Description: Limpets have flattened, cone-shaped or cap-shaped shells with concentric growth lines and radial ribs that extend from the edge of the shell to the apex. Their size ranges from about 5 to 200 millimetres in length.
They usually adhere strongly to rocks using pedal mucus and a muscular "foot", enabling them to attach securely to the rocks, in spite of the intensity of the wave action. This strong attachment also protects them from desiccation during low tide for though they may be in full sunlight, they are able to efficiently retain water, being tightly clamped onto the rocks. Although mostly sessile, they move through muscular contractions of the foot when there is the need to move towards food or change a microhabitat. Limpets generally feed on algae found on the shores, and are prey to a variety of organisms such as starfish, shore-birds, and fish.
Hypothesis
The size of the limpets will decrease as the height above chart datum increases. The lower shore is exposed to greater wave action, hence the limpets probably have a larger surface area for a better grip on the rocks. Intensity of wave action decreases as height of the shore increases, hence the size of the limpets should decrease accordingly.
METHODOLOGY
We use an interrupted belt transect of 0.5m2 quadrats across and up the shore at different height intervals , 2.0m, 4.0m and 6.0m above chart datum. These heights are chosen as they fall into the lower shore, middle shore and upper shore respectively. Five quadrats are placed along each height at intervals of 1m, and 5 limpets were selected to be measured from each quadrat. In total, we collected 25 samples from each height.
The independent variable in this investigation is the height on the shore above chart datum, and the dependent variable is the width of the limpet shell.
Equipment: A 0.5m2 quadrat, a metre ruler, an optical level, a pair of Vernier callipers and a tape measure.
ANALYSIS
We used a t-test to see if the means from our data are significantly different. The null hypothesis (Ho) for this t-test is that there is no significant difference between the mean widths of Common Limpets at different heights on the shore above chart datum.
Calculated ‘t’ values:
t1 (comparing lower shore and middle shore) = 0.6541
t2 (comparing middle shore and upper shore) = 2.900
t3 (comparing lower shore and upper shore) = 1.1987
We now compare our calculated ’t’ values (0.6541, 2.900 and 1.1987) with a critical ‘t’ value from statistical tables.
Calculating degrees of freedom: (n1 + n2) – 2 = 25 + 25 – 2 = 48. Since 48 is not quoted in the table of critical values, we use the next smallest value, 40.
The critical ‘t’ value for the 5% significance level [df = 48] is 2.021.
Concluding statement:
The calculated ‘t’ values ‘t1’(0.6541) and ‘t3’(1.1987) is less than the critical ‘t’ value (2.021), hence we accept the null hypothesis with 40 degrees of freedom at the 5% significance level. The calculated t value ‘t2’ (2.900) is greater than the critical ‘t’ value (2.021), hence we can reject the null hypothesis with 6 degrees of freedom at the 5% significance level, and accept the alternative hypothesis that there is a statistically significant difference between the mean widths of Common Limpets at 4.0m and 6.0m on the shore above chart datum.
INTERPRETATION
CONCLUSION
EVALUATION
Limitations and Improvements:
- Our selection of limpets to measure from each quadrat was not random and was subject to human bias. To improve, we could have produced random numbers with a calculator and measured the limpet that is closest to each random value along the tape measure.
- The width of the limpet is not solely dependent on the height of the shore above chart datum and is also dependent on other factors such as the age of the limpet and its microhabitat.
- On the upper shore, the number of limpets is very small and some of our quadrats had not enough limpets for us to measure.