influence_of_wave_exposure_tidal_height_and_migratory_behaviorhttp://www.researchgate.net/publication/236946487_Body-size_variation_exhibited_by_an_intertidal_limpet_Influence_of_wave_exposure_tidal_height_and_migratory_behaviorhttp://climate.nasa.gov/effects/http://www.marlin.ac.uk/biotic/browse.php?sp=4220


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Distance along

sampling line Land facing rock (mm) Running mean Sea facing rock (mm) Running mean

0 2.40 2.40 5.00 5.00

10 4.60 3.50 5.10 5.05

20 4.50 3.83 4.70 4.93

30 7.60 4.78 14.40 7.30

40 6.10 5.04 4.00 6.64

50 6.20 5.23 5.40 6.43

60 5.90 5.33 11.00 7.09

70 4.60 5.24 9.00 7.33

80 4.80 5.19 4.90 7.06

90 6.00 5.27 5.00 6.85

100 4.40 5.19 9.20 7.06

110 4.00 5.09 12.00 7.48

120 5.60 5.13 11.40 7.78

130 2.90 4.97 9.00 7.86

140 5.20 4.99 5.60 7.71

150 4.40 4.95 10.00 7.86

160 5.20 4.96 6.00 7.75

170 5.00 4.97 7.70 7.74

180 5.50 4.99 8.00 7.76

190 4.20 4.96 9.20 7.83

200 3.30 4.88 6.70 7.78

210 3.60 4.82 12.70 8.00

220 5.30 4.84 8.00 8.00

230 3.00 4.76 10.70 8.11

240 6.10 4.82 10.40 8.20

250 4.90 4.82 7.30 8.17

260 4.20 4.80 11.40 8.29

270 3.70 4.76 8.30 8.29

280 4.00 4.73 7.90 8.28

290 4.20 4.71 11.10 8.37

300 6.00 4.75 6.90 8.32

310 5.30 4.77 9.30 8.35

320 3.90 4.75 11.70 8.45

330 4.20 4.73 7.80 8.44

340 4.10 4.71 6.40 8.38

350 4.80 4.71 8.90 8.39

360 4.50 4.71 6.50 8.34

370 5.10 4.72 9.50 8.37

380 6.00 4.75 7.00 8.34

390 5.20 4.76 8.30 8.34

400 4.10 4.75 11.10 8.40

410 5.50 4.76 8.70 8.41

420 4.90 4.77 7.90 8.40

430 4.60 4.76 8.10 8.39

440 4.30 4.75 6.90 8.36

450 5.10 4.76 10.10 8.40

Averages 4.76 8.40


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