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Investigating the colour variation of Littorina littoralis and their abundance across the upper, middle and lower zones of a rocky shore

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Introduction

Abstract The objective of this study was to see whether the abundance of different shell colours of Littorina littoralis were distributed in any significant way over the upper, middle and lower zones of a rocky shore. The study took place on Porth Cwyfan, Wales, and involved taking readings of shell colours over each zone using an interrupted belt transect. Using the chi-squared it was shown that there was significance in how the different shell-coloured Littorina littoralis were distributed. Section 1 - Introduction Littorina littoralis are gastropod molluscs distributed widely in north-west Europe [1]. The species is known colloquially as the 'flat periwinkle', due to its shell's flattened spire. Littorina littoralis exist in a variety of colours, usually appearing olive-green, but yellow, brown, banded and criss-cross patterned varieties are also common [1]. Figure 1.1 shows a selection of Littorina littoralis shell colours. These colours are related to the degree of exposure to wave action, with olive-green shells being dominant on sheltered shores and criss-cross brown shells on exposed shores [1]. These differences are believed to be maintained by visual selection of predators, such as shore fishes [1]. Throughout the day, most coastal areas experience two high tides and two low tides. High tides cause the sea to flood in, submerging the entire rocky shore while low tides expose the shore to harsh winds and extremes in sunlight The exposure to sunlight varies along the rocky shore. The lower zone of the rocky shore is predominantly covered by the sea for most of the day. British shores are particularly muddy-brown in colour, and so the majority of sunlight will be blocked before it can reach the sea bed. The upper zone, however, is exposed to air and sunlight for the majority of the day, only becoming submerged at full tide. Even at full tide, the sea would be very shallow and so significantly more sunlight would reach the sea bed here than in the lower zone. Littorina littoralis live in association with certain fucuoid algae. ...read more.

Middle

Once the samples were finished with, they were returned safely into the same quadrat in which they were found. Table 3.1: The number of Littorina littoralis in the upper zone of the rocky shore Number of individuals found in each 0.25m2 quadrat Colour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Total Black 0 0 2 0 1 0 0 0 0 0 0 0 0 0 0 3 Yellow 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 2 Green 0 2 1 0 0 1 0 1 0 0 0 0 0 0 0 5 Orange 0 0 0 1 0 0 0 0 0 0 2 0 0 2 0 5 Brown 0 0 0 1 0 0 0 0 1 0 1 0 0 0 0 3 Table 3.2: The number of Littorina littoralis in the middle zone of the rocky shore Number of individuals found in each 0.25m2 quadrat Colour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Total Black 5 1 0 0 4 2 0 1 0 3 3 1 0 5 0 25 Yellow 2 0 1 0 0 0 0 3 0 0 0 0 0 0 0 6 Green 0 0 0 0 1 7 3 1 1 0 0 0 0 0 1 14 Orange 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 2 Brown 0 0 2 0 1 0 0 0 0 0 1 0 0 0 0 4 Table 3.3: The number of Littorina littoralis in the lower zone of the rocky shore Number of individuals found in each 0.25m2 quadrat Colour 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Total Black 8 8 7 13 8 1 5 2 3 8 5 9 5 4 6 92 Yellow 0 0 0 2 1 2 0 ...read more.

Conclusion

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. ...read more.

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