Flat periwinkle Investigation
Brown Flat Periwinkles: Raw Data Collected from the Sheltered Shore of Angle Point
Zone
Quadrat number
Abundance
Running Mean
LOWER
0
0.000
LOWER
2
0
0.000
LOWER
3
0
0.000
LOWER
4
0
0.000
LOWER
5
0
0.000
LOWER
6
0
0.000
LOWER
7
0
0.000
LOWER
8
0
0.000
LOWER
9
0
0.000
LOWER
0
0
0.000
LOWER
1
0
0.000
LOWER
2
0
0.000
LOWER
3
0
0.000
LOWER
4
0
0.000
LOWER
5
0
0.000
LOWER
6
0
0.000
LOWER
7
0
0.000
LOWER
8
0
0.000
LOWER
9
0
0.000
LOWER
20
0
0.000
LOWER
21
0
0.000
LOWER
22
0
0.000
LOWER
23
0
0.000
LOWER
24
0
0.000
LOWER
25
0
0.000
LOWER
26
0
0.000
LOWER
27
0
0.000
LOWER
28
0
0.000
LOWER
29
0
0.000
LOWER
30
0
0.000
LOWER
31
0
0.000
LOWER
32
0
0.000
LOWER
33
0
0.000
LOWER
34
0
0.000
LOWER
35
0
0.000
LOWER
36
0
0.000
LOWER
37
0
0.000
LOWER
38
0
0.000
LOWER
39
0
0.000
LOWER
40
0
0.000
LOWER
41
LOWER
42
LOWER
43
LOWER
44
LOWER
45
LOWER
46
LOWER
47
LOWER
48
LOWER
49
LOWER
50
Zone
Quadrat number
Abundance
Running Mean
Introduction: Aims and Hypothesis
The flat periwinkle exists as two distinct species littorina obtusata and littorina mariae, (http:/lineone.net/wildlife/molluscs_flat_periwinlkle.html) both species are common to the sheltered shores of Great Britain and both species can be found in range of different coloured shells (Fish JD and Fish S 1996). The aim of this investigation is to determine if this difference in shell colour will have a significant effect on the distribution of the flat periwinkle on the sheltered shore of Angle point. The distribution and abundance of the flat periwinkle will ultimately be determined by the interactions that the organism experiences between its environment and other organisms. Thus, if changes in shell colour result in the interactions that the flat periwinkle experiences between its biotic and abiotic environment changing, or if different colours of flat periwinkle carry unique genes that effect the species vulnerability to disease or behavioural patterns for example, then differences in shell colour will have a significant effect on the distribution of the flat periwinkle.
The flat periwinkles interactions with the abiotic environment could change as a result of shell colour. As different colours absorb heat at different rates, with darker colours absorbing more heat then lighter colours, there could be a relationship between the internal temperature of a flat periwinkle and its shell colour. This is significant because the internal temperature of an organism determines the efficiency that its enzymes operate at. Therefore, as the flat periwinkle will compete better when working at an optimum metabolic rate, and as different zones along the sheltered shore can be assumed to be at different temperatures (due to aspect, height, and the relative position of the sea); so it would make sense to predict that rather then an even distribution of different colour of flat periwinkle along the sheltered shore, what will be observed will be distinct zones in which one colour of flat periwinkle dominate to the detriment of the others.
Changes in colour could also effect the biotic interactions that the flat periwinkle will experience. Simply put, it was observed from a previous trip to Angle point that different zones along the sheltered shore had a different visual appearance. Therefore it makes sense to assume that different colours of flat periwinkle would experience different degrees of camouflage depending on where they were located on the shore. Therefore, due to the effect of predation it makes sense to assume that what will be observed will be distinct zones in which one colour of flat periwinkle dominates as a result of its colour that enhances its ability to avoid predator attack.
Hypothesis: As a result of what has been discussed above it is possible to predict that following the collection of data from angle point the subsequent chi squared test will disprove the null hypothesis. I.E the results should show that the ratio of different colours of the flat periwinkle will vary with zonation.
Null Hypothesis: The ratio of different colours of flat periwinkles will be independent of zonation.
Site Justification: To test the hypothesis a site had to be chosen in which flat periwinkles would be abundant. Background Reading (Fish JD and Fish S 1996) revealed that flat periwinkles generally preferred sheltered environments, and as the nearest sheltered shore available was at Angle Point. It was decided that data would be collected from this site. Moreover as the investigation relied on a detailed knowledge of the structure of the shoe (with reference to the upper, middle, and lower shore), a shore had to be chosen for which the zonation was known. Simply put for Angle point it was very easy to find the heights at which the different zones ended and began (A Williams Pers Comm).
Method: How Data was Collected From Angle Point
In conjunction with data provided that listed the heights at which each zone (upper, middle, and lower) began and ended at Angle point (A.Willaims Pers Comm) a 60cm cross staff was used to determine the midpoint of 1st the lower shore then middle shore and finally the upper shore. At this point markers were laid down at the midpoints of the three zones (upper, middle and lower). This was not stated in the planed method, but this amendment had to made because the heights at which each zone began and ended was provided relative to the seas position at a certain time, clearly as the day progressed and the sea came in it would be difficult to accurately account for this and thus assessing the midpoint of each of the zones at a later point would not have been practical.
With the midpoint of each zone clearly identified using markers, a horizontal belt transect of the lower shore was then conducted. The 0.25 meter squared quadrat was laid on the shore and then the numbers of flat periwinkles were counted, noting how many fell into each different colour category. Flat periwinkles were identified by their teardrop shaped operculum, flattened spire and rounded almost spherical shape. Each quadrat was thoroughly checked, ensuring that both the top and bottom of each piece of seaweed was thoroughly examined. Background reading (Charles, 1982) just prior to the experiment revealed that flat periwinkles were likely to be found on Fucus spiralis, F. vesiculosus, Ascophyllum nodosum, and F. Serratus so these species of seaweed were checked with even more thoroughness.
Once the total number of each of the different colours of flat periwinkles had been counted, the quadrat was flipped over on its side 180 degrees to the right ensuring that the right edge remained in contact wit the shore and the above method for counting the numbers of yellow, green and brown flat periwinkles was repeated, and the belt transect repeated of the lower shore in this manner until the tide cam in to such a level that work on the lower shore had to be stopped for that day. At this point the experiment proceeded to the middle shore as indicated by the marker, and a belt transect was conducted at this point in exactly the same manner as it had been on the lower shore. Data was collected until the incoming tide made the collection of data no longer possible and then the same method was repeated on the upper shore. Data was collected in this manner on 2 consecutive days: the 15-16 July 2001.
Method: Amendments made to the Planned Method
Whilst conducting the experiment it was noted that in addition to yellow and brown flat periwinkles, green flat periwinkles existed also, and so the abundance of green flat periwinkles was noted. Whilst counting the numbers of flat periwinkle that were green it was noted that if the surface of some of them was scratched, a green algal covering was removed resulting in a yellow periwinkle being identified instead of a green one. This became apparent after the third quadrat and so results for the first three quadrats of the lower ...
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Method: Amendments made to the Planned Method
Whilst conducting the experiment it was noted that in addition to yellow and brown flat periwinkles, green flat periwinkles existed also, and so the abundance of green flat periwinkles was noted. Whilst counting the numbers of flat periwinkle that were green it was noted that if the surface of some of them was scratched, a green algal covering was removed resulting in a yellow periwinkle being identified instead of a green one. This became apparent after the third quadrat and so results for the first three quadrats of the lower shore should be treated with caution. NB: After the third quadrat of the lower shore all flat periwinkles were carefully checked and the surface scratched to check for the real colour of the flat periwinkle.
Method: The Control of Variables
Ideally the weather would be a constant, as the weather effects the position of organisms within the zones of the shore. However, ultimately the weather is not something that can be controlled, but observations as to what changes in the weather occur will be noted so that any anomalous results can be accounted for.
The other variable that could effect the data is the time at which the data is collected. Simply put, it could be the case that certain colours of the flat periwinkle redistribute themselves along the shore at certain times of the year. However, because the study is being conducted over 2 days it is very unlikely that such redistribution will be evident in the results.
Analysing Evidence: Summary Tables, Graphs & Chi Squared
Table showing the total Number of Yellow, Green, and Brown Flat Periwinkles at each Zone:
Zone
Total Number of Yellow Flat Periwinkles
Total Number of Brown Flat Periwinkles
Total Number of Green Flat Periwinkles
UPPER SHORE
25
0
33
MIDDLE SHORE
40
0
306
LOWER SHORE
200
0
40
Table showing the Total Number of Flat Periwinkles in each Zone:
ZONE
TOTAL NUMBER OF FLAT PERIWINKLES
UPPER SHORE
58
MIDDLE SHORE
346
LOWER SHORE
240
Table listing observations made whilst conducting the experiment:
Upper Shore
Middle Shore
Lower Shore
Dominant seaweed type: F. spiralis
Relatively low percentage seaweed cover 30%; seaweed was very dry.
Crabs observed in this region were larger then those form middle and lower shore.
Dominant Seaweed type: Ascophyllum nodosum; estimated percentage seaweed cover 90%, in quadrats where there was low seaweed cover G. umbilicalus dominated to the detriment of the flat periwinkle. Green Flat periwinkles resemble the bladders on the seaweed. Yellow flat periwinkles were present on F. Serratus, but there was very little F. Serratus present. In areas were no flat periwinkles were present there was little seaweed and G Umbilicus was found on the rocks surfaces.
Dominant Seaweed type: F Serratus
The dominant seaweed appeared yellow when held up to the light.
General observations: 6 crabs were spotted, the majority in the middle shore whilst data was being collected. Lower down the shore the shells of the flat periwinkles collected tended to be very small and often the shells much thicker and almost always yellow. This fits the description of L. mariae (Charles, 1982). Higher up the shore the dominant shell colour was green and these appeared to be much larger then that of the L. mariae found lower down the shore, leading to the conclusion that these were in fact l. obtusata (again conclusion based on descriptions from the Comparative Ecology of Flat winkles).
The ?2 Test: Determining if Distribution is Dependant on Zonation
) Each piece of Raw Data was assigned a letter:
Zone
Total Number of Yellow Flat Periwinkles
Total Number of Brown Flat Periwinkles
Total Number of Green Flat Periwinkles
UPPER SHORE
A: 25
B: 0
C: 133
MIDDLE SHORE
D: 40
E: 0
F: 306
LOWER SHORE
G: 200
H: 0
I: 40
2) Corresponding EXPECTED FREQUENCIES (E) were calculated:
E = ((TOTAL OF ROW CELLS) MULTIPLIED BY (TOTAL OF COLUMN CELLS)) DIVIDED BY (TOTAL OF ALL CELLS)
Cell A: ((133+25) MULTIPLIED BY (25+40+200)) DIVIDED BY (25+40+200+133+306+40)
= (158 MULTIPLIED BY 265) DIVIDED BY 744
= 41870 / 744
= 56.277 (To 3dp)
Cell B: ((158) MULTIPLIED BY (0)) DIVIDED BY 744
= (0) DIVIDED BY 744
= 0
= 0
Cell C: ((158) MULTIPLIED BY (133+306+40)) DIVIDED BY 744
= (158 MULTIPLIED BY 479) DIVIDED BY 744
= 75682 / 744
= 101.723 (To 3dp)
Cell D: ((40+306) MULTIPLIED BY (25+40+200)) DIVIDED BY 744
= (346 MULTIPLIED BY 265) DIVIDED BY 744
= 91690 / 744
= 123.239 (To 3dp)
Cell E: ((346) MULTIPLIED BY (0)) DIVIDED BY 744)
= (346 MULTIPLIED BY 0) DIVIDED BY 744
= 0 / 744
= 0
Cell F: ((346) MULTIPLIED BY (133+306+40)) DIVIDED BY 744
= (346 MULTIPLIED BY 479) DIVIDED BY 744
= 165734 / 744
= 222.761 (To 3dp)
Cell G: ((200+0+40) MULTIPLIED BY (265)) DIVIDED BY 744
= (240 MULTIPLIED BY 265) DIVIDED BY 744
= 63600 / 744
= 85.484 (To 3dp)
Cell H: ((240) MULTIPLIED BY (0)) DIVIDED BY 744
= (240 MULTIPLIED BY 0) DIVIDED BY 744
= 0 / 744
= 0 (To 3dp)
Cell I: ((240) MULTIPLIED BY (479)) DIVIDED BY 744
= (346 MULTIPLIED BY 265) DIVIDED BY 744
= 91690 / 744
= 123.239 (To 3dp)
3) Data calculated in observed cells ' O ' and Expected Cells ' E ' values are then put in the formula
Chi2 = ? ((O-E )2/ E )
a) subtract E from O
b) square the result of step A
c) divide the result of step b by E
d) Add the results of step c
Cell
Step a
Step b
Step c
(O - E)
(O - E) 2
(O - E) 2 / E
A
(25 - 56.277)
= -31.277
978.251
978.251 / 744
= 1.315
B
(0 - 0)
= 0
0
0 / 744
= 0
C
(133-101.723)
= 31.277
978.251
978.251 / 744
= 1.315
D
(40 - 123.239)
= -83.239
6928.731
6928.731 / 744
= 9.313
E
(0 - 0)
= 0
0
0 / 744
= 0
F
(306 - 222.761)
= 83.239
6928.731
978.251 / 744
=1.315
G
(200 - 85.484)
= 114.516
3113.914
3113.914 / 744
= 17.626
H
(0 - 0)
= 0
0
0 / 744
= 0
I
(40 - 123.239)
= -83.239
6928.731
6928.731 / 744
= 9.313
Step d: ?2 = 1.315 + 0 + 1.315 + 9.313 + 0 + 1.315 + 17.626 + 0 + 9.313
= 40.197
4) Find the Degrees of Freedom (df):
df = (number of rows - 1) (number of columns - 1)
Including Brown Flat Periwinkle Data:
df = ( 3 -1 ) ( 3 - 1 )
= 2 times 2
= 4 degrees of freedom
Excluding Brown Flat Periwinkle Data:
df = ( 3 - 1 ) ( 2 - 1 )
= 2 times 1
= 2 degrees of freedom
5) Find the Critical Value:
If df = 2 (excluding brown flat periwinkles from the calculation)
then the critical value = 13.82 (level of significance = 0.001)
If df = 4 (including brown flat periwinkles from the calculation)
then the critical value = 18.46 (level of significance - 0.001)
?2 = 40.197
6) Make a significance decision:
A ?2 value is higher then the critical value (18.46) this allows us to reject the null hypothesis and conclude that the ratio of green to yellow to brown flat periwinkles is independent of zonation. I.E different colours of the flat periwinkle are not evenly distributed along the sheltered shore but instead different zones on the sheltered shore have different flat periwinkle demographs; with each zone have unique ratios of yellow : green : brown flat periwinkle.
A ?2 value 21.737 higher then the critical value of 18.46 means that there is a 99.999% chance of the above statement being correct. This is a very high level of significance and strongly supports the above conclusions.
NB: It is clear form the collected data that brown flat periwinkles WERE NOT PRESENT AT ALL on the sheltered shore of angle point. Hence, when the chi-squared test was preformed not including brown periwinkle data the chi squared value was 4.64 higher (compared to the critical value for two degrees as opposed to four degrees of freedom). What should be noted is that despite the chi squared result, clearly the distribution of brown flat periwinkles was equally spread over the sheltered shore, it didn't matter where on the shore one sampled, there simply were not any brown flat periwinkles.
(Method for chi squared including critical values, taken from Research and Statistics in Psychology Second edition)
(Hugh Coolican; Research Methods and Statistics in Psychology 2nd Edition; Hodder & Sloughton)
The Distribution of Green Flat Periwinkles Per Quadrat across the Sheltered Shore:
Frequency
Lower Shore
Middle Shore
Upper Shore
0
6
2
3
27
0
0
2
7
0
5
3
0
0
3
4
0
1
5
0
0
8
6
0
0
7
0
7
0
8
0
8
0
9
0
9
0
0
0
0
0
1
0
2
0
The Distribution of Yellow Flat Periwinkles Per Quadrat across the Sheltered Shore:
Frequency
Lower Shore
Middle Shore
Upper Shore
0
0
7
5
0
26
25
2
0
7
0
3
0
0
0
4
4
0
0
5
4
0
0
6
2
0
0
Running Means Obtained for Each Colour of Flat Periwinkle (to 2 Significant figures):
Upper Shore
Middle Shore
Lower Shore
Yellow Flat Periwinkle
6.6
.0
5.0
Green Flat Periwinkle
3.3
7.6
.0
Brown Flat Periwinkle
0.0
0.0
0.0
A Graph Showing the Distribution of Yellow Flat Periwinkles Per Quadrat across the Sheltered Shore
A Graph Showing the Distribution of Green Flat Periwinkles Per Quadrat across the Sheltered Shore
Green:
The graph shows that in the majority of quadrats on the lower shore very few green flat periwinkles were found. Significantly, in no quadrat was more then three flat periwinkles found. In the middle shore the graph shows that in most quadrats over six green periwinkles were found with the mode result being eight and the highest number found 11. However, there were anomalous results obtained, the graph shows that whilst in the majority of quadrats high numbers of green periwinkles were found, there were 3 instances in which less then 4 flat green periwinkles were found with 2 quadrats having no flat green periwinkles present at all. Data obtained from the upper shore revealed that on no occasion were more then 6 flat green periwinkles found in a quadrat. The mode result was three with the next most frequent results being four and five. There appear to be three anomalous results where zero flat periwinkles were found.
Yellow:
The graph shows that the upper shore population of yellow periwinkles was very low indeed. The mode result was one with the next most frequent result being zero. In no instance was more then one yellow flat periwinkle found in a quadrat. The population of yellow periwinkles in the middle shore is similarly low, again the mode result is one per quadrat, although significantly perhaps the total number of yellow periwinkles is higher then at the upper shore (for a more clear demonstration of this see the graph showing the relative abundance of flat periwinkles at the upper middle and lower shore). however, on the lower shore the population of flat yellow periwinkles is at it's highest. The mode result was 4.5 and in no quadrat was less then 4 yellow flat periwinkles found.
Discussion and Evaluation
The Chi squared test showed that different colours of flat periwinkles were not evenly distributed along the shore. From the graphs is was clear that on the lower shore yellow flat periwinkles outnumbered green flat periwinkles and no brown flat periwinkles were present at all (200 : 40 : 0). However, on both the upper, and middle shore it was the green flat periwinkle that outnumbered the yellow flat periwinkle. The ratio on the middle shore being 306 Green flat periwinkles compared to just 40 yellow flat periwinkles; whilst on the upper shore the ratio was 133 flat green periwinkles compared to just 25 flat yellow periwinkles).
Whilst previously it was suggested that shell colour was linked to the organisms internal temperature and so could explain the distribution of different colours of the organism along the shore; background reading (Charles 82) revealed this to be unlikely stating " Colours may effect thermal properties of shells but this has not been investigated in L. obtusata and is likely to be unimportant in a mid-shore to low-shore species that shelters among algae during low tide. " On the basis of further research the explanation seems to be based around 3 main themes, visual selection by predators and the distribution of seaweed along the shore and behavioural differences between L. mariae and L. obtusata.
On the lower shore based on the observations made, the smaller yellow flat periwinkles belonged to the species L. mariae, whilst the larger green flat periwinkles were clearly L. obtusata. (Fish JD and Fish S 1996) Now one must ask why L. mariae out compete L. obtusata on the lower shore and the answer is based on study of both organisms innate behaviour.
L. mariae actively selects Fucus Serratus, living in symbiosis with the plant, eating the micro algae and sessile invertebrates found on the fronds of F. Serratus, preventing the plant from obtaining disease (G A Williams; The Comparative Ecology of the Flat Periwinkles).
However, L. obtusata whilst capable of grazing on F. Serratus does not actively select it, instead it's preferred food plant is Ascophyllum nodosum (G A Williams; The Comparative Ecology of the Flat Periwinkles). On the lower shore the dominant seaweed type is F. Serratus and so it makes sense to assume that whilst L mariae graze on F Serratus, L. Obtusata would still be attempting to locate Ascophyllum nodosum. Only when L. obtusata has given searching for Ascophyllum nodosum would it then attempt to feed on the epiphytes found on F Serratus. However, by this time the amount of food available would be significantly reduced, with a significant amount consumed by L. mariae. This would result in L. obtusata being unable to out compete L. mariae and this is one possible explanation for their low abundance on the lower shore as reflected by the running mean on 1.0.
The results can also be explained in terms of visual selection. The lower shore is for the majority of the day covered by the sea. The result of this is that any species found on the lower shore are potentially vulnerable to predation from fish. Bellow is a picture from http://web.uvic.ca/~reimlab/Snails.html. It shows how a yellow and a darker morph of the flat periwinkle would be seen from the common foraging position of Blennius, a common predator on the snails.
- all proportion of F. Serratus it is not going to significantly enhance the darker morphs chances of survival. Similarly when light is reflected from (rather then transmitted through) F. Serratus the seaweed is darker favouring the chances of the darker morph surviving. However, as for the majority of time light is transmitted through the
plant (From information in Reimchen 1979) on the lower shore lighter morphs will always stand a better chance of surviving. Explaining, the data obtained form the lower shore.
REFLECTED LIGHT TRANSMITTED LIGHT
On the middle shore the trend was different. This time green flat periwinkles L. obtusata outnumbered yellow flat periwinkles L. mariae with the ratio of yellow : green being 40 : 306. This can be explained because on the middle shore the dominant seaweed type was Ascophyllum nodosum with only some F. Serratus present. The key point is that whilst L. mariae was able to feed by grazing on F. Serratus it was unable to graze on Ascophyllum nodosum, the reason being that Ascophyllum nodosum produces noxious chemicals that repel L. mariae (Comparative ecology of Flat winkles). L. obtusata is not repelled by these chemicals, it is attracted by them and actively selects Ascophyllum nodosum over other food sources. As most grazers are repelled by these chemicals L. obtusata has a virtual monopoly over the food source (G A Williams; The Comparative ecology of flat winkles). This explains the high running mean of 7.6 green flat periwinkles obtained per quadrat obtained.
The middle shore's high abundance of green flat periwinkles and low abundance of yellow flat periwinkles can also be explained with reference to visual selection by predators. In the observation section it was noted that green periwinkles resembled the air bladders of the algae. This was conformed by the website: http://www.itsligo.ie/biomar/LIT_ROCK/BIOT0050.HTM It stated " L. obtusata - these winkles are found usually on Fucus Vesiculosus and Ascophyllum nodosum where they mimic the air bladders of the algae. " The reason for this is presumably some sort of camouflage. Many species of Bird and Crab are predators of the flat periwinkle (Reimchen, T. E. 1982) and 6 were observed whilst the experiment was being conducted on the middle shore. The same source conformed that both crabs and birds were more likely to notice the yellow flat periwinkle as opposed to the green flat periwinkle, hence further explaining why there was such a high abundance of green periwinkles and relatively low abundance of yellow periwinkles.
Whilst this explains the general trends on the middle shore, at this point it is important to account for the significance of crab predation on Littorina Mariae on the lower shore. Surely if crabs can notice L. mariae on the middle shore and hence limit their population then why doesn't the same apply to the lower shore? The answer is that L. mariae is an annual and its innate reproductive behaviour is adapted to the threat from crab predation. Simply put when the threat from crabs is lowest it reproduces rapidly meaning that there are enough young L. mariae to replenish the population. L. mariae also has a temporal refuge when the crabs have migrated offshore (Reimchen, T. E. 1982).
There were anomalous results obtained from the middle shore. In two quadrats zero flat periwinkles (of any type) were observed. There were three quadrats in which four or less flat periwinkles were observed. In these quadrats instead of flat periwinkles another species, Gibbula umbilicalis dominated the quadrat. Significantly as noted form the observations, in these quadrats there was significantly less seaweed of whatever type present. The following website (http://biology.soton.ac.uk/bs311/swan.shtml) conformed that these species were not uncommon to the sheltered middle shore. Gibbula umbilicalis was found often on the surface of a rock layer and didn't appear to be present on the seaweed at all. However, I have been unable to find a source to confirm this. It seems to be the case that with less seaweed present, the area was more illuminated, and with no seaweed in which L. obtusata could hide itself, the result appears to be that any flat periwinkle species that could have been present in the quadrat would be easily found by predators and eaten. Reasons as to why Gibbula umbilicalis were dominant in such regions were not found nor are they relevant to this investigation.
Below is a summary of the data obtained for the upper shore:
Total Number of Yellow Flat Periwinkles
Total Number of Brown Flat Periwinkles
Total Number of Green Flat Periwinkles
UPPER SHORE
25
0
33
On this shore the dominant seaweed type was observed to be F. spiralis it was also noted that it was very dry. The seaweed cover (over the entire zone) was observed to be the lowest (approximately 30%) and crabs in the region were noticeably larger then those found in the middle or lower shore. As nether L. mariae or L. obtusata actively seeks this species of seaweed the low abundance in terms of the total number of flat periwinkles makes sense (158 the lowest value obtained from any shore). Also because this area represents the highest zone on the shore the effect of desiccation on the flat periwinkle will be at its greatest, again explaining the low abundances, as do the presence of large crabs.
There are two possible reasons as to why the green flat periwinkle outnumbers the yellow flat periwinkle in this zone. The first relates to camouflage with green periwinkles being more readily hidden in the green / brown dry F. spiralis and the second relates to the organism's ability to survive desiccation. It is the later of the two points that is the harder to explain; as an explanation would imply that that the 2 species L mariae and L. obtusata have developed different mechanisms to deal with the effect of desiccation and in order to understand that someone would have to conduct a dissection of the two organisms and analyse their internal structures looking for which organs and or tissues were involved in helping the organism to survive desiccation. I had neither the skill nor the equipment to conduct such a detailed analysis of L mariae and L. obtusata and I have been unable to find any information elsewhere regarding this. Cleary this is an area worthy of further investigation, as it will explain why L. obtusata appear to be more resistant to desiccation, or if they are not, then that the above trend is purely the result of visual selection by predators. However, at this point it is impossible to tell without further investigation.
There is also another question that needs answering: Why were there no brown flat periwinkles present anywhere on the shore? Again no information was found regarding this point, and again it could be the result of slight differences in its internal structure. This is another area worthy of further work and one that I am particularly interested in.
Limitations of the Study & Reliability of Results
In terms of the reliability of the data collected the graphs showing the results collected against the frequency with which they were collected reveal that the data collected was accurate and reliable, the reason being that there are clear ranges that the collected data for each organism falls into in terms of its abundance per quadrat and where there were exceptions (such as on the middle shore) this did not weaken conclusion drawn from the data but was separately analysed and explained.
The reason why the study was limited was because insufficient data was available to conclude why on the upper shore green flat periwinkles outnumbered yellow flat periwinkles. However, details of this have been outlined previously, and it is an area worthy of further investigation.
There were many ways in which the experimental techniques employed were limited. The method involved the manual counting of all the green flat periwinkles and all the yellow flat periwinkles within the quadrat. To do this ever piece of seaweed had to be thoroughly searched and what is important to note is that some flat periwinkles could have been accidentally counted twice or not counted at all. The most significant area fro errors would have occurred when counting the abundance of L. mariae as the species were often so tiny that they were very difficult to find. The next most significant error could have occurred when trying to find green flat periwinkles on the middle shore, as a result of their similar appearance to the air bladders on the see weed; perhaps least significantly another source of error was that other biology students could have caused movement of periwinkles, out of their natural environment.
Another potential source of error occurred out of an attempt to be thorough, in some instances when counting the abundance of flat periwinkles in that quadrat, the top layer of seaweed had to be pushed to one side in order to count the abundance of flat periwinkles on the remaining seaweed. However sometimes, this resulted in seaweed from other regions being dragged into the quadrat being counted. This occurred on very few occasions and ultimately despite these potentials for error, the results of the investigation still made scientific sense when analysed. It is because of this that it is safe to assume that the results collected from Angle point's sheltered shore were both accurate and reliable.
Perhaps most significantly, it is important to note that the experiment was conducted over a period of 2 days only. It would make more sense in order to gain a fuller understanding of the flat periwinkle to conduct the study over a period of several years, to allow for the reading collecting to take fully into account things such as when the population demograph changes I.E when the population consists of mostly adults or when the population consists of mostly juvenile flat periwinkles and the effect that has on the total population. The results obtained from this experiment are akin to a snapshot; they are not the complete picture. What is ideally needed is a thorough investigation that takes into account seasonal variations.
Abstract: An Outline of the Investigation
The aim of the investigation was to examine, and later find the reasons for the ratio of different colours of flat periwinkle (Green : Yellow : Brown) on the middle, upper and lower shores along the sheltered shore of Angle point. The method used to obtain the results was a belt transect of each zone, with the abundance of each colour being counted separately and any additional observations noted. On the upper shore, 25 yellow flat periwinkles were found and 133 green flat periwinkles were found. On the middle shore 306 green flat periwinkles were found and 40 yellow flat periwinkles were found. On the lower shore 200 yellow flat periwinkles , and 40 green flat periwinkles were found. No brown flat periwinkles were found on any part of the shore. Having obtained the results, a chi-squared result of 40.197 (greater then the critical value of 18.46) revealed that there was not an even distribution of different colour of flat periwinkles along the shore. Initially it was thought that different colours would give rise to different thermal properties and hence that would explain the distribution, but this hypothesis was rejected on the basis of stronger evidence.
On the lower shore it was found that the yellow flat periwinkles dominated because they were able to hide more effectively from predators by exploiting the yellow colour that F. Serratus adopts when light is transmitted through it. On the middle shore it was found that L. mariae were unable to feed on the dominant seaweed in the zone leaving L. obtusata with a monopoly over the zone. On the upper shore Green periwinkles dominated because they were better camouflaged in the green / brown F. spiralis and it was thought because they were more resistant to desiccation then the yellow periwinkles were, although more data was needed to confirm this.
In summary the trends in distribution were the result of differences in innate behaviour, visual selection by predators and tolerance to desiccation. However, in the end it was concluded that further work, in particular a detailed dissection of the different species of flat periwinkle (L. obtusata and L. Maria) would be of benefit in order to better understand the reason why green periwinkles out number yellow periwinkles on the upper shore and so determine if it was the result of an increased resistance to desiccation or purely the result of a better ability to hide from predators, or a combination of both.
References:
Fish JD and Fish S 1996; A Students Guide to the Seashore; Cambridge University Press
Gray A Williams; The Comparative Ecology of the Flat Periwinkles, Zoology Department University of Bristol, Bristol BS8 1 UG
http://lineone.net/wildlife/molluscs_flat_periwinlkle.html
http://web.uvic.ca/~reimlab/Snails.html.
http://biology.soton.ac.uk/bs311/swan.shtml
http://www.itsligo.ie/biomar/LIT_ROCK/BIOT0050.HTM
Hugh Coolican; Research Methods and Statistics in Psychology 2nd Edition; Hodder & Sloughton
Reimchen, T. E. 1982. Shell size divergence in Littorina mariae and L. obtusata and predation by crabs
Charles, 1982; Systematics and Evolution of Littorina
Reimchen, T. E. 1979. Substrate heterogeneity, crypsis, and colour polymorphism in an intertidal snail
An Explanation of the Running Mean:
When determining the running mean, first the point at which the data started to level out was noted. This was shown by the raw data as the value where this started to occur is highlighted in yellow. The quadrat number at which the variance of the data seemed to stop, was noted and then doubled. The running mean was the running mean obtained from the quadrat number that was double the point at which it originally started to level out. This value is highlighted also.