My pilot study and extended investigation is to be conducted at Hunt's Bay, Gower.
INTRODUCTION
My pilot study and extended investigation is to be conducted at Hunt's Bay, Gower.
Hunt's Bay or Deep Slade is situated on the Gower Peninsula facing south. The OS (ordinance survey) grid reference of hunt's bay, from a mumbles and south Gower map is 564 868 (see appendix 1). It is a rocky beach. The rocks are made up of carboniferous limestone, a sedimentary rock. We decided that the bedding planes at Hunt's Bay are about 45 degrees from each other, with smooth fronts due to erosion by the sea. This will affect the type and amount of sea life at the beach.
Cliff face
Sea
45 degrees
Aim of Pilot study
The aim of my pilot study is to conduct a transect of Hunt's Bay. From this, I will be able to produce a zonation pattern for the beach, draw a profile of the beach, calculate the exposure rating and discuss the distribution of various organisms found on the beach. This will help me with my extended investigation.
Method for conducting a transect at Hunt's Bay
The method I am going to use to conduct a transect at Hunt's Bay is the Cross-Staff method. (Refer to appendix 2)
Tides
A tide is a periodic rise and fall of all ocean waters, including those of open sea, gulfs and bays. The tide will rise and fall twice approximately every 25 hours, giving two high tides and two low tides daily. Tides are the result from the gravitational attraction of the moon and the sun upon the water and upon the earth itself.
During the periods of new and full moon, when the sun, moon, and earth are directly in line, the moon and sun pull in the same direction. This results in the condition known as spring tides, in which the high tide is higher and the low tide is lower than usual. When the moon is in first or third quarter, however, it is at right angles to the sun relative to the earth and the moon and sun are pulling at right angles to each other. This condition produces neap tides, in which the high tide is lower and the low tide is higher, than normal.
Tides
Spring tides
Neap tides
Tidal range
MHWS (mean high water spring) to MLWS (mean low water spring) in the Bristol Channel covers a rise and fall of 14 metres. Around Gower, including Hunts Bay the range is about 8 metres. It has been highlighted that low water of spring tides occurs between 12.00 and 3.00pm in this area. It takes 6 hours for the tide to come in and 6 hours for the tide to go out. The speed at which the tide covers and uncovers the shore is described as the 'rule of twelve'. In the first hour the tide rises and falls by 1/12th of its range, 2/12th in the second hour, 3/12th in the third hour, 3/12th in the fourth hour, 2/12th in the fifth hour and 1/12th in the sixth hour. Therefore, in the third and fourth hour there is most danger because the water is rising or falling faster.
Zonation pattern
The distribution of organisms on a rocky shore consists of a series of overlapping bands called the zonation pattern. It is a response to the amount of time each part of the shore is immersed in seawater and/or emersed.
The immersion/emersion cycle
The zonation pattern of plant and animal distribution is a response to the frequency and length of period of immersion and emersion, when the tide rises and falls. Immersion and emersion times change depending on the relative positions of the moon and the sun i.e. whether it is a spring or a neap tide. This week at Hunt's Bay (21st-25th September 1998) was a week of spring tides. The week before was a period of neap tides; therefore the organisms at the bottom of the beach were uncovered and exposed to the air. The previous week they remained completely immersed in seawater. At the top of the beach there will be organisms that will be briefly covered this week by the sea. Last week they spent the whole week emersed from seawater.
Environmental conditions during emersion
The most important factors affecting the upper distribution of shore organisms are:
. HEAT
2. DESICCATION
3. STRONG LIGHT (light intensity and u.v light)
Under these conditions all metabolic activities are reduced- feeding, respiration and movement. Toxic products may accumulate and the shore is open to terrestrial predators, for example birds. It is evident that organisms that occupy the upper parts of the shore need to have mechanisms to allow them to resist these factors.
Environmental conditions during immersion
When the tide is in the temperature is cool and stable. There is no worry of water loss (desiccation). The water contains dissolved oxygen, carbon dioxide, and nutrient salts together with organic detritus. Animals can move around easily. Toxic excretory products can be removed from the body. It is possible to release gametes for fertilisation.
The most important factor affecting the lower distribution of shore organisms is:
. THE LEVELS OF LIGHT
The levels of light may fall below the required compensation for photosynthesis. This will limit the lower distribution of some algae. However, different algae have overcome this problem by using different photosynthetic pigments.
The immersion/emersion cycle is affected by a number of modifying factors, the most important of which is:
. Exposure- the size and strength of the waves are the result of wind speed and FETCH i.e. the distance over which the wind and waves travel before hitting the beach. Therefore the exposure rating of the beach is affected by the geographical location of the beach.
Increased exposure will increase the vertical extent of the shore above and below its theoretical limits. The emersion factors associated with the MHWN level of a normal beach will be encountered at MHWS and above on an exposed beach. Zonation patterns will be enclosed by the theoretical limits MHWS and MLWS.
Increased exposure will also change the types of organisms to be found on the beach. The physical force of waves can dislodge organisms, crush them and smash them against the rocks. Therefore, it is likely that exposed beaches will be devoid of plant life because the violent waves and wind would destroy these organisms. It could be argued that an exposed beach would contain fewer animals than a sheltered beach and contain organisms that can survive in the harsh conditions.
2. Topography- this will affect wave action. Rough shallow slopes will dry out less rapidly than steep slopes.
3. Aspect- South-facing beaches will dry out more quickly than north facing beaches. Hunt's Bay faces south. This suggests that this will affect the extent up the beach of the zonation pattern.
4. Time of low water springs- Emersion factors will be at their strongest between 12.00 and 3.00 p.m. during the summer months. Hunt's bay experiences Low water spring tides at this time.
5. Biotic factors- These are the inter-relationship between organisms on the shore, for example:
a. Competition between and within species
b. Predation
c. Overcrowding
d. Grazing
PILOT STUDY
After calculating the exposure rating for Hunt's Bay, I have come to the conclusion that it is a semi-exposed beach to an exposed beach (see appendix 3).
After analysing my zonation pattern for Hunt's Bay (see appendix 4), I am able to describe and explain the distribution of various organisms along the beach. I am also able to draw a profile of Hunt's Bay (see appendix 5).
Distribution of the Littorinid species
The common name for the Littorinid species is periwinkles. There are four main species. These are as follows:
Littorina neritoides (small periwinkle)
Littorina saxatilis (rough periwinkle)
Littorina littorea (edible periwinkle)
Littorina obusata (flat periwinkle)
Distributed at the top of the beach, MHWN to MHWS, is Littorina neritoides. They have no ctenidium but the cavity wall is greatly folded and acts as a lung. They, therefore, do not need water all the time for respiration and this is one reason for their distribution at the top of the beach. Reproduction is via external fertilisation. They release their gametes into the water and so need a high tide. Their main excretory product is uric acid. Littorina neritoides are able to withstand desiccation and are therefore found at the top of the beach in crevices and empty barnacle shells.
Littorina saxatiliis is found on the middle of the beach near to the top, MLWN to MHWN. A ctenidium is present for respiration, but it is much reduced. The cavity wall is much folded. This periwinkle can breathe air. They do not require water all the time for respiration but need more than Littorina neritoides because their ctenidium is reduced and therefore are situated slightly further down the beach than Littorina neritoides. They possess internal fertilisation (viviparous) and the young emerge direct from the parent. This is clearly an adaptation to more of a terrestrial existence and an increased protection for the young. Their main excretory product contains some uric acid crystals, which again suggests that they are adapted to a terrestrial existence. They live in crevices and on stones on upper and upper middle rocky shores, feeding on seaweed.
Found towards the bottom of the beach, MLWN to MLWS, is Littorina littorea. A ctenidium or gill lies in a cavity, which has little folding of the wall for respiration. Reproduction is via external fertilisation and their main excretory product is ammonia, requiring a large volume of water for dilution of toxic material. It is evident that they cannot breathe, reproduce or excrete without the presence of water. Therefore, edible periwinkles are found on rocks, stones and seaweed on the middle and lower parts of rocky shores, feeding on plant debris and algae.
Littorina obtusata is found at the bottom of the beach, MLWS. A ctenidium or gill that lies in a cavity, which has little folding of the wall, is present. During fertilisation eggs are laid (oviparous) in gelatinous on seaweed. Young are shelled and they emerge directly. Their main excretory product is ammonia. Water constitutes 90% of urine whereas urea constitutes 10%. Therefore, water is needed for both respiration and excretion and that is why flat periwinkles are found right at the bottom of the shore, at the intertidal zone, where it feeds on brown seaweed. They are mainly found on sheltered shores and would be washed away on an exposed shore.
The resistance to desiccation (without water) on the Littorina species after 7 days at 18°C has been investigated. The results are as follows:
SPECIES
% MORTALITY
Littorina littorea
70
Littorina obtusata
80
Littorina saxatilis
8-17
Littorina neritoides
0
This shows that none of the Littorina nertoides died when exposed to the effects of desiccation. It suggests that they be found at the top of the beach, MHWS, because they can survive without water for long periods of time.
This also shows ...
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The resistance to desiccation (without water) on the Littorina species after 7 days at 18°C has been investigated. The results are as follows:
SPECIES
% MORTALITY
Littorina littorea
70
Littorina obtusata
80
Littorina saxatilis
8-17
Littorina neritoides
0
This shows that none of the Littorina nertoides died when exposed to the effects of desiccation. It suggests that they be found at the top of the beach, MHWS, because they can survive without water for long periods of time.
This also shows that only 8- 17% of the Littorina saxatilis died. It indicates that they are found towards the top of the beach, MHWN to MHWS, because a high percentage of them can survive without water. However, 70% of the Littorina littorea species died in the absence of water. This provides evidence that they are found towards the bottom of the beach, MLWS to MLWN, because they cannot survive without water for a long time. Littorina obtusata are found at the bottom of the beach, MLWS. This is suggested by the fact that 80% of them died when deprived of water and cannot, therefore, survive without water for long periods of time.
Distribution of Thias lapillus (dog whelks)
My kite diagram for Hunt's Bay (see appendix 4) shows us that Thias lapillus is found towards the bottom of the beach (MLWS to MLWN). An investigation into the effects of desiccation on Thias lapillus, after 7 days at 18°C helps explains this. The result was as follows:
SPECIES
% MORTALITY
Thias lapillus
100
This shows that all of the dogwhelks died in the absence of water. This is evidence that they are found at the bottom of the beach near the water, MLWS to MLWN, because they have a very low tolerance of the effects of emersion.
Distribution of Barnacles
There are two main types of species:
Balanus balanoides (Northern barnacle)
Chthamalus stellatus (Southern barnacle)
From my zonation pattern of Hunt's Bay (see appendix 4) it is evident that the Balanus balanoides are the most abundant at MLWN and the Chthamalus stellatus are the most abundant at MHWN. This suggests the southern barnacle prefers warmer conditions towards the top of the beach whereas the nouthern barnacle prefers cooler conditions at the bottom of the beach near the water.
An investigation into the temperature tolerance in water of various seashore organisms shows that Patella vulgata (limpet) and Thias lapillus (dogwhelk) die within a few hours at temperatures of 40?C and above. The four species of periwinkle can survive only a few degrees more than 40?C before they begin to die. However, barnacles can survive at a higher temperature for longer than the other species. This suggests that more barnacles will be found at the top of the shore where the temperature can become high during emersion.
Fucaceae and other algae
The two species of Fucaceae found at Hunt's Bay are as follows:
Fucus evesiculosis (bladderless bladder wrack)
Fucus serratus
The zonation pattern of Hunt's Bay shows that Fucus serratus are most abundant at the bottom of the beach, MLWS. Fucus evesiculosis are also found towards the bottom of the beach, a little further up the beach than Fucus serratus, MLWS to MLWN.
General adaptations of algae
. Water content of algae and loss of water in the air
It is evident that the rate of water loss from different species of fucaceae and algae after exposure to air has an effect on their distribution along the beach. The species that loses most water is found towards the bottom of the beach, MLWS to MLWN, because it needs to replace this loss. Whereas, the species that loses least water when exposed to air is found at the top of the beach, MHWN to MHWS, further away from the water. An experiment into the rate of water loss, measured by a decrease in fresh weight after exposure to air, shows that Fucus serratus has a very high water loss.
2. Cell wall thickness
It is also possible to relate cell wall thickness to the position on the shore. Species with a thicker cell wall are found at the top of the shore, MHWN to MHWS, whereas thinner celled-walled species are found at the bottom of the beach, MLWS to MLWN. This is because species with a thick cell wall are more resistant to desiccation, wind and heat. Fucus serratus has a very thin cell wall.
SPECIES
CELL WALL THICKNESS/u
Fucus serratus
0.42+/-0.03
3. Growth rates in Fucaceae
Measurements of growth rates of different species of algae under different cycles of immersion/emersion give us an indication of the distribution of the different species along the beach.
. Fucus serratus grow best at a cycle of 1 hour dry and 11 hours wet. Therefore are found at the bottom of the beach (MLWS).
4. Air bladders
Some algae possess air bladders. These bladders help to keep the plant upright when the tide is in, so presenting a larger surface area for photosynthesis. However, this habitat is not suited to exposed shore conditions, like that of Hunt's Bay, as the bladders are ripped by the bad weather and rough sea. Therefore, no algae that possessed bladders were found on Hunt's Bay. They are more likely to be present on a sheltered calmer beach. Fucus evesiculosis was found which is a bladderless bladder wrack.
2. Holdfasts
Some species possess well developed holdfasts (hapteron). For example, Laminaria. This help them to be strong, flexible and redgrow.
3. Mucilage
The 'slime' characteristic of many littoral algae provides protection against desiccation. The slime is mucilage, which is produced by special mucilage glands opening out on to the surface of the thallus.
4. Mechanical strength
The thallus is often leathery and very tough and flexible which help the algae withstand the surge and tug of the tide.
5. Dichotomous branching
The thallus branches in only one plane. This minimises resistance to water movement.
6. Reproductive adaptations
Algae release their gametes into the sea. This release is often synchronised with the tides. Male gametes are motile and chemotactic, i.e. attracted by a chemical secretion of the female gametes.
7. Light and algae
Light has an effect on the distribution of some algae. On the debit side levels of light at the bottom of the shore may fall below the required compensation point for photosynthesis. This will limit the lower distribution of some algae. Different algae have overcome this problem by using different photosynthetic pigments.
Normally, in clear sea water red light penetrates the least and green light penetrates most deeply.
Green algae utilise red and blues lights most and green light the least. It is likely; therefore those green algae will be found at the top of the beach.
Brown algae utilise red and blue light with a wider extension into the blue/green part of the spectrum. The main photosynthetic pigment is fucoxanthin, which strongly absorbs blue light. It is likely, therefore, that a high percentage of brown algae, for example Laminaria, will be found at the bottom of the beach.
Red algae, however, make use of green light with extension into the red. It is likely, therefore, that red algae will mostly be found at the bottom of the beach.
INTRODUCTION-Limpets
Patella vulgata is the common limpet. It belongs to the phylum mollusca and is a member of the class gastropoda. The are all characterised by a conical, rough, ribbed shell whose apex lies towards the front of the animal rather than centrally. They have transparent or translucent marginal tentacles and variable foot colour, which can be quite orangey, usually drive-green, yellow or grey.
TIDAL LEVEL : Eulittoral (most numerous mid-shore)
EXPOSURE : Found on sheltered to exposed beaches
GEOGRAPHY : All coasts
Identification notes
The marginal tentacles are the only reliable characteristic. Foot colour is variable and often depends on the ripeness and set of the underlying gonads.
Ecological notes
Patella vulgata is the most common species. All are grazers and Patella's activities prevent seaweed growth on exposed and moderately exposed shores in the eulittoral zone from high to low level.
Distribution
Found on all British shores between MLWS and MHWS if conditions are suitable. It is the most highly adapted of intertidal molluscs. It moves in search of food. It has a firm attachment to the by means of a sucker-like foot which creates a 'vacuum'. Its broad based conical shell permits a wide area for attachment with a minimum resistance to water movement. It resists desiccation by pulling the shell down onto the rock. This reaction also prevents removal by predators. The margins of the shell make a perfect fit with the rock of the 'home base'. Water is retained in a narrow groove between the shell and foot. The limpet moves about when the tide is in but returns to its homebase when the tide ebbs. It will also move and feed on damp nights when the tide is out. Coming back to homebase prevents barnacles taking over the rock.
Life cycle of Patella vulgata
Structure and adaptations shown by limpets
- Molluscs has three body sections
. Muscular foot.
2. Dorsal position is a visceral mass containing most of the organs including the head.
3. Mantle- produces shell.
- Limpets have flattened conical shells that can be up to 7cm tall.
Outside is a slightly flexible layer of conchiolin (a substance chemically related to chitin) mixed with calcium carbonate in the form of calcite, which cushions the brittle shell against external erosion.
The middle layer is the main thickness of the shell against erosion. It is made of crystals of lime built up into a prism. This prismatic layer has on its inner surface thin 'mothers of pearl' layer in which plates of hard calcium carbonate (about fifty-thousandth of an inch thick), alternate with equally fine films of conchiolin. The various forms of lime make up about 99% of the total shell weight, small amounts of calcium phosphate and magnesium carbonate, together with the conchiolin, make up the rest.
They provide armour for the limpet. It has been suggested that their shells become less tall the further down the beach they occur and the creatures exposed to air for the longest periods have the thickest shells. Thick cell walls offer many fixed animals protection from overheating, and some cool off by releasing water trapped beneath their shells. Limpets trap air beneath their shells.
- Limpets defence at low tide is to pull their shell against the suction pad of foot, fitting the rim of shell so tightly to the rock surface that the damp cannot 'ooze' out. Therefore, preventing desiccation. This also acts as a defence against predators.
- Limpets move via a muscular organ called the foot. They move by the means of a muscular contraction that flows forward along the sole of the foot, which also makes a fine anchor.
- A ring of gills (or ctenduim) is another special feature. Limpets use their gills to collect fine kinds of food (browse on algae and diatoms) as well as molecules of oxygen.
- They also have a band of teeth (radula) in their forgut. For a diagram of a limpet's radula and teeth see appendix 6.
Feeding
Limpets usually feed at night and return to the same 'home'. If the rock is soft, then over a period of time, because of erosion, the limpet return to a marked depression.
When the limpet feeds, the head is exposed and the radula or lingual ribbon is extended beyond the shell as the animal grazes the young algal sporelings and lichens on the rock around it, often leaving radula marks on the rocks. The radula consists of a long strip carrying rows of teeth, up to 2000 teeth in all. Most of the radula is rolled up inside the sac. It is gradually unrolled as the radula is worn away from the tip. (For diagram of radula and teeth, see appendix)
Reproduction
Young limpets are male, and become female as they grow older (after about 30mm in shell length and one year old). Limpets breed during the period January to March. Gametes are released into the sea (external fertilisation). The zygote develops to form a minute free swimming larva. These are temporary members of the plankton before they 'settle' on the shore. Mortality in young limpets can be very high if seas are rough or if there is a spell of frosts on the shore.
AN INVESTIGATION INTO THE RELATIONSHIP BETWEEN RADULA LENGTH, SHELL THICKNESS AND VERTICAL ZONATION OF A COMMON LIMPET, PATELLA VULGATA ON AN EXPOSED ROCKY SHORE.
Aim
I aim to investigate the relationship between radula length, worn part of the radula length, the thickness of the shell and the vertical zonation (position) of Patella vulgata (the common limpet) on an exposed rocky beach, Hunt's Bay.
Hypothesis
I believe that the thickness of the limpet shell will increase as you move up the beach, from MLWS to MHWS, because at the top of the beach the limpets are more susceptible to the effects of emersion. These include desiccation, heat and strong u.v. light. I believe that the top of the beach is affected more by these emersion factors, as it is emersed for a higher percentage of the time compared to the bottom of the beach. Therefore, the thicker shells will act as a protective layer to protect the limpet from desiccation, overheating and u.v. light.
I believe that limpets with thicker shells survive at the top of the beach because they are protected from the effects of emersion. However, I believe that limpets with thinner shells do not survive at the top of the beach but are eliminated, because they are not protected as much from the effects of emersion. Therefore, I believe that the thickness of the limpet's shell is environmentally induced variation.
The null hypothesis states that any variation in shell thickness along the vertical height of the beach is biological variation and is not significant.
I believe that the radula length of the limpet should vary against the position on the beach.
The radula length of the limpet could decrease as you move up the beach because at the top of the beach the rocks are rougher and jagged due to the covering by barnacles and the decrease in tidal activity. I believe that this roughness has an effect on radula length because the radula will wear away more quickly as the limpet feeds from the rocks. At the bottom of the beach where there is constant tidal activity with pebbles being washed in and out and eroding away the rocks making them smoother, the limpets will have a longer radula. This is because when feeding, their radula will not be worn away as much by the smoother rock.
The worn part of the radula could be longer at the top of the beach because the rougher rocks wear it away, increasing the amount of worn part on the radula.
On the other hand, the radula length could increase as you move up the beach because at the top of the beach the limpets will have less time to feed as they are immersed for a smaller percentage of time of the tidal cycle. I believe that as you move towards the bottom of the beach where there is more food, the radula length will become shorter because the feeding time available for the limpets will increase, as they will be immersed for a higher percentage of time of the tidal cycle. The limpet will eat food more frequent and the radula will become worn away by the increased amount of food it consumes.
The worn part of the radula could be longer at the bottom of the beach because the radula will wear away faster where the limpet eats more food more frequent, increasing the amount of worn part on the radula.
The null hypothesis states that any difference in radula length or worn part of the radula length along the vertical height of the beach is insignificant and is due to biological variation.
Apparatus
Limpets
Blunt kitchen knife
Ruler
Vernier calipers
Scalpel
Method
. Firstly, I will choose 7 suitable intervals along Hunt's Bay. To do this I will use the profile of Hunt's Bay (see appendix 5). Mid tidal level will be station 4. I will then estimate 3 stations above and below mid tidal level by calculating the average vertical distance between each station, using the vertical distance between the 14 stations from the transect of Hunt's Bay (see appendix 2), which was 0.75 metres.
Therefore,
0.75m x 14 = 10.5m
10.5 = 1.5m
7
The average distance between each station will be 1.5m.
2. At each station, I will collect 10 limpets within the size range of 25-30mm. This will ensure that all the limpets collected are the same size, resulting in a fair test. We will remove them from the rocks using a blunt kitchen knife.
3. I will dissect each limpet using a scalpel and remove its radula.
4. For each limpet, I will measure and record:
a. The length of its radula in mm.
b. The length of the worn part of the radula in mm, by placing it on a ruler.
5. I will then remove the limpet from its shell and measure and record: c. The length of the shell in mm using a ruler.
6. Using a set of pliers, I will break the shell until I can measure and record: d. The thickness of the shell near the apex using the vernier calipers.
Safety precautions
. It is very important to carry out our investigation under strict safety precautions. They are as follows:
2. Wearing of suitable stout footwear and clothing at the beach, to ensure that when moving from one end of the beach to the other you do not fall on the jagged rocks. Care also needs to be taken when moving across the rocks, again not to have a nasty fall.
3. Take care when removing limpets from the rocks with the kitchen knife, ensuring that the knife is pointing away from your body and anybody else's body!
4. Special care must taken when dissecting the limpets with a scapule. This is to ensure that you do not cut yourself because it could become infected
Variables
Dependent
. Length of limpet's radula.
2. Length of worn part of limpet's radula.
3. Length of limpet's shell.
4. Shell thickness.
Independent
. Position on the shore
Control
. Ensure that the limpets collected are all at the same vertical height for each station.
2. Same number of limpets collected at each station.
3. Size of each limpet collected, approximately the same size, in the range of 25mm-30mm.
Results
For raw data, see appendix 7.
Length of shell:
Station-
Limpet
No.
1
(Bottom of beach- MLWS)
2
3
4
5
6
7
(Top of beach- MHWS)
Length
Of
Shell/ mm
1
34
28
27
38
44
36
35
2
26
31
39
44
44
37
49
3
31
24
29
42
37
46
37
4
29
26
34
34
42
29
38
5
33
38
38
40
36
39
34
6
22
32
29
47
42
28
50
7
28
28
34
35
47
33
35
8
28
25
39
48
51
30
31
9
29
27
22
41
48
35
26
10
27
29
30
28
49
50
43
Length of radula:
Station-
Limpet
No.
1
(Bottom of beach- MLWS)
2
3
4
5
6
7
(Top of beach- MHWS)
Length
Of
Radula/mm
1
77
53
46
67
73
43
50
2
52
58
65
80
78
45
73
3
67
44
62
73
54
65
56
4
53
61
63
72
65
47
59
5
69
72
66
74
50
56
50
6
48
66
55
80
63
40
69
7
50
59
58
60
69
42
47
8
58
50
70
82
83
44
43
9
61
50
46
57
66
57
35
10
50
54
50
50
70
77
52
Length of worn part of radula:
Station
Limpet
No.
1
(Bottom of beach- MLWS)
2
3
4
5
6
7
(Top of beach- MHWS)
Length of Worn Part of Radula/mm
1
19
7
7
10
10
6
10
2
8
9
10
8
3
6
12
3
11
8
16
10
8
10
13
4
8
8
13
10
10
8
10
5
12
6
8
9
11
8
11
6
10
11
10
6
8
9
8
7
8
12
11
8
5
7
12
8
8
6
15
12
7
6
6
9
6
11
7
8
6
7
6
10
10
5
12
8
8
8
11
Shell thickness:
Station-
Limpet
No.
1
(Bottom of beach- MLWS)
2
3
4
5
6
7
(Top of beach- MHWS)
Shell
Thick-
Ness/ mm
1
1.6
1.2
1.2
2.1
3.0
1.9
2.0
2
1.5
1.8
3.0
2.8
3.4
1.9
3.6
3
2.0
0.9
1.2
2.7
1.9
3.2
3.1
4
1.2
1.6
1.5
2.2
3.0
1.0
2.0
5
1.9
1.4
2.3
1.5
1.4
2.1
2.1
6
1.1
1.0
1.3
4.0
5.1
2.3
3.0
7
1.0
1.9
1.9
2.0
4.0
1.9
1.5
8
1.0
1.1
2.3
3.2
2.0
2.5
1.9
9
2.0
1.2
1.2
2.9
2.1
2.9
1.9
10
1.0
1.8
1.6
1.9
2.5
5.0
2.1
Statistical analysis of results
The limpet shell lengths varied along Hunt's Bay and it was difficult to collect limpets of the same size, as stated in my method, especially where limpets were scarce. Therefore, as we were not collecting the same size limpets, it is important that from our results I take a ratio of mean radula length and shell thickness against mean shell length, so that I can compare my results.
Radula length
Station number
Mean radula length/mm
Mean shell length/mm
Mean radula length/shell length ratio (3d.p)
Mean radula length/shell length ratio
(2d.p)
(bottom of beach, MLWS)
58.5
28.7
2.038
2.04
2
56.7
28.8
1.969
1.97
3
59.1
32.1
1.841
1.84
4
69.5
39.7
1.751
1.75
5
67.1
44.0
1.525
1.53
6
51.6
36.3
1.421
1.42
7 (Top of beach, MHWS)
53.4
37.8
1.413
1.41
As we were not able to collect limpets of the same size at each station, it is important to take a ratio of mean shell thickness against mean shell length so that I can compare my results.
Shell thickness
Station number
Mean shell thickness/mm
Mean shell length/mm
Mean shell thickness/shell length ratio (3d.p)
Mean shell thickness/shell length ratio (2d.p)
(Bottom of beach, MLWS)
1.43
28.7
0.050
0.05
2
1.39
28.8
0.048
0.05
3
1.75
32.1
0.055
0.05
4
2.53
39.7
0.064
0.06
5
2.84
44.0
0.065
0.06
6
2.47
36.3
0.068
0.07
7 (Top of beach, MHWS)
2.32
37.8
0.061
0.06
As we were not able to collect limpets of the same size, it is important to take a ratio of mean length of worn part of radula against mean shell length so that I can compare my results.
Worn part of radula
Station number
Mean length of worn part of radula/mm
Mean shell length/mm
Mean length of worn part of radula/shell length ratio (3d.p)
Mean length of worn part of radula/shell length ratio (2d.p)
(Bottom of beach, MLWS)
10.0
28.7
0.348
0.35
2
8.3
28.8
0.288
0.29
3
10.9
32.1
0.340
0.34
4
8.9
39.7
0.224
0.22
5
7.6
44.0
0.173
0.17
6
7.5
36.3
0.207
0.21
7 (Top of beach, MHWS)
9.9
37.8
0.262
0.26
Numerical processing of results
Using Spearman's rank correlation coefficient (see appendix 7) will help determine whether my results show any correlation.
Applying Spearman's rank to my results for the shell thickness/shell length ratio shows:
STATION NUMBER
SHELL THICKNESS
RATIO RANK
DIFFERENCE
1
6
-5
2
7
-5
3
5
-2
4
3
+1
5
2
+3
6
1
+5
7
4
+3
TOTAL = 0
rs = 1- 6 D²
n(n²-1)
rs = 1-6((-5)²+(-5)²+(-2)²+(1)²+(3)²+(5)²+(3)²)
7(7²-1)
rs = 1-6(25+25+4+1+9+25+9)
7(49-1)
rs= 1-6(98)
7(48)
rs= 1-588
366
rs = 1- 1.75
rs= -0.75
The critical value for rs with n=7 is 0.786 at the significance level of 5% and 0.714 at the 10% significance level. The calculated value of rs lies between these two values, so I have to accept the null hypothesis of no correlation between shell thickness and position on the shore, as the significance level was greater than 5%.
Applying Spearman's rank to my results for radula the length/shell length ratio show:
STATION NUMBER
RADULA LENGTH RATIO RANK
DIFFERENCE
1
1
0
2
2
0
3
3
0
4
4
0
5
5
0
6
6
0
7
7
0
TOTAL = 0
rs = 1-6 D²
n(n²-1)
rs = 1-6((0)²+(0)²+(0)²+(0)²+(0)²+(0)²+(0)²)
7(7²-1)
rs = 1-6(0)
7(49-1)
rs = 1- 0
7(48)
rs = 1-0
336
rs = 1-0
rs = +1
The critical value for rs with n=7 is 0.786 at the 5% significance level. The critical value of rs is greater than this value so we can reject the null hypothesis of no correlation because the significance level is greater than 5%. The results show a positive correlation.
Applying Spearman's to my results for the length of the worn part/shell length ratio of the radula shows:
STATION NUMBER
WORN PART OF RADULA LENGTH RATIO RANK
DIFFERENCE
1
1
0
2
3
-1
3
2
+1
4
5
-1
5
7
-2
6
6
0
7
4
+3
TOTAL = 0
rs = 1-6 D²
n(n²-1)
rs = 1-6((0)²+(-1)²+(1)²+(-1)²+(-2)²+(0)²+(3)²)
7(7²-1)
rs = 1-6(1+1+1+4+9)
7(49-1)
rs = 1-6(16)
7(48)
rs = 1-96
336
rs = 1-0.286
rs = 0.714
The critical values for rs with n=7 are 0.786 at the significance level of 5% and 0.714 at the 10% significance level. The calculated value of rs lies at the 10% significance level, so there is no correlation between the worn part of the radula and position on the shore, as the significance level is greater than 5%.
Conclusion
According to Spearman's rank any variation in the thickness of the limpet's shell against its position on the beach is biological variation and is insignificant, as we have to accept the null hypothesis.
This seems likely to be the case when analysing my results and graphs. From results, the mean shell thickness/shell length ratio is 0.05 at stations 1-3, it increases at station 4 to 0.06, and it remains at 0.06 at station 5, increases to 0.07 at station 6 but at station 7 it decreases to 0.06. This does not indicate a steady increase from station 1 (bottom of beach) to station 7 (top of beach). There is no correlation.
Thus, it rejects my prediction that the shell thickness of the limpet increases as you move up the beach from station 1 to 7. Therefore, it rejects the theory that the limpets at the top of the beach have a thicker shell to protect them effects of desiccation, heat and u.v. light, as they are emersed for the longest periods of time. Therefore, proving my prediction that the thickness of a limpets shell is environmentally induced variation incorrectIt suggests that limpets are one of the most adaptable of seashore organisms.
According to Spearman's rank, I have to reject the null hypothesis that states that any variation in radula length is biological variation and is insignificant because my results show a negative correlation.
In my hypothesis for radula length, I effectively stated two predictions. The first one was that the radula length would increase as you move up the beach from station 1 to 7and the other that it will decrease. My results have enabled me to eliminate one of these hypotheses.
After analysing my results and graphs, I have come to the conclusion that the mean radula length of the limpet decreases as you move up the beach from station 1 to station 7. This is clearly evident in my radula length/shell length ratio results. At station 1, this value was 2.04, at station 2 it decreased to 1.97, it decreased to 1.84 at station 3, continued to decrease between stations 4 to 6 from 1.75 to 1.53 and 1.42 respectively and at station 7, the value was 1.41. They show a negative correlation of decreasing when you move from station 1 (at the bottom of the beach) to station 1 (at the top of the beach). This is supported by my research. When a limpet feeds, its head is exposed beyond the shell and it scraps the rock. Therefore, it suggests that the radula length will be shorter at the top of the beach where there are rougher, more jagged rocks due to the covering of barnacles and the decrease in tidal activity. These rocks will wear away the radula more quickly than the smoother rocks at the bottom of the beach.
According to Spearman's rank, I have to accept the null hypothesis that any variation in the length of the worn part of the radula is biological variation and is insignificant.
This seems likely to be the case, since the mean length of the worn part of the radula/shell length ratio in my results fluctuates from station 1 at the bottom of the beach, to station 7 at the top of the beach and there is no pattern. From stations 1 to 7, the mean length of the worn part of the radula/shell length ratio values are 0.35, 0.29, 0.34, 0.22, 0.17, 0.21,and 0.26. There is no correlation. This rejects my two predictions. That the length of the worn part of the radula could increase as you move up the beach due to wearing away by the jagged rock. The length of the worn part of the radula could decrease as you move down the beach because the limpets are immersed for most of the time, have more time to feed and will therefore eat more food wearing away its radula. Any difference in the length of the worn part of the radula is biological variation and is insignificant.
Discussion
After bringing my experiment to a conclusion, it was very interesting to find out that any difference in the shell thickness of the limpet is biological variation and is insignificant. Research into this had suggested that a limpet shell would be thicker at the top of the beach where the limpet is more affected by the emersion factors. Numerical processing of my results, using Spearman's rank has rejected this theory. Therefore, my prediction that the thickness of the limpet's shell is environmentally induced variation is also proved wrong as any difference in shell thickness along the shore is biological variation and is insignificant. Limitations in my experiment may have been responsible for these results (see page 32- Reliability/Limitations).
My results and graphs show that, the radula length of the limpet decreases as you move up the beach from station 1 to 7. When a limpet feeds, its head is exposed beyond the shell and it scraps the rock. The radula length could be shorter at the top of the beach where there are rougher, more jagged rocks due to the covering of barnacles and the decrease in tidal activity. These rocks could wear away the radula more quickly than the smoother rocks at the bottom of the beach. It proves my other hypothesis, which states that the length of the radula increases as you move up the beach incorrect.
With my results for the length of the radula, I thought that the worn part of the radula would have increased as we moved up the beach because the radula was becoming shorter and worn away by the rougher rocks, increasing the amount of worn part on the radula. However this was not the case as our results for the length of the worn part of the radula did not show any trend in decreasing or increasing as we moved up the beach. It proved my other hypothesis that stated the length of the worn part of the radula could decrease as you moved up the beach incorrect. Instead our results indicates that any variation in the worn part of the radula is biological variation and is insignificant. Again, limitations in our experiment could have caused these results (see page 32-Reliability and Limitations).
Limitations/Reliability
There were some limitations to our experiment.
We only collected 10 limpets at each station because we were limited to the number of limpets that we could dissect at each station. It would have meant us killing more limpets, which could have affected other shore life. We therefore used small sample sizes, decreasing the reliability of our results because we were limited to the number of results we could obtain.
It was very difficult to collect limpets of the same size at each station, especially where limpets were scarce, as there was such a wide variation in the sizes of each limpet. The limpets we collected were not of the same size. Therefore, I had to take ratios of the mean shell thickness, mean radula length and mean worn part of radula length against the mean shell length to allow our results to be compared. This could have been a contributing factor in the inaccuracy of our results because there were large differences in the size of each limpet. For example, at station 1 the mean shell length was 28.7, at station 4 it was 39.7 and at station 7 it was 37.8.
In our experiment, due to our time limitation, we divided the beach into 7 stations and collected 10 limpets from each station. I believe that this could have led to inaccuracies in my results because at each station there was a large area from which the limpets could be collected and different vertical heights. It was difficult to collect 10 limpets from the same vertical height, especially where limpets were scarce. The limpets at one station could have had big differences in shell thickness, radula length and worn part of the radula length if they were from different vertical heights.
When working with a small number of stations, random variation is likely to obscure any differences in shell thickness, radula length and the length of the worn part of the radula of the limpets along the beach. It has been suggested to avoid working with sample sizes of less than 7 or 8, when working with the Spearman rank correlation coefficient. We used 7 stations, which is, therefore, the minimum advised to use and could have led to inaccuracies in our results.
It was difficult to measure the worn part of the radula even though the non-worn part of the radula was brown in colour and the worn part was a white colour. This is because there is no distinct cut off point between the non-worn radula and its worn part, but they seemed to merge into one and gradually changed from a white colour to brown. We had to estimate where the worn part of the radula ended and the non-worn radula begun; hence the length of the worn part. Our estimations could have led to inaccuracies in our results for the worn part of the radula because it may not have been the true measurement.
It was also difficult to measure shell thickness using the vernier calipers because of the ridges that the limpet shells possess. This could therefore have led to inaccuracies in our results because it may not have been the true measurement of shell thickness.
Modifications
To increase the accuracy of our results there are certain modifications that could be carried out in this investigation.
. I would need to repeat our results using a larger number of limpets at each station. Instead of collecting 10 limpets at each station, I would collect 30. We would have more results, increasing the accuracy of our results when taking averages and ratios.
2. I would increase the number of stations along the beach, from which I would collect the limpets. We would therefore have more samples, which would lead to more accurate results when working out averages and ratios. Increasing the number of samples would also decrease the likelihood of random variation obscuring a genuine association with a small number of samples. 7 stations are the minimum numbers of samples suggested to use when working with the Spearman rank correlation coefficient. I would therefore use more than 7 stations but less than 30 stations since with a large sample numbers the procedure of working out the ranks becomes very tedious. I would use 14 stations instead of 7.
3. The worn part of the radula was white and the non-worn part was brown. However, there was a gradual change from white to brown and there was no definite cut off point between the worn part and the non-worn part. To make our measurement the worn part of the radula as accurate as possible, we ensured that took the measurement at the same point throughout the experiment. This point was the last tooth on the worn part of the radula where it was clearly white and had not begun to change brown. This increased the accuracy of our results.
4. It was difficult to measure shell thickness due to the ridges that the limpet shells possess. To make this measurement as accurate as possible we ensured that we took the measurement at the same point throughout the experiment. This point was the apex.
Bibliography
Radula length
STATION 1 (Bottom of beach, MLWS)
Mean radula length = 585 ? 10 = 58.5mm
Mean shell length= 287 ? 10 = 28.7mm
Therefore,
Mean Radula ratio
Shell length
= 58.5 ? 28.7 = 2.04 (2 d.p)
STATION 2
Mean radula length = 567 ? 10 = 56.7mm
Mean shell length= 288 ? 10 = 28.8mm
Therefore,
Mean Radula ratio
Shell length
= 56.7 ? 28.8 = 1.97 (2 d.p)
STATION 3
Mean radula length = 591 ? 10 = 59.1mm
Mean shell length = 321 ? 10 = 32.1mm
Therefore,
Mean Radula ratio
Shell length
= 59.1 ? 32.1 = 1.84 (2 d.p)
STATION 4
Mean radula length = 695 ? 10 = 69.5mm
Mean shell length = 397 ? 10 = 39.7mm
Therefore,
Mean radula ratio
Shell length
= 69.5 ? 39.7 = 1.75 (2 d.p)
STATION 5
Mean radula length = 671 ? 10 = 67.1mm
Mean shell length = 440 ? 10 = 44.0mm
Therefore,
Mean Radula ratio
Shell length
= 67.1 ? 44 = 1.53 (2 d.p)
STATION 6
Mean radula length = 516 ? 10 = 51.6mm
Mean shell length = 363 ? 10 = 36.3mm
Therefore,
Mean Radula ratio
Shell length
= 51.6 ? 36.3 = 1.42 (2 d.p)
STATION 7 (Top of beach, MHWS)
Mean radula length = 534 ? 10 = 53.4mm
Mean shell length = 378 ? 10 = 37.8mm
Therefore,
Mean Radula ratio
Shell length
= 53.4 ? 37.8 = 1.41 (2 d.p)
Shell thickness
STATION 1 (Bottom of beach, MLWS)
Mean shell thickness = 14.3 ? 10 = 1.43mm
Mean shell length = 287 ? 10 = 28.7mm
Therefore,
Mean Shell thickness ratio
Shell length
= 1.43 ? 28.7 = 0.05mm (2 d.p)
STATION 2
Mean shell thickness = 13.9 ? 10 = 1.39mm
Mean shell length = 288 ? 10 = 28.8mm
Therefore,
Mean Shell thickness ratio
Shell length
= 1.39 ? 28.8 = 0.05mm ( 2 d.p)
STATION 3
Mean shell thickness = 17.5 ? 10 = 1.75mm
Mean shell length = 321 ? 10 = 32.1mm
Therefore,
Mean Shell thickness ratio
Shell length
= 1.75 ? 32.1 = 0.05mm (2 d.p)
STATION 4
Mean shell thickness = 253 ? 10 = 2.53mm
Mean shell length = 397 ? 10 = 39.7mm
Therefore,
Mean shell thickness ratio
Shell length
= 2.53 ? 39.7 = 0.06mm (2 d.p)
STATION 5
Mean shell thickness = 28.4 ? 10 = 2.84mm
Mean shell length = 440 ? 10 = 44.0mm
Therefore,
Mean shell thickness ratio
Shell length
= 2.84 ? 44.0 = 0.06mm (2 d.p)
STATION 6
Mean shell thickness = 24.7 ? 10 = 2.47mm
Mean shell length =363 ? 10 = 36.3mm
Therefore,
Mean shell thickness ratio
Shell length
= 2.47 ? 36.3 = 0.07mm (2 d.p)
STATION 7 (Top of beach, MHWS)
Mean shell thickness = 232 ? 10 = 2.32mm
Mean shell length = 378 ?10 = 37.8mm
Therefore,
Mean shell thickness ratio
Shell length
= 2.32 ? 37.8 = 0.06mm (2 d.p)
30
