Introduction
Rough Periwinkles are herbivorous snails that are found in the upper shore and the splash zone of a beach. To be able to survive in harsh condition the rough periwinkle has an impermeable hard shell which protects them from damage and also stops them drying out as they have a reduced surface area. They breathe air through the mantle cavity (the small hole on the shell) and excrete uric acid which removes waste from the body without losing water allowing them to stay moist without the tide being up to the upper shore. They glue themselves to the rock which allows them to not be disrupted and pushed off the rock when the tide is high and waves crash against the rock. They also give birth to live young as the young need to be able to hold onto the rock immediately.
Exposed shore
Safety
Rough Periwinkles are herbivorous snails that are found in the upper shore and the splash zone of a beach. To be able to survive in harsh condition the rough periwinkle has an impermeable hard shell which protects them from damage and also stops them drying out as they have a reduced surface area. They breathe air through the mantle cavity (the small hole on the shell) and excrete uric acid which removes waste from the body without losing water allowing them to stay moist without the tide being up to the upper shore. They glue themselves to the rock which allows them to not be disrupted and pushed off the rock when the tide is high and waves crash against the rock. They also give birth to live young as the young need to be able to hold onto the rock immediately.
Exposed shore
- Exposed shores are shores that face into prevailing winds and into large stretches of sea. This means that waves that hit the coast are larger and quicker as they can build up. This will result in a more saline environment further up the beach as there is a large splash zone. This results in a harsher environment for organisms to live in
- Sheltered shores are shores that face away from prevailing wind and only have small bodies of water running up to them. This means that slower, softer waves hit the beach resulting in a smaller splash zone. This generally results in more biodiversity at the site as the area is more hospitable
- Hypothesis
- The periwinkles will be of a larger average length (mm) at the sheltered shore compared to the exposed shore
- Null hypothesis
- There will be no difference in the length of periwinkle at each of the shores
- Independent Variable
- Exposure to wave action, our two sites being Jetty beach and Castle beach. Jetty beach is sheltered from a large amount of the waves as it faces south-south-east and Castle beach which is exposed to many of the waves faces north-north east
- Dependent Variable
- The length of the rough periwinkle (mm) measured with a caliper from the apex of the shell to the bottom
- Control Variable
- Height above sea level, for this practical we used 6m. This the conditions the periwinkles have are the same except for exposure of the shorelines
- Rocky shore, both shores are rocky which means that they are generally impermeable and have many microhabitats
- Sampling Strategy
- We used stratified random to randomly choose periwinkles to measure
- Calipers
- Tape Measure
Safety
- The rocks on which the periwinkles are found on are very slippery when wet, care should be taken when finding periwinkles to measure and appropriate footwear should be worn
- Lie a tape measure parallel to the shore edge at 6 metres.
- We used stratified random to find out which periwinkles to measure. This involved using a calculator or website generate a random 3 figure number. The number generated should be used to find a point along the tape. This is done to ensure that the periwinkle data gathered is random
- At the spot corresponding to the random number measure one periwinkle. When measuring the periwinkle you should measure from the apex (or pointy part) of the shell to the bottom. The measurement should be given in mm. It is important to understand that periwinkles are living animals and are affected greatly when moved and we should be as quick as possible when measuring so human interference is to a minimum. This is the most ethical approach when trying to gather data for research on live animals that require movement and interference
- Gather 10 measurements of periwinkles each at a random spot along the tape using step 2 to work out where to measure
- Repeat step 4 at the other beach and compile results in a table
T test and the statistical significance of our results
- Calculating standard deviation
Each location will have have a unique standard deviation which indicates that the true value for the mean is within that particular range. This should be indicated on the graph in the form of error bars. Error bars also allow for an interpretation into how precise the results and methodology are.
- S1 = 2.0714
- S2 = 3.0374
- Calculating statistical significance
Conclusion
The calculated value of 2.777 is greater than the critical value of 2.101 (p=0.05, degrees of freedom = 18). Therefore we can reject the null hypothesis of no statically significant differences with 95% certainty.
Analysis
Our t test shows that we can accept our hypothesis meaning that the periwinkles are larger at Jetty beach compared with Castle beach with a statistically significant difference. This is because Jetty beach is a sheltered rocky shore with an exposure rating of 6. This more suitable for the periwinkles, allowing them to grow as they are disrupted less by powerful waves. This is because the peninsula on which Jetty beach is one covers the beach from the Atlantic waves meaning more hospitable conditions and less disruption to growth.
Evaluation
I believe that this experiment and the results gathered from it are accurate and reliable. I do not believe that we experienced any anomalies but different results were natural variation in periwinkle size. If I were to repeat the experiment, I would return to the beach multiple times and then average out my results. I would return every week as you can assume that is a long enough time scale that many of the periwinkles in the following counts are different. I could also add additional beaches that were exposed and sheltered to see if this small case study fit in to a general trend.
References
http://www.sciencedirect.com/science/article/pii/0022098181901040
http://www.sciencedirect.com/science/article/pii/002209819500131X
http://www.tandfonline.com/doi/pdf/10.1080/00445096.1968.11447358
- 1 = 11.34 and 2 = 8.11
- N1 and N2 = 10
- S1 = 2.0714 and S2 = 3.0374
- T = 2.777
- Critical value = 2.101
Conclusion
The calculated value of 2.777 is greater than the critical value of 2.101 (p=0.05, degrees of freedom = 18). Therefore we can reject the null hypothesis of no statically significant differences with 95% certainty.
Analysis
Our t test shows that we can accept our hypothesis meaning that the periwinkles are larger at Jetty beach compared with Castle beach with a statistically significant difference. This is because Jetty beach is a sheltered rocky shore with an exposure rating of 6. This more suitable for the periwinkles, allowing them to grow as they are disrupted less by powerful waves. This is because the peninsula on which Jetty beach is one covers the beach from the Atlantic waves meaning more hospitable conditions and less disruption to growth.
Evaluation
I believe that this experiment and the results gathered from it are accurate and reliable. I do not believe that we experienced any anomalies but different results were natural variation in periwinkle size. If I were to repeat the experiment, I would return to the beach multiple times and then average out my results. I would return every week as you can assume that is a long enough time scale that many of the periwinkles in the following counts are different. I could also add additional beaches that were exposed and sheltered to see if this small case study fit in to a general trend.
References
http://www.sciencedirect.com/science/article/pii/0022098181901040
http://www.sciencedirect.com/science/article/pii/002209819500131X
http://www.tandfonline.com/doi/pdf/10.1080/00445096.1968.11447358