Before I can determine my method I need to make a valid cut off size for what is a juvenile or adult limpet.
Limpets begin to reach sexual maturity at around 9 months. To determine whether a limpet has reached sexual maturity requires detaching it from its home scar and dissecting to inspect the gonad. As this may be seen as ethically and environmentally unacceptable I have deciphered another method to estimate whether or not it is sexually mature. At sexual maturity the size of the limpet is between 15-20mm, so to establish if the limpet is adult or juvenile I will simply measure using callipers.
In preliminary experimentation I took 1 quadrat at low shore and then sampled a further quadrat at higher shore height and measured initially every limpet found within each quadrat and gained the following results;
Low shore height
There is a significant drop after 20mm showing that this is a good cut-off point.
Excluding the 2 that occurred below 20mm the remaining 8 of 10 limpets found at the higher shore were above 20mm. This pattern in my preliminary results suggests to me that 20mm is a reasonable cut off point to distinguish adults and juveniles. My results seem to have bi-modal distribution with 2 peaks and 2 troughs verifying it to be a functional cut off size.
In order to prove or disprove or prove my hypothesis I will investigate limpet distribution using the following method.
Method
- Before I can gather any results I will first have to determine what is low and what is high shore heights. I will also need to construct transect lines.
- To construct my transect lines I will have one tape measure vertical to the sea level and another parallel to the sea level. Using a crosstaff and pole I will measure the height vertical between my transect lines and what I will determine as high and low shore. I will measure height above chart datum. To show exactly where I am above the tide, on the shore my low shore height transect is. Using height above chart datum will give me 2 accurate measurements to show my exact position of transect lines.
- 10 quadrats may be needed to give a broad enough set of results. Transect lines are constructed to give me a precise place on the shore. Using quadrats placed randomly using telephone numbers is a helpful technique to remove bias. Where the quadrat is placed is where I will start to take my results. If the quadrat I empty or the numbers of limpets are not quite as I expected, the results will not be disregarded as this is bias. Using every result without discrimination will give fair true results. I will use a minimum of 10 quadrats. Depending on the amount of data collected, I may take more to increase quantity of my data. However many quadrats are used at one shore height the corresponding will be taken at the other shore height so they have equal numbers if quadrats sampled.
- Once I have placed the quadrat I will be recording the frequency of juveniles and the frequency of adults in the quadrat. Although I will be using the callipers to measure if the limpets are >20 mm or<20 mm, I will not be recording the individual size of each limpet I find in order to save time therefore gather more data. I feel based on the conclusion I drew from my preliminary work that 20 mm is a fair cut off size to decide whether a limpet is a adult or a juvenile. I will measure the limpet and either record it as adult or juvenile as a tally (see example recording sheet).
Apparatus
- Quadrat (0.5m)
- Tape measure x2
- Callipers
- Cross-staff and pole
- Clipboard
- Pencil
- Recording sheet
- Suitable footwear and clothing
- Mobile phone
Once I have carried out his method in order to analyse my data statistically I will carry out a chi square test. I feel a chi square test is most suitable as I am not questioning the size variation of the limpets but rather the observed ratio’s of adult to juvenile limpets at different shore heights. My chi square will prove whether or not there is a significant difference in the 2 ratios observed at low and high shore heights.
I will also do a histogram to show the frequencies overall at each shore height and also to show the proportion of adults to juveniles in that frequency.
Safety
- Always work from low shore to high shore. It is important to know the tide times and to be aware of the sea’s position.
- Always work within a pair so if there was any form of accident one person can contact help.
- Sensible footwear and clothing are important when working on the shore.
- Never walk with hands in pockets as rock surfaces can be very slippery.
- Take a mobile phone for emergencies.
Limiting factors.
With the nature of the tides it is difficult to control any limiting factor, those that could occur we should prevent by carrying out the experiment all on the same day. However I should take into account the following.
- I should ensure I use the same shore for my experiment.
- The shore I will use will be exposed to the same climate, it will also be the same rock type.
- It will be the same time of year and therefore the same time in the limpets settlement and lifecycle.
- It will be carried out with the same tide and at round about the same time of day.
Analysis
Before attempting to analyse my results I first had to prove the significance of my findings and demonstrate whether my hypothesis could be supported. So to do this I carried out, like talked about in my plan, a chi square test and calculated the following;
Observed frequencies
Expected frequencies
Row total x column total / sum total
WORKING;
RESULTS;
Given that X² =∑ (O - E)² / E
My degree of freedom is 1 (number of classes -1) (2-1) * (2-1) = 1 therefore using the table of X² distribution X² = 215.67 can be determined as very highly significant (almost certain). Thus supporting my hypothesis, I can now go on to analyse and give explanation for my result.
Patella Vulgata- a limpet has a reproductive cycle that comprises a stage from October to November (Trocophore) when the planktonic larval offspring are released into the sea. When I carried out my research on the shore itself was the beginning of October therefore at the very beginning of this stage, or to be looked at differently as the very end of the final stage of the cycle
