Patella Vulgata and Neritoidea Interdependence

Introduction

The Isles of Scilly, located 28 miles southwest of the Cornish coast, are an interesting marine tidal ecosystem, provided by the open Atlantic's nutrient-rich, saline waters. A diverse environment that is home to many species of fish, invertebrates and marine mammals defines the islands and offshore reefs. As a nature reserve, they are known by many sailors and vacationers to be a haven of Natural Life and untouched beauty. The Island group consists of four islands, namely St Mary's, Tresco, St Martin's, St Agnes and Bryher. I chose to Analyse a remote Part of the Rocky Coastline of Bryher, which is an Island with 90 inhabitants. The shoreline is very flat and Rocky around it, so a change in Tides results in the Island revealing a large area of Land. Sea snails (Neritoidea) and Limpets (Patella Vulgata) dominate the Beach. It interests me whether these two species depend on one another, since they are both quite common in this area. This experiment was conducted within a natural tidal environment of the Isles of Scilly (49˚ 57’ 32.8464” N, 6˚ 21’ 5.256” W) over the span of a week.[1]

Patella Vulgata is generally found close to the coastal shore, at the edge of the sublittoral zone on firm surfaces. Its shell is conical and is up to about 6 cm wide. Even though Limpets look like mussels, they are a type of Sea Snail. They feed on Layers of Algae which is found on rocks, which they scrape away from the surface with their Radula, a contraption of muscles with array of over 100 teeth, that is comparable to a tongue. [2]

Figure 1 Patella Vulgata

The more snail shaped sea snail in this case is the (Neritoidea) which can be found in western Europe and like Limpets feeds on Algae. They do however not have Radula like the Patella Vulgata, so they cannot scrape away algae from hard rock as well as Patella Vulgata. Therefore, they also attach themselves to soft surfaces such as Seaweed and plants in addition to the rocky surfaces.[3] Sea snails make great animals to observe over time, as they seem virtually stationary while counting which simplifies that process. As they were observed in the same habitat frequently, the question whether there is a correlation between the two arrives.

Figure 2 Neritoidea

Research Question

Is there a distinct correlation between the occurrence of Patella Vulgata and Neritoidea in the tidal movements in the ecosystem found on the island of Bryher?

Hypothesis

H0: The occurrence of the two species indicates a dependence on one another.

H1: The occurrence of the two species indicates no dependence on one another.

Variables

Independent Variables

• Natural tidal change

Dependent Variables

• Amount of Sea snails/ m₂
• Amount of Limpets/ m₂

Method [4]

To collect the data, I laid down a Rope and measured a trajectory of 50m (red line in Fig.1) from the coast into the Low Tide. Every 10 metres, I then performed a quadrat sample and wrote down pH, temperature of the sediment, as well as the percentage of Limpets and sea Snails. I then waited around 24 hours which equals two tidal cycles and proceeded to the next data collection for a total of four days. (Maben, 2020). The quadrant I used has 100 squares dividing one square metre into smaller quadrants. Using this methology we can assume that when a small quadrant contained either a Limpet or Sea Snail, it would account for 1% of Specimen coverage within the large quadrant. To determine the percentage under water I accounted based on how far away from the waterline at low/ high tide the location was and how far it was elevated from the ground (rocks, piles of algae).

The resulting data can be counted in percentage, as each quadrat has 100 sections, so the amount in % is equal to the number of sections covered.

Raw data

Day 1
Date	Thursday, 23.7.2020
Time	13:30 - 15:30
Low Tide	14:48
Tide height (low)	0.83m
Lux	12500
Air temp	20°C
Weather	Overcast some rain but mainly dry
	Q1	Q2	Q3	Q4	Q5	Q6
	50m	40m	30m	20m	10m	0m
% under water	70%	60%	40%	20%	10%	5%
Sediment	Alg/Sand	Alg/Rock	Alg/rock	Alg/rock	Rock/alg	Rock/moss
pH	6.5	9.5	7.5	8	7	6
Temp	16	19	19	20	20	20
%[5] of Snail D1	6%	13%	11%	9%	4%	0%
% of Limpet D1	0%	0%	5%	3%	10%	0%

The data for the first day shows a stark increase in Sea snails in the first two Quadrants, followed by a steady decline in Snails up to the highwater shoreline where No Sea snails can be found as seen by the second last Row (% of Snail Day 1). For the Limpets, the spread along the trajectory line is much narrower. Limpets can only be found as far as 30 metres away from the highwater shoreline. They do however increase in occurrence to a maximum of 10% in Quadrant 5 which is only 10 metres away from the highwater shoreline, before also completely disappearing at the high-water shoreline.

A further interesting observation to make is that the sediment further out on the trajectory line is at a cooler temperature than the Sediment closer to the highwater shoreline.

Day 2
Date	Friday, 24.7.2020
Time	14:00 - 15:30
Low Tide	15:32
Tide height (low)	0.84m
Lux	7500
Air temp	17°C
Weather	Overcast and rain
	Q1	Q2	Q3	Q4	Q5	Q6
Dist. to shore	50m	40m	30m	20m	10m	0m
% under water	70%	50%	40%	25%	15%	5%
Sediment	Grav/sand	Rock/alg	Rock	Alg/rock	Rock	Rock/moss
pH	7	7	7.5	7.5	7	6.5
Temp	16°C	16°C	16.5°C	16°C	15.5°C	17°C
% of Snail D2	5%	7%	9%	11%	18%	1%
% of Limpet D2	0%	0%	3%	9%	12%	0%

The data for the second day shows a more gradual increase in Sea snails in the first two Quadrants, followed by a steady decline in Snails up to the highwater shoreline where no Sea snails can be found. For the Limpets, the percentage follows a similar pattern to the previous day, in the sense that they show a peak in occurrence just below the high-water waterline.

The weather on this day was wetter on this day in comparison to the previous day. Rain is more prevalent, and the overcast is a lot denser. Temperature was also lower than the past day, whereas the pH level remained closer to neutral this time round.

Day 3
Date	Saturday, 25.7.2020
Time	14:00-15:30
Low Tide	16:16
Tide height (low)	0.93m
Lux	20’000
Air temp	20°C
Weather	Sunny
	Q1	Q2	Q3	Q4	Q5	Q6
Dist. to shore	50m	40m	30m	20m	10m	0m
% under water	75%	50%	45%	35%	30%	10%
Sediment	Grav/Alg	Rock/alg	Alg/rock	Rock/alg	Sand/alg	Rock/alg
pH	7.5	8	7.5	7.5	6.5	7
Temp	16°C	16.5°C	20°C	20°C	19°C	20°C
% of Snail D3	6%	11%	15%	7%	3%	0%
% of Limpet D3	0%	0%	0%	10%	2%	0%

The data for the third day’s Sea snails follow a similar pattern to the first day. Gradual growth up to 20 metres from the shoreline, and for the remaining 30m a gradual decrease. The Limpet’s spread has become even tighter than before, as only two Quadrants even contain Limpets, of that, Quadrant five only holding 2% coverage with Limpets)

The weather on this day is sunny. Temperatures have risen again as the surrounding temperature has also assumed 20˚C.

Day 4
Date	Sunday, 26.7.2020
Time	15:30 - 16:30
Low Tide	17:03
Tide height (low)	1.08m
Lux	12’000
Air temp	19°C
Weather	Sunny
	Q1	Q2	Q3	Q4	Q5	Q6
Dist. to shore	50m	40m	30m	20m	10m	1m
% under water	75%	50%	45%	35%	30%	10%
Sediment	Grav/Alg	Rock/alg	Alg/rock	Rock/alg	Sand/alg	Rock/alg
pH	7.5	8	7.5	7.5	6.5	7
Temp	16°C	16.5°C	20°C	20°C	19°C	20°C
% of Snail D4	4%	8%	12%	6%	7%	0%
% of Limpet D4	0%	0%	13%	9%	9%	0%

On the Last day, the Sea snails showed to gradually increase up to 12% on the third quadrant, followed by a gradual decrease. The Limpets again can only be found from Quadrant 3 to Quadrant 5.

The two species seem to be able to coexist on Rock and algae, but not on Sand, as They have no grip on sand and are thus rendered defenceless, as they cannot move or hold on and will be taken by the tide.

Processed data and data analysis

$x^2=\sum \frac{{\left(O_i-E_i\right)}^2}{E_i}$

Day 1	L+	L-	Marginal Row Totals
S+	4 (3.5) [0.07]	3 (3.5) [0.07]	7
S-	1 (1.5) [0.17]	2 (1.5) [0.17]	3
Marginal Column Totals	5	5	10 (Grand Total)

By performing a Chi squared test for every day, we can check whether there is a correlation between the occurrence of the two species. Please Note that the value “1” Has been added to every occurrence to avoid having zero’s in the calculation. i.e. an occurrence of zero becomes one and an occurrence of three becomes four. The table will thus give us the following information: For S+ (Sea Snails are present), S- (Sea Snails are absent), L+ (Limpets are present) and L- (Limpets are absent) we fill out the interface area from our Raw data. Using the formula above, we can calculate (the expected cell totals) and [the chi-square statistic for each cell] using the total observed cells, for a significancy level at 0.05.

The Table below would yield following results: The chi-square statistic is 0.4762, with a p-value of 0.490153. This Chi-Squared Test result is not significant at p < 0.05.

The chi-square statistic is 0, with a p-value of 1. This Chi-Squared Test result is not significant at p < 0.05.

Day 2	L+	L-	Marginal Row Totals
S+	4 (4) [0]	4 (4) [0]	8
S-	1 (1) [0]	1 (1) [0]	2
Marginal Column Totals	5	5	10 (Grand Total)

	L+	L-	Marginal Row Totals
S+	3 (2.8) [0.01]	4 (4.2) [0.01]	7
S-	1 (1.2) [0.03]	2 (1.8) [0.02]	3
Marginal Column Totals	4	6	10 (Grand Total)

The chi-square statistic is 0.0794, with a p-value of 0.77816. This Chi-Squared Test result is not significant at p < 0.05.

The chi-square statistic is 0.4762, with a p-value of .490153. This Chi-Squared Test result is not significant at p < 0.05.

	L+	L-	Marginal Row Totals
S+	4 (3.5) [0.07]	3 (3.5) [0.07]	7
S-	1 (1.5) [0.17]	2 (1.5) [0.17]	3
Marginal Column Totals	5	5	10 (Grand Total)

Figure 3

In figure 3 one can quickly see that the increase in Limpets in Quadrant 3 has no detrimental influence on the occurrence of Sea snails. Neither does the depletion of Sea snail occurrence have an influence on Limpet occurrence. This still does not explain why Patella Vulgata and Neritoidea are such dominant species on this beach. Neither does it explain their consistent placement on the beach. For this we must investigate more biotic factors and abiotic factors. The statements that we can make about the occurrence of the Limpets and Sea Snails is that they cannot live outside of the tidal region as seen in quadrant 6. This type of Limpet also does not occur on the deep end of the Tidal region, as there are few rocks, to hold on to. Limpets’ natural predators are Sea stars, some Seals and during low tide even Birds, although the latter being the weaker enemy. Thus, Limpets take to the higher regions of the Tide to stay away from The Stronger enemies. During Low tide they can clamp themselves onto the substratum which they reside on. The smaller Sea snails can be found on a wider spread on the beach. As they attach themselves and feed off Kelp and Seaweed, they can allow themselves to venture further out in the tide to stay away from the birds.

Evaluation

The Chi Test Data suggest that hypothesis 1 is correct. The occurrence of the two species indicates no dependence on one another. This can also be visualized if the data is viewed in another format. The following graph shows the average percentage of Limpets over the 4 days and the average percentage of Sea snails (y-Axis) for each quadrant (x-Axis)

Here one can quickly see that the increase in Limpets in Quadrant 3 has no detrimental influence on the occurrence of Sea snails. Neither does the depletion of Sea snail occurrence have an influence on Limpet occurrence. This still does not explain why Patella Vulgata and Neritoidea are such dominant species on this beach. Neither does it explain their consistent placement on the beach. For this we must investigate more biotic factors and abiotic factors. The statements that we can make about the occurrence of the Limpets and Sea Snails is that they cannot live outside of the tidal region as seen in quadrant 6. This type of Limpet also does not occur on the deep end of the Tidal region, as there are few rocks, to hold on to. Limpets’ natural predators are Sea stars, some Seals and during low tide even Birds, although the latter being the weaker enemy. Thus, Limpets take to the higher regions of the Tide to stay away from The Stronger enemies. During Low tide they can clamp themselves onto the substratum which they reside on. The smaller Sea snails can be found on a wider spread on the beach. As they attach themselves and feed off Kelp and Seaweed, they can allow themselves to venture further out in the tide to stay away from the birds.

To conclude the placement on the beach of the two species is a result of abiotic factors such as the terrain/ sediment and biotic factors such as natural predators. The species show no correlation to one another.

Bibliography

BBC. (2021, 03 25). Organisation of an ecosystem. Retrieved from BBC Bitesize: https://www.bbc.co.uk/bitesize/guides/zqskv9q/revision/3

Maben, A. (2020, 10). enviroliteracy.org. Retrieved from Rocky Intertidal Transect Survey – Teacher/Student: https://enviroliteracy.org/teaching-resources/ap-environmental-science-course-material/rocky-intertidal-transect-survey-teacherstudent/

marlin. (2021, 2 11). Patella Vulgata. Retrieved from marlin: http://www.marinespecies.org/aphia.php?p=taxdetails&id=196314

naturalhistoryofscilly. (2020, 11 14). wildlife. Retrieved from naturalhistoryofscilly: http://naturalhistoryofscilly.info.websitebuilder.prositehosting.co.uk/wildlife

Species, M. (2021, 2 13). Neritoidea. Retrieved from marinespecies: http://www.marinespecies.org/aphia.php?p=taxdetails&id=196314

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[1] http://naturalhistoryofscilly.info.websitebuilder.prositehosting.co.uk/wildlife (naturalhistoryofscilly, 2020)

[2] https://www.marlin.ac.uk/species/detail/1371 (marlin, 2021)

[3] http://www.marinespecies.org/aphia.php?p=taxdetails&id=196314 (Species, 2021)

[4] https://www.bbc.co.uk/bitesize/guides/zqskv9q/revision/3 (BBC, 2021)

[5] 70% = 70 squares covered