When the theory of natural selection is linked to the idea that the environmental conditions are varied along a rocky shore, it may be possible that different shell-coloured Littorina littoralis have each become adapted to best suit one of these environments.
The habitat of the Littorina littoralis is related to the location of fucuoid algae, in particular Ascophyllum nodosum, Focus vesiculosus and Focus serratus. The Littorina littoralis feeds upon these seaweeds [3], and also uses them to maintain moisture when exposed to the air. Therefore the distribution of these algae will also be a factor of Littorina littoralis distribution on a rocky shore.
The size of the Littorina littoralis population is affected by a set of ‘limiting factors’. These are conditions that limit the size of the population. These factors can be either density-dependent or density-independent.
Density-dependent factors are affected by the population density itself. For example, if there was fatal disease affecting Littorina littoralis, the disease will spread more easily in a dense population. Therefore, disease is a density-dependent factor. Other density-dependent factors include parasites, competition, and predation. These are generally biotic factors. [4]
Density-independent factors are not affected by the population density. These factors are generally abiotic factors, such as the weather or a destructive fire. These are factors in which any one member of the species has the same chance of survival as any other [4].
If the general population of Littorina littoralis ever increased, any of their predators will themselves thrive due to an increased food source. Hence at one point in time, the numbers of both predator and prey will increase. There will however come a time where the predators will be consuming large numbers of Littorina littoralis, and so their population decreases. Logically, a decrease in food source will be followed by a decrease in predator numbers through intraspecific competition. The decrease in predator numbers will cause the population of Littorina littoralis to increase, as there is less threat of predation, and so this cycle repeats. See figure 1.2 below.
Hypothesis
There is a significant difference between the shore zone and the abundance of different shell colours of Littorina littoralis.
Null hypothesis
There is no significant difference between the shore zone and the abundance of different shell colours of Littorina littoralis.
Predictions
In this investigation, it was predicted that predict that the abundance of living Littorina littoralis would decrease when moving from the lower zone to the upper zone. This may be the case as the environment becomes drier, the Littorina littoralis will themselves dry out and die, known as desiccation.
It was also predicted that towards the lower zone, the darker coloured shells would be more abundant. This may be because the environment is predominantly dark coloured, with a longer duration of sea cover and a thicker layer of sea weeds than that of the upper zone. This would provide better camouflage for those shell colours which can best mimic this range of colours, leading to a better resistance to predation. Similarly, as the upper zone will consist of more dry sand and some lighter coloured pebbles, it was predicted that lighter coloured shells like yellows and orange, which best mimic these colours and are therefore more camouflage, so predators would have more difficulty seeing them.
It was also predicted that throughout the stretch of the rocky shore, a higher proportion of Littorina littoralis will have dark coloured shells. The majority of the rocky shore is covered in dark grey coloured rocks, and wet sand. The Littorina littoralis with shell colours of grey or black will appear more camouflaged, and so have increased protection against predation
Section 2 - Materials and Methods
The apparatus used in this investigation is discussed below in Table 2.1
When investigating on a rocky shore, certain aspects of the landscape must be recorded. To level the rocky shore, a clinometer and two measuring poles are used. The measuring poles should ideally have identical markers. This is so that the clinometer can be placed a set distance up the pole, and then a reference point is created to observe through the clinometer
First, the measuring poles are held vertically at the top and bottom of the shore. The clinometer is held level with a reference point on one of two measuring poles See figure 2.1 below.
Looking through the clinometer, the reference point is lined up, and a reading is taken
After this, the distance between the two poles is measured, giving the length of the rocky shore. With the length of the beach measured, it is then divided into the three zones: upper, middle and lower.
Before beginning to take results, one side of each zone was declared ‘sacred’ – that is, it must not be stood on or disturbed in anyway before results were taken.
In this investigation, results were taken along each zone in an interrupted belt transect; every 2m, a 0.25m2 quadrat was used to take results.
Risk assessment
This investigation involved working on a rocky shore. There were many hazards which had to be addressed before starting to collect data
The rocky shore was understandably very slippery, especially while wet. This being the case , it was necessary to wear appropriate outdoor footwear to ensure sound grip to walk across the rocky shore. It was still necessary to take care while walking along the rocky shore, as there may have been uneven and possibly unsecured rocks.
On the rocky shore, there was the risk of the tides coming in quickly unexpectedly. To ensure this was not the case, it was planned beforehand the time of the investigation in accordance with local tide logs. This ensured ample time to carry out the investigation without the worry of tides coming in.
There was always possibility of extreme weather making it unsuitable to carry out the investigation. Heavy rain may have significantly reduced visibility which would lead to difficulty collecting results, and would have affected those supervising the activity. To overcome this, there were multiple days possible to take the investigation, and using local weather forecasts, the most appropriate day was chosen.
In this investigation, there were others around undergoing similar investigations. Each action may have affected other people’s investigation in ways that may be damaging to both their results and welfare. It was therefore established the areas in which it were possible to walk, and areas to avoid.
Working on the rocky shore meant that it was possible, to become out of sight of those supervising the activity. The supervisors therefore, made regular checks on those carrying out investigations
This investigation involved the handling of organisms, and therefore the temporarily removing them from their natural habitat. To ensure their wellbeing, they were all unharmed and handled with care. Once the samples were finished with, they were returned safely into the same quadrat in which they were found.
Graph 3.1 shows that in the lower zone of the rocky shore, black-shelled Littorina littoralis were the most abundant of all the colours. This was followed by green, brown then yellow. There were no orange-shelled Littorina littoralis present in the lower zone of the rocky shore.
Generally, it is shown here that the more abundant shell-colours were those of a darker colour. There were much fewer recordings of the brighter shell colours, namely yellow and orange, than there were of darker shell colours, namely black and greens.
Graph 3.2 shows that black-shelled Littorina littoralis were the most abundant shell colour in the middle zone of the rocky shore. This was followed by the green-shelled, then the yellow-shelled, brown-shelled and finally the orange shelled Littorina littoralis.
Graph 3.3 shows that the two most abundant shell colours of Littorina littoralis in the middle zone of the rocky shore were the green-shelled and orange-shelled of which there were 5 organisms recorded. This is then followed by black-shelled and brown-shelled Littorina littoralis, both with 3 recordings. The yellow-shelled Littorina littoralis were the least abundant colour found in this zone of the species.
Generally, this graph shows that there was a more even mix of the different shell-colours in the upper zone. It is not apparent that any one shell-colour is considerably more abundant than any other.
From this graph it can be seen that by moving closer to the land-side of the rocky shore, the general trend was that each shell colour decreased in number. This was the case for every shell-colour except for the orange-shelled Littorina littoralis. The number of orange-shelled Littorina littoralis actually increased when moving closer to the land-side of the rocky shore.
There were 91 black-shelled Littorina littoralis in the lower zone, but only 3 were found in the upper zone. This means that there are nearly 31 times more black-shelled Littorina littoralis in the lower zone than there are in the upper zone. This is a very significant difference, and suggests a harsh decrease in black-shelled Littorina littoralis closer to the upper zone. With 25 black-shelled Littorina littoralis being found in the middle zone, this gives a ratio of 1 : 8.3 : 30.7 of upper : middle : lower.
The abundance of yellow-shelled did not vary as much as the black-shelled Littorina littoralis. The ratio of yellow-shelled Littorina littoralis is simply 1 : 3 : 4 of upper : middle : lower, with there being just 2 recordings on the upper zone.
The green shelled Littorina littoralis were significantly more abundant in the lower zone than they were in the upper zone. There were 60 green-shelled Littorina littoralis in the lower zone and 5 in the upper zone. This means there were 12 times more green shelled Littorina littoralis found in the lower than in the upper. The ratio of these were 1 : 2.8 : 12 of upper : middle : lower.
The orange-shelled Littorina littoralis did not match the pattern of all the other shell colours. Whereas other colours decreased as they got closer to the land-side of the shore, orange-shelled Littorina littoralis actually increased in abundance. However, there were no orange organisms found in the lower zone, 2 in the middle zone and 5 in the upper zone. It is the only shell colour to be most abundant in the upper zone.
Brown-shelled Littorina littoralis were more abundant in the lower zone. There were 18 in the lower zone and 3 in the upper zone, meaning there were 6 times as many in the lower zone than the upper zone. The ratio of these were
1 : 1.3 : 6 of upper : middle : lower.
Graph 3.5 shows how the average density of each quadrat
The Chi-squared test aims to observe differences between comparable sets of data grouped into classes. The chi-squared value was compared to a value taken from a significance table to confirm whether any deviation was through to 'chance', or whether it was statistically significant. The value taken from the significance table was 5%, which was a significant amount for this investigation.
The null hypothesis for this investigation was that there is no significant relationship between shell colour of Littorina littoralis and its location on a rocky shore
The alternate hypothesis for this investigation was that there is a significant relationship between shell colour of Littorina littoralis and its location on a rocky shore
Degrees of freedom = (3-1) x (5-1) = 8
The critical value taken from a chi-squared table provides a 95% certainty towards supporting the hypothesis. The critical value for this investigation was 15.51
Table 3.2b shows how I have calculated the chi-squared value.
54.40 > critical value (15.54)
Therefore reject the null hypothesis and accept the alternate hypothesis:
There was a significant relationship between shell colour of Littorina littoralis and its location on this particular rocky shore, Porth Cwyfan.
Section 4 - Discussion and Evaluation
The distribution Littorina littoralis and their abundance along the rocky shore is affected heavily by predation. Shell colour provides the most variability between the Littorina littoralis. The species increases its ability to camouflage by adapting shell colours to mimic their environment. Contrasting or vivid colours will be much less camouflage than those which can mimic their environment. A member of the species which makes itself more easily available to predators will have less chance of survival, and therefore through natural selection will have these undesirable phenotypes gradually removed from the gene pool.
The results of this experiment indicate that the abundance of every shell colour except orange decreases by moving closer to the land side of the rocky shore. The orange-shelled Littorina littoralis actually increase in number the closer they are towards the land side.
The lower zone of the rocky shore is covered in sea water, and so the sea floor there has very low levels of light. Dark shell colours therefore have an advantage here, as their colours are a better mimic of this environment. Looking at Graph 3.1, there is strong evidence to support the idea that black-shelled Littorina littoralis are the more dominant in the lower zone. Their shell colour is better adapted to the lower zone than any other shell colour in this investigation. A possibility is that natural selection has increased the chances of survival for the black-shelled Littorina littoralis found in the lower zone, and so as a result they have become the dominant shell colour here.
In the low light levels of the lower zone, those shell colours which were more contrasting would therefore be much more easily identified by predators. Yellow and orange-shelled Littorina littoralis would therefore have been at a significant disadvantage in the lower zone. As Graph 3.1 shows, there were very low numbers of yellow-shelled recorded in the lower zone, and actually no orange-shelled Littorina littoralis. This provides evidence to suggest that brighter shell colours contrasting with dark environments caused significant decrease in the chances of survival of the Littorina littoralis.
Graph 3.4 shows an increase in the abundance of orange-shelled Littorina littoralis moving closer to the land side of the shore. I believe that this is the case because of the changing environmental conditions closer to the land side. The upper zone contained significantly more sunlight and so had the introduction of brighter coloured sand, which appears faint orange in colour. This creates an environment much better matching the orange shell-colour.
It may be the case that those organisms with this shell colour have slowly adapted to better mimic the sand-like colour found on the rocky shore. Therefore, any of the species found in the upper zone has an increased chance of survival compared to those whose shell colour contrasts this colour. Graph 3.3 provides evidence to suggest that those Littorina littoralis with orange shells are overall best suited to live in areas which are predominantly covered by sand. The upper zone of this rocky shore predominantly covered in sand, and so their chances of survival here are increased.
It may be the case that because those Littorina littoralis are more camouflaged to the upper zone and that terrestrial species cannot identify them as easily, they have very little worries about predation while living in the upper zone when it is exposed. Living in the upper zone significantly reduces the exposure that the species has to marine predators.
The reason that orange and yellow-shelled Littorina littoralis numbers are generally quite low could be due to one of two reasons. Firstly, in order to live in the upper zone, the species must be able to tolerate the dry conditions and high levels of sunlight, of which the Littorina littoralis cannot. Secondly, as the tide comes into the upper zone, the orange-shelled Littorina littoralis are again covered in sea water, reintroducing the low levels of sunlight and marine predators, making them much more identifiable to predators.
The distribution of orange or yellow-shelled Littorina littoralis could also be directly affected by the distribution of Fucus vesiculosus. These contain gas-filled vesicles which appear olive in colour, on which it may be possible for the species to appear camouflaged against.
Green-shelled Littorina littoralis have the advantage of being able to mimic the green colour of many types of seaweed which appear over most of the rocky shore. This adaptation to mimic its green surrounding would lead to these being less easily identifiable by predators and so increase their chances of survival. However, in areas with no seaweed cover, this species may appear more contrasting, and hence the reason that the green-shelled Littorina littoralis are not the most dominant shell colour overall.
Graph 3.3 shows that the green-shelled Littorina littoralis are joint first in being the most dominant shell colour for the upper zone.
The brown-shelled Littorina littoralis are in relatively low numbers compared to the black or green-shelled Littorina littoralis. The brown-shelled Littorina littoralis are actually reddish-brown in colour, meaning they would stand out slightly against a grey rock or green seaweed. Throughout certain areas of the middle zone, there existed dark brown coloured seaweed, which could help the brown-shelled Littorina littoralis appear more camouflaged.
In this investigation, I feel an unanswered issue is the natural movement of this species along the shore. It is not known whether some or all Littorina littoralis actively seek out their optimum environment, or whether they are randomly distributed by factors such as wave action. Both of these processes would lead to those Littorina littoralis unsuited to the environment to be found in low numbers there, and those best suited to be found in higher numbers. Further work into this would provide considerable additional evidence to help support this investigation.
Although every effort was made make sure that the data collect is valid, this does not mean that the data itself is a true reflection of this species everywhere. The data collected was limited to one particular rocky shore over a period of roughly three hours at one time of the year. I cannot be certain of how the data from this rocky shore is a reflection of all Littorina littoralis. The results for each zone of the rocky shore deviate considerably in places, meaning that I cannot claim their truth to a one hundred per cent certainty. Any conclusions drawn from these results will therefore be a reflection of this variability in data.
Section 5 - References
