The average lifespan of a limpet varies considerably depending on environmental conditions. Where growth is fast, such as on a sheltered shore, the lifespan can be 4 to 5 years while in places of slow growth this can be up to 15 to 17 years. The limpet species Patella vulgata is said to be a ‘protandrous hermaphrodite’ as younger individuals are predominantly male until the age of about 1 year when they undergo a sex change and progressively become more female. Therefore the male limpets of this species are usually smaller than the females, and their size usually increases as they become older and more female.
Hypothesis:
There will be a statistically significant difference between the sizes of limpets found on the upper shore than on the lower shore
Null Hypothesis:
There will be no statistically significant difference between the sizes of limpets found on the upper shore than on the lower shore.
Planning
Preliminary Work
My preliminary work enabled me to work out where above chart datum I should do my investigation. I did this using a graph showing the distribution of organisms in transect at Frenchman’s Steps. Then using the profile graph (Appendix) I managed to work out that the highest number of Common Limpets will be found between 3M and 6M at vertical height above chart datum. Then I looked at the “Plot of the tidal heights predicted for Milford Haven for the 8th of October 2003” graph, to work out what time would be best for me to go and do my investigation in between 3M and 4M above chart datum, I decided from this that I would lay my tape measure at 3.5M above chart datum. I decided that I would do my investigation on the 8th of October and I would visit Frenchman’s steps at 12.15pm, as this would be the time when the tide would be low on the shore.
Apparatus:
- Tape measure
- Callipers
- Tide tables
- Calculator
- Data record table
- Metre rule
- Spirit level
- Chart datum graph
- Clipboard and pencil
- Clear plastic bag
- Gloves
- All safety equipment, (waterproof clothing and shoes, first aid kit and risk assessment copy)
Variables
Included in this section are the output variables, which will enable us to quantify the
shape of a limpet, as well as the factors affecting limpet dimensions.
I will measure the limpets completely at random to give me a reliable and accurate result. The word random meaning no specific criteria not large not small but any, the spaces between each limpet must also be random. I must be random as this will be fair and give truthful accurate results.
The width of the base, along with the height perpendicular to the rock surface of the limpet will be measured. This will allow calculation of the width: height ratio – a value that quantifies the shape of the limpet thereby enabling a comparison to be made between sets of data by statistical analysis.
In order to carry out an experiment that is as fair and unbiased as possible, all variables other than the manipulated should be kept constant. Due to the nature of the investigation, continuity of some variables is impossible to achieve and this will unavoidably hinder the overall accuracy of the procedure. Although the common species of limpet is Patella vulgata, other species of different shapes can be found on the coast of Wales such as Patella depressa, see figure 3 (page 4). However, it would be practically impossible to determine each limpet before any measurements are taken and so it will be assumed that all limpets measured are of the species Patella vulgata.
Another variable that is known to affect limpet size is vertical height along the shore. High-shore limpets grow much larger than low-shore limpets, as the risk of desiccation is greater up the shore. Larger limpets have a lower surface area to volume ratio reducing water loss. However, in this case it is possible to keep the variable constant by sampling at the same vertical height on both shores.
Safety Assessment:
- Never work alone on the shore.
- Sign out and in.
- Check the tides; do not get cut off by incoming tides. Preferably work on a falling tide. Do not work on Castlebeach Bay 2 hours either side of Low water.
- RISK ASSESS YOUR SHORES- fill in an assessment for your shores before you go and take it with you.
- First aid kit must be carried with you.
- Make sure waterproof and warm clothing is worn.
- Waterproof or walking shoes must be worn.
- Be careful of dangerous animals such as Barnacles, which can cause scratches and cuts and seaweed, which can be very slippery.
- Weather must be checked beforehand, as rain and strong winds can make the shore a very dangerous place to work on.
Gloves should be worn as there is contact with living animals and hands must be washed afterwards, so no contamination is caused.
Ethical implications:
In this investigation ethical implications have to be considered as a living animal is being investigated with. I will have to make sure that Limpets are not removed from the shores, and when counting Limpets it is important that I do not push or trample over the Limpets or their habitat; if accidentally a Limpet is moved from its natural habitat I must replace it where it is found. While moving around to count Limpets I must make sure that I am not endangering the life of another animal or plant, by trampling over them or their habitat or by moving them.
In general collecting of specimens should be avoided unless if it is absolutely necessary. Do not collect anything from Frenchman’s Steps and Castlebeach Bay, as they are used regularly for surveys and monitoring. Habitats of animals must be protected, if stones or boulders are turned over, they must be replaced.
Method
Results:
In order to show that there is a statistically significant difference between the shapes of limpets, as calculated, on lower and upper shores it is necessary to perform further mathematical analysis. When testing for any statistically significant difference between two sets of data, being the width and height ratio on lower and upper shores in this case, a T – Test is usually carried out. To calculate the t value the mean and standard deviation (a value for the spread of the data) of each set of results are worked out. Next, the value of t is calculated using the formula given. In order to interpret the resulting t value the critical value must first be determined. This takes into account the level of significance of the results used, that is the probability that the differences between the measured values and the actual values are due to chance, and the number of comparisons made between measured and actual values also known as the degrees of freedom, similar to the chi-squared test learnt at A2 level. In this experiment, the level of significance is equal to 0.05; the standard value used in biological experiments, and the degrees of freedom is 30. Using the table of T-distribution, see appendix 1, the critical value for this experiment can therefore be shown to be 2.042. The t value was calculated to be 8.242.
Mean x Mean x
= x = x
= 9.82 = 18.29
30 30
= 0.328 = 0.61
Variance (s²) = Σx² - (Σx) ²
n
n – 1
= 4.0362 – (9.82) ²
30
30 - 1
= 4.0362 – 96.4324 = 3.214
30
= 4.0362 – 3.214 = 0.8222
- 29
= 0.028
Variance (s²) = Σx² - (Σx) ²
n
n – 1
= 11.3573 – (18.29) ²
30
30 - 1
= (11.3573 – 11.151)
29
= 0.00712
t = | x – x |
= s + s
n n
t = |0.328 – 0.16|
= 0.028 + 0.00712
30 30
= -0.282
(1.170666667 x 10 )
= 0.282
0.03405006
t = 8.242
Conclusion
Looking at all the results, graphs 1 and 2 and the statistical tests, I can propose that my hypothesis is partly correct. The sizes of limpets on the upper shore are bigger than the sizes of limpets found on the lower shore at Frenchman Step’s at two different heights on there vertical range at 3M and 6M above chart datum. The height/base ratio calculated for both shores showed us that the limpets found on the upper shore had a much larger height/base ratio than the limpets found on the lower shore. After doing the T-Test using my results, I was able to reject my null hypothesis, that there will be no statistically significant difference between the sizes of limpets found on the upper shore than on the lower shore at two different heights on there vertical range at 3M and 6M above chart datum at 5% significance level and this supported my hypothesis further.
Graph 1 shows that the sizes of limpets are smaller on the lower shore as the majority of limpets that are found there have a small reading compared to the sizes of limpets found on the upper shore which have a much higher reading. Looking at the graph we can see that most limpets found on the lower shore, 3M above chart datum have a reading between 0.11 – 0.40. The most common size for the limpets found on the lower shore was 0.21 – 0.30, as a total of 10 limpets were found with this size.
The graph also shows that the limpets found on the upper shore, 6M above chart datum have a very high reading of 0.41 – 0.80 as most limpets were found at this size.
The most common size of limpets that were found on the upper shore was 0.61 – 0.70 where a number of 15 limpets were found.
Therefore the graph clearly tells us that the majority of limpets on the lower shore have a smaller size compared to the majority of limpets found on the upper shore.
The reasons for this are that the effects of wave action are more at the exposed shore site. As a result of the force of the water’s acceleration increasing at a faster rate than the organisms ability to hold on as an organism grows, wave exposure prevents the distribution of larger limpets. When limpets are mobile their adhesive tenacity is much less than when they are stationary. This suggests that due to their reduced tenacity with movement, the Patella spp. may be limited in size by waves at an exposed shore.
It was mentioned in the scientific background, that there is usually more algae present on the upper shore resulting in increased growth rates and so larger limpets. This statement correlates with the evidence so one can conclude that the presence of more algae result in larger limpets. Also limpets on the upper shore need to store water in their shells which makes them larger as they are not so near it, on the other hand limpets on the lower shore do not need to store water in their shells as they already have enough water surrounding them as they are near the sea, which makes them smaller in size compared to the limpets found at the top.
The analysis of the T – Test shows us that there is a statistically significant difference between the sizes of limpets found on the upper shore and those found on an lower shore, as was originally stated in the hypothesis. This is because the calculated t value of 8.242 is above the critical value (2.042) confirming that there is a statistically significant difference. Therefore the earlier conclusion stating that limpets found on the upper shore are bigger in size than those found on the lower shore is shown to be valid.
Discussion and Evaluation
“The size of limpets will be higher on the upper shore then the limpets on the lower shore at Frenchman’s Steps at 3M and 6M above chart datum.”
Looking at the results tables, graphs and statistical tests I can conclude that my hypothesis has been supported. The limpets found on the upper shore had a higher height/base ratio than the limpets found on the lower shore. This is because the limpets have adapted to the conditions that are found on the upper shores and lower shores. On the upper shore the limpets are bigger because there is more food found on the upper shore than on the lower shore and the limpets on the upper shore also have to store water in their shells as they don’t get as much water as the limpets on the lower shore which means that the limpets do not need to store water in their shells resulting in smaller shell sizes.
It was seen that there was also a lot of seaweed present at 3M above chart datum, which means that there could have been competition between the seaweed and limpet for food on the lower shore. This means that the limpet’s get less food compared to the limpets on the upper shore that have plenty of food without any competition. This also supports my hypothesis that the limpets are larger on the upper shore than on the lower shore as the limpets on the upper shore have more food available to them without competition. The limpets on the lower shore have competition with seaweed and therefore don’t get as much food making them smaller in size.
The results showed that there was a higher frequency of smaller limpets and a lower frequency of larger limpets at the wave-exposed site (lower shore) than at the sheltered site (upper shore). Therefore wave exposure has a profound effect on the size distribution of this intertidal limpet species. The reasons for this are that the effects of wave action are more at the lower shore, as it is closer to the splash zone. As a result of the force of the water’s acceleration increasing at a faster rate than the organisms ability to hold on as an organism grows, wave exposure prevents the distribution of larger limpets. When limpets are mobile their adhesive tenacity is much less than when they are stationary. This suggests that due to their reduced tenacity with movement, the Patella spp. may be limited in size by waves at the lower shore.
On the lower shore more diversity of species and high biomass was found on the where it would be harder for Limpets to survive because of strong wave action and high tides. However on the upper shore at Frenchman’s Steps there is more diversity on the and biomass increases down the shore, however and Limpets can stay in the middle and upper shore away from high tide and strong wave action. To survive on the lower shore the Limpets need strong muscles, to stay attached to the rocks, but not all Limpets can have strong muscles, especially new born young limpets. This was supported by my observations, as I saw that there were more young, smaller Limpets on the lower shore then on the upper shore, Frenchman’s Steps where more adult and large Limpets were found. This could relate to the fact that Limpets might die young on the lower shore, due to the strong wave action and high tides, as young Limpets may not have enough muscle attachment to the rocks.
On the upper shore there is not strong wave action and high tide and therefore, the Limpets survive to become adults and large in size and therefore have strong muscles to attach themselves to the rock. This means more Limpets are able to survive on the because of the lower wave action compared to the exposed rocky shore, where wave action is high. Due to this size of Limpets is higher on the upper shore compared to the lower shore.
On the upper shore the species are more mobile and therefore the Limpets are able to migrate up and down the shore according to the conditions. In my observations I noted that many Limpets had moved up to a steep rock to survive the strong wave action and high tides. But on the lower shore the Limpets are much more attached to the rocks surface and are not able to migrate in severe wave action and weather conditions and therefore may not survive but die at a young age.
Overall, I think that the investigation went well, as I have managed to obtain results and graphs, which supported my hypothesis and statistical tests, which rejected my null hypothesis. All the variables that I said I would control were controlled, the same animal, Limpet was measured, the distance above chart datum was 3M and 6M, the method was the same on both vertical heights and the time was controlled as a the investigation on both heights were carried out on low tide.
To extend this investigation or to provide additional evidence for the conclusion I could:
- Compare the abundance of Limpets on a sheltered and exposed rocky shore, 3M and 6M above chart datum. (As I saw in my observations that number of Limpets did vary considerably on both shores)
- I could compare the size of Limpets on both shores, at another chart datum, for example 5M above chart datum, to see how the results differ.
- I could compare the size of Limpets 3M and 6M above chart datum, with another animal such as rough periwinkles, on the exposed and sheltered rocky shores.
- I could compare the upper, middle and lower shores for their sizes of Limpets on two different shores.