Field Trip project
Anna Motylova, Date of experiment 19/07/2019
Investigation of the size of Nucella lapillus shells in relation to their vertical distance from Chart Datum on a sheltered rocky shore.
1. Introduction
N. lapillus has a conical shell with spiral ridges and a pointed apex, usually up to 3 cm high by 2 cm broad and white, but can be grey, brown or yellow with spiral banding (1). The aperture is distinctly oval with a channel leading out of the shell. On sheltered shores, the muscular foot tends to be smaller, therefore the aperture is smaller and the shell also tends to be longer compared to exposed shores (2). The longer shell is a result of less wave action and thus increased growth rate, compared to exposed shores (3). It also acts as protection from crabs, which are more abundant on these sheltered shores (2), as it often exceeds the preferred size range and thus makes them less susceptible to predation (3). They are found in the intertidal zone, from the mid shore downwards. They are gregarious and often found amongst barnacles and mussels on which they feed (1) by boring through their plates and shells using a combination of radula and chemical secretions, such as carbonic anhydrase (4), which help digest the prey (2).
The site is a sheltered rocky shore, Sawdern Point, one of the largest natural harbours in the world with a tidal range of maximum 7.75 metres. GPS 51.687962, -5.056523
2. Hypothesis
N. lapillus shells increase in length as their vertical height above Chart Datum decreases, because the feeding time increases and risk of desiccation decreases.
3. Method
1. Identify the lower shore by locating Fucus serratus.
2. Shoot the Welly from low tide twice (or more/less according to width of the low shore) to reach a vertical distance of 1.6 m above low tide and mark the spot with chalk. Repeat on the other horizontal end of the shore and mark the spot with chalk.
3. Place a tape measure stretched horizontally between the two marked spots to create a transect line (station 1).
4. Place a quadrat on one end of the transect line. Measure the maximum length of each N. lapillus shell with callipers within the quadrat. Record results in a suitable table.
5. Move the quadrat to sample an area adjacent to the previous one, i.e. no spaced sampling. Continue along the whole transect line until 30 specimens are sampled.
6. Shoot the welly 5 times to reach a height of 4 m above station 1 and mark the spot using chalk. Repeat on the other horizontal end of the shore and mark the spot using chalk. Stretch a tape measure between the marked spots to create a transect line (station 2). Make sure the station is in the barnacle zone.
7. Sample all N. lapillus specimens using the same method as in station 2 until 30 specimens are sampled.
8. Calculate the mean length in both transect belts.
Piece of equipment Justification
Shoot the welly To measure the vertical distance of transect lines above chart datum.
Tape measure To maintain a constant height of the transect line above Chart Datum.
Chalk To mark points above Chart Datum and stretch tape measure between them.
Quadrat To avoid bias sampling.
Vernier Callipers To measure the length of shells to the accuracy of 0.05 mm.
Pencil To protect raw data from water damage.
Macroalgae and N. lapillus To correctly identify the lower shore area and N. lapillus
identification guide. specimens.
4. Risk assessment
Hazard Risk Control measure
Microalgae Slippery rocks Walking boots
Sun Sun burn Sun cream
Wet and windy weather Hypothermia Water proof layered clothing
Lack of water intake Dehydration Water bottle always ready
Tide Being trapped on a tall rock Be aware of low tide time surrounded by water
Digestive enzymes and Skin irritation Minimise contact with the
acid secreted by dog whelk muscular foot of specimens.
5. Null hypothesis
There is no difference in the mean size of the N. lapillus shells on the lower shore and the upper shore of a sheltered rocky shore.
6. Results
time of low tide = 13:32
height of low tide above chart datum = 1.25 m
height of station 1 above chart datum = 1.25 + 1.6 = 2.85 m
height of station 2 above chart datum = 1.25 + 4 = 5.25 m
difference between the height of station 1 and station 2 above chart datum = 2.4 m
7. Analysis of results
Lower shore specimens Mid shore specimens
Mean 27.00 25.58
n 30.00 30.00
s 3.78 2.57
s^2 14.32 6.59
s^2÷ n 0.48 0.22
t1=1.70
degrees of freedom = 30+30-2 = 58
critical value at p 0.05 = 2.00
The calculated value t1 ˂ critical value, therefore the null hypothesis must be accepted. There is more than 95% probability that the difference in the mean shell length of Nucella lapillus on the lower and mid shore is due to chance.
8. Conclusion
A general trend of a smaller shell length on the mid shore than on the lower shore was observed. This trend can be explained by the fact that as the height above chart datum increases, the feeding time of the animal increases, as shells and plates of mussels5 and barnacles6 (main prey of dog whelk) open slightly when submerged, making it easier to reach the inside of the prey. More nutrition means a faster growth rate and results in a longer shell3. Furthermore, feeding underwater means a lower risk of desiccation.1
All specimens were found exclusively in rock pools and often under rocks, which can be explained as protection from desiccation, but as it involved all specimens, it can be assumed that this fact had no effect on the results.
According to a preliminary study, the ratio of the maximum shell length and the maximum diameter of aperture proved to be constant. It was therefore decided to only include the measurement of shell length in the method, as it is more accurate and less intrusive to the animal. It also proved to be indicative of its age and feeding frequency. Other methods of expressing “size” are available, such as weighing or the water displacement method. However, these do not account for the water content of the specimen, therefore the shell length approach was favoured.
To increase the potential difference in feeding time, one or two additional 0.8 m steps of vertical height between the stations could be included, provided station 2 would still be located in the barnacle zone.
On station 1 (lower shore), four specimens (see results table*) that were noticeably shorter and had a thinner outer lip1 of the aperture than the other specimens found. This gives some evidence to support the theory that they might be juveniles spawned in spring two years ago based on the measurements of growth rate of N. lapillus 3, but there are many other factors that might affect the morphology of the specimens, therefore they cannot be confidently classified as juveniles.
However, a second t-test that was performed excluding these values gave a significant value.
Lower shore specimens Mid shore specimens
Mean 28.21 25.58
n 26 30
s 1.38 2.57
s^2 1.89 6.59
s^2 ÷ n 0.07 0.22
t2=4.88
degrees of freedom = 26+30-2 = 54
critical value at p 0.05 = 2.00
critical value at p 0.001 = 3.46
The calculated value t2 ˃ critical value, therefore the null hypothesis must be rejected. There is more than 95% probability than the difference in the mean shell length of Nucella lapillus on the lower and mid shore is not due to chance.
In fact, the calculated value t2 ˃ critical value even at p = 0.001, therefore there is more than 99.99% probability that the difference in the mean shell length of Nucella lapillus on the lower and mid shore is not due to chance.
Therefore, perhaps the greatest factor that decreased the accuracy of the results is sample size. The fact that only 30 samples from each station were collected due to time pressure meant that the four individuals of substantially shorter length on station 1 affected the standard deviation significantly, and therefore the calculated t value was below the critical value. The second t-test gave a dramatically greater t value. Therefore, I suggest that increasing the sample size to 50 on each station would decrease the effect on the overall mean and standard deviation of the substantially shorter individuals and give a more reliable value.
Further research investigating the relationship of the outer lip thickness of N. lapillus and its age could provide a reliable method of excluding any immature individuals.
9. Bibliography
1. Tyler-Walters, H., 2007. Nucella lapillus Dog whelk. In Tyler-Walters H. and Hiscock K. (eds) Marine Life Information Network: Biology and Sensitivity Key Information Reviews, [on-line]. Plymouth: Marine Biological Association of the United Kingdom. [accessed 19-06-2019]. Available at: https://www.marlin.ac.uk/species/detail/1501
2. Richard, B., Barker, J. and Rickayzen, S. (2007). Ocean: Revealing the Secrets of the Deep. Parragon Publishing India Pvt Limited, p.104
3. Tyler, L. (n.d.). BIOTIC Species Information for Nucella lapillus. [online] BIOTIC - Biological Traits Information Catalogue. Available at: http://www.marlin.ac.uk/biotic/browse.php?sp=4288 [Accessed 19 Jun. 2019].
4. Crothers, J. (1985). Dog-whelks: An Introduction to the Biology of Nucella lapillus (L.). Field Studies, (6), pp.291-360.
5. Nordsieck, R. (2019). The Blue or Common Mussel (Mytilus edulis). [online] The Living World of Molluscs. Available at: http://www.molluscs.at/bivalvia/index.html?/bivalvia/common_mussel.html [Accessed 19 Jun. 2019].
6. Richard, B., Barker, J. and Rickayzen, S. (2007). Ocean: Revealing the Secrets of the Deep. Parragon Publishing India Pvt Limited, p.103
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