Colour variation in Littorina littoralis on the upper, middle and lower zones of a rocky shore

A2 Biology Coursework

2008

SECTION 1 - PLAN

Introduction

In this investigation, I will be looking at the abundance of different colours of Littorina littoralis and their distribution over the upper, middle and lower zones of a rocky shore. Figure 1 shows a typical yellow-shelled Littorina littoralis.

The Littorina littoralis are more commonly known as the ‘flat’ periwinkle, due to their shell’s flattened spire. They are common to all British and Irish shores where brown seaweeds are found [1]. Its shell is variable in colour, usually appearing olive-green, but yellow, brown, banded and criss-cross patterned varieties are also common .

I will collect my data from Porth Cwyfan, Wales (Grid Reference SH 338 683) on the morning and early afternoon of Thursday the 9th of October, 2008.

Theory

Throughout the day, tides cause the sea level to fluctuate up and down, causing the shore to be cyclically exposed to air and submerged by the sea. By doing this, varying environmental conditions are formed in one stretch of shore. Most coastal areas experience two high tides and two low tides each day, each causing the environmental conditions of the shore to change.

There are many conditions which are altered by the tides. The duration of exposure to sunlight will vary along the rocky shore, with the lower zone being submerged by seawater throughout most of the day, blocking – partially or otherwise – sunlight reaching the sea floor. The upper zone of the rocky shore will be predominantly exposed to sunlight for most of the day. Although sea water does reach the upper zone, its coverage is very shallow, and would not completely block sunlight reaching the sea floor.

Prolonged exposure to sunlight can cause Littorina littoralis to dry out and die, known as desiccation. However, a zone on the rocky shore which has no exposure at all to sunlight will be very unlikely to have any plants or seaweed there, as sunlight is needed for plants to photosynthesize. The seaweeds are the main source of food for periwinkles. Therefore, no sunlight would lead little or no plant life, further leading to little or no Littorina littoralis. If it were the case that the upper zone has 100% exposure to sunlight, and the lower zone has 0% exposure to sunlight, it is very likely that most periwinkles would be found in between these two extremes. Although this is not the case, this provides a rough idea of how I will expect the Littorina littoralis to be distributed.

Being submerged by the sea can also help species hide from terrestrial predators. Predation is an important biotic factor on rocky shores especially for Littorina littoralis, whose only defence mechanism is its shell.

In any environment, organisms will struggle to survive, and compete for resources. Competition between a single species is referred to as intraspecific competition. Competition between different species is called interspecific competition, and this type of competition will affects the population distribution of a species.

If an organism’s phenotype is well adapted to suit the environment it is in, it will have a better chance of survival, and hence a better chance to reproduce, passing on its desirable phenotype to its offspring. The offspring should then have the same increased chances of survival and reproduction. Organisms with undesirable phenotypes will have decreased chances of survival, and hence are less likely to reproduce. When this organism dies, its undesirable phenotypes are then removed from the gene pool.

This process, by which favourable phenotypes become more common and unfavourable phenotypes become less common, is called natural selection. Over many years, it can cause a species to tend towards an optimum phenotype to suit one particular environment. Phenotypic variation can come from freak mutations in an organism’s genotype, which cause it to have an advantage over other members of its own species. The chance, however, of a freak mutation having a positive effect on an organism’s chances of survival is very rare.

In the case of Littorina littoralis, its variation in shell colour will have been the result of ...

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In the case of Littorina littoralis, its variation in shell colour will have been the result of natural selection. The present ranges of shell colours in existence have been favourable towards its chances of survival.

By linking natural selection with the idea of varying environmental conditions on a rocky shore as a result of tides, it may be the case that different shell colours of Littorina littoralis have each become adapted to suit a different environmental condition on the rocky shore. This investigation looks to see if this is the case, and if so, will attempt to justify the reasons.

If the population of Littorina littoralis were to increase, predators which feed from them will themselves thrive due to the increase in food source. Hence, at one point in time, both predator and prey populations 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. Figure 2 below represents this idea.

The population size of Littorina littoralis will be limited by factors such as food availability, water supply, environmental conditions and living space [2]. Density-dependent factors will affect the size of population the Littorina littoralis population and will be dependent on the size of the population. These have a greater impact as the population gets more dense [3]. For example, a disease can spread much faster between Littorina littoralis if there are large numbers living in close proximity to one another compared to low numbers living far apart. These factors are usually biotic.

Density independent factors will affect the size of the Littorina littoralis population, and will not relate to population density itself [3]. These are generally abiotic factors such as the weather or freak events like fires.

The carrying capacity of an area is defined as follows:

“The maximum number of a species that can be supported indefinitely by a particular habitat, allowing for seasonal and random changes, without degradation of the environment and without diminishing carrying capacity in the future” [G. Hardin, 1977] [4].

If the population of Littorina littoralis were to reach the carrying capacity, the species would undergo interspecific competition over things such as food, water and habitat. In turn, this causes the population to drop slightly. This would then decrease the level of competition and so numbers would increase. The overall effect of this would be that population of Littorina littoralis would fluctuate slightly above and below the carrying capacity. See Figure 3 for a representation of this process.

The term niche refers to the biological ‘role’ of an organism in its environment. This describes behaviours such as how the organism functions, what kind of food it eats, its habitat and so on. In the case of Littorina littoralis, its niche can be described as a gastropod molluscs found on the shore feeding from brown seaweeds. Each species must have its own unique niche; otherwise the two species sharing the same niche will compete with each other, eventually causing one of the species to become dominant.

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, I predict that the number of living Littorina littoralis will decrease as I move closer to upper zone. I believe this is the case because as the environment becomes drier, the Littorina littoralis will themselves dry out and die.

I also predict that towards the lower zone, the darker coloured shells will be more abundant. This is 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, I predict that it will be the lighter coloured shells like yellows and orange which best mimic these colours will more well camouflage and so predators would have more difficulty seeing these shell colours.

I also predict 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
Control Variables

Preliminary work

The day before my investigation, I visited Porth Cwyfan. Here, I decided upon a suitable length of shore to carry out my investigation. I measured the length of the rocky shore and left a number of markers to identify the next day.

The location of each of the 3 zones is proportional to the length of the rocky shore. The entire length of the shore is approximately 180 meters. The upper and lower zones are located 30 meters inwards from each end of the rocky shore, and the middle zone is located between the two. See Table 3 and Figure 4 below.

It should be noticed that the distances represented here are only approximate values.

Levelling

Secure a level pole vertically at the top of the shore (see Figure 5, ‘A’)
Attach one end of a tape measure to the bottom of the pole.
Holding the other end of the tape measure, walk to the bottom of the rocky shore.
When the bottom of the shore is reached, secure another level pole vertically (see Figure 5, ‘B’)
Attach the other end of the tape measure to the bottom of this pole.
Measure the distance between the two poles.
Use a clinometer (located at Figure 5, ‘C’) to measure the angle of decline/incline between the two poles using a set marker on each of the identical poles.

Collecting data

Place the 0.25m2 quadrat at the start of the measuring tape that stretches along one zone of the shore
Collect any Littorina littoralis present and place them carefully into the tray. Discard any dead samples
Divide the Littorina littoralis within the tray into groups based on the list of possible colours.
Record the results
Return the Littorina littoralis into the 0.25m2 quadrat in which they were found
Move the quadrat 2m along an interrupted belt transect and repeat steps 1 to 5 until the end of the measuring tape is reached
When the end of the measuring tape is reached, move to another zone and repeat steps 1 to 6

For this investigation, I do not feel that there is a need to repeat each zone. I feel that one accurate recording of each tidal zone will be an accurate representation of this particular rocky shore. Repeating each tidal zone would also be time consuming, and may not be possible to do within the duration of 1 tide.

During the investigation, Littorina littoralis will be collected and then returned to within 0.25m2 of where they were found. This, however was not their exact location. This means that if I did repeat a zone, some species will have been moved from their original location, which has the potential to provide slightly altered data.
Risk assessment

In this investigation I will be working on a rocky shore. There are many hazards which must be addressed.

The rocky shore I will be working on will be very slippery, especially while wet. To ensure my own safety, appropriate outdoor footwear will be worn and properly secured before I enter the rocky shore. This will ensure sound grip for me to walk across the rocky shore. However, care must still be taken as the rock surfaces will be uneven and possibly unsecure. If I come across rocks which are unsecure, I will make those working along me aware of it and will avoid using it myself.

On the rocky shore, there is the risk of the tides coming in quickly and catching me off guard. To ensure this is not the case, I will plan beforehand the time of my investigation in accordance with local tide logs. This will ensure that I have ample time to carry out my investigation without the worry of tides coming in.

There is always the chance of extreme weather making it unsuitable for me to carry out my investigation. Heavy rain may significantly reduce visibility which would affect both me and those supervising the activity. To overcome this, there will be more than one day in which it is possible for me to take the investigation, and using local weather forecasts, the day which has the best suited weather will be chosen.

In my investigation, there will be others around me undergoing similar investigations. My actions may affect their investigation in ways that may be damaging to both their results and welfare. As this is the case, good communication between me and others will help decide the course of action. It will be planned where we as a group can and cannot walk and who will be working where. I will also take consideration for how my actions will affect others.

Working on a rocky shore may mean that at times I may be isolated in collecting data. To ensure my safety, there will be supervisors making regular checks on the safety of both myself and others on the rocky shore.

This investigation involves the handling of organisms. To ensure their wellbeing, they will all be unharmed and handled with care. Once I have finished with any organisms, I will return them safely to within the same 0.25m2 quadrat in which they were found.

Statistical test

I will be using the Chi-squared test, which aims to observe differences between comparable sets of data. The data itself is collected and grouped into classes. The 'null' hypothesis for this investigation will say that that the data is distributed randomly and has no trend.

The chi-squared value is compared to a value taken from a significance table to confirm whether any deviation is through to 'chance', or whether it is statistically significant. The value taken from the significance table will be of 5%, which is a significant amount for this investigation.

References