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Cockle Size
Variation at Different Intertidal Ranges

Introduction,

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Due to the variation of bivalve in shore
heights, there has been a significant spike in interest for researchers in
recent years. In order to protect themselves from predators, bivalves live in
sediment along the shore lines. The Common Cockle (Cerastoderma edule) is the focus of the studies, native to North
Western European shores. These mollusks are well adapted to surviving in the
changing conditions of the shore lines they reside in. Studies have suggested
that the primary reason for the variation in their sizes is the level of
predators present in their habitats.

 

One
potential cause for the decline in mollusks is the haematopus ostralegus, using various methods to catch and kill c.edule using its hard shell against it the
oyster catcher will drag the mollusk deeper into the sand until it reaches an
area dense enough in order to crack the shell. If there are more vulnerable
mollusks in the area, those whose shells are already cracked or exposed
already, it will instead stab them. This affects the hunting patterns of the
predator.

 

Another predator
is the carcinus maenas of the
Atlantic Ocean is able to hunt the mollusks and is present in many of the areas
which c.edule also inhabits. In
addition to their hunting of c.edule they
hunt a large variety of other organisms. It is reported that they are the cause
of many declines in populations across mollusks, and is able to

survive
outside of the water for large amounts of time. This ability allows them to
take advantage of the changing conditions in a way that c.edule cannot, creating an abundance of food and shelter.
Subsequently, the population of the mollusks is heavily influenced by the
presence of these predators.

 

Method,

 

A study was conducted on 27th of
October 2017 on the tip of Anglesey to the south, in North Wales. The bay is
called “Traeth Melynog”, the lower shore is rocky, changing to sand further up
the beach, most of the area is exposed. Abermenai point provides shelter to support
stable conditions to study.

 

The study
aimed to measure the population of cockles from a wide variety of areas along
the shore. Due to the environment, there were numerous problems which had to be
overcome in order to ensure a successful study. The biggest issue facing the
students was the changing tides, the exposed and flat surface of the beach
leads to fast moving waters, reaching up to 8 knots (kts). As such, the timing
of the study had to be perfect. The vast area covered by the beach meant that the
study had to be conducted in different areas in order to meet the requisite
heights of the shorelines, high mid and low. The distanced between these
resulted in areas where data was not collected, creating an increase in
variability that could have potentially created variance in the data.
Compounding this, certain previously designated areas were impossible to survey
as the sediment was too hard. This caused some groups to have to deviate from
their originally planned areas, creating more variation.

 

The
population of the cockles was measured at similar distances for each shore
level from west to east. One metre squared quadrats were thrown randomly, and
this section was studied by digging out the sediment from surface level using a
metal spade. To ensure the data was reliable, each group would throw three
quadrates per subgroup. 10cm of sediment was removed from surface level before
being placed into a sieve. All mollusks and other organisms that were found
were noted, and the living organisms were discarded. The c.edule were counted before being placed into bags to be frozen to
preserve the samples for examination. This method was repeated.

 

The
collected c.edule were then examined
for their length, height, weight and width. Measured accurately using a Vernier
caliper and an electronic scale were used with an accuracy of two decimal
places. The measurements were recorded for graphs and tables comparing the
variables in order to determine any correlation with the abundance of c.edule and its measurements.

 

Results,

 

The study was carried out to determine the
quantity and distribution of the cockles. Figures 1, 2, and 3 demonstrate the
findings of October 2017. There was a large amount of cockles through Treath
Melynog as they were found at very high and very low shore levels. (Figures 1,
2 and, 3). There was also a high level of variation in the densities at
different shore heights. There were twenty cockles at the high shore level, and
34 on the lower level. The highest level was in the middle with 649 c.edule accounted for (figure 2).
Therefore we can conclude that the heighest level is the middle.

 

In
addition, we observed (figures 1, 2, and 3) that there is a variation in the
length of the cockles at each shore height. The mid shore was between 20 and 21.9mm
whilst the high shore had 12 to 13.9mm in length. Low shore also had a larger size,
being higher than mid shore at 28 and 29.9mm in length.

 

However,
due to the significant samples found in the mid section, the majority being
over 20mm, it was decided that the average population was within the smaller
range sampled.

 

Figure 1 –
The number of cockles found at high shore and their lengths.

 

Figure 2 –
The number of cockles found at high mid shore and their lengths.

 

 

Figure 3 –
The number of cockles found at Low shore and their lengths.

 

 

Figure 4

 

 

Discussion,

 

The main difference between the low shore and
high shore is that the high shore is far more exposed than the lower shore. The
low shore is covered by the sea for much of the time, leading to far less
predation from predators such as the oystercatcher. However, it is more exposed
to the crabs that prey on it.

 

Previous
research on the feeding behavior of oystercatchers suggests that there is a
link between the exposure of the region, and the effects of predation on the
mollusks.

In the
months of October, a large influx of oystercatchers occurs on the Welsh shores.
The higher area had therefore been largely impacted by this increase in
predators, suggesting that the elevated section was far more vulnerable to the
birds. The hammering method that the birds allowed them to fracture stronger
shells, so long as they are not under the water (11, 15). The oystercatchers
prefer to feed on the larger cockles for energy efficiency (14). Mid and low
shore are only exposed to this bombardment during specific times of the day,
and are therefore less touched by the predators. There is a link between the
birds preferring larger cockles, and there being an abundance of smaller
cockles in the mid shore.

 

Carcinus maenas (shore crab) live on the shore and move with
the tide (10), feeding off of bivalve based on their size. Another study (10)
suggests that 72.8% of C.Maenas are
abundant as their prey, c.edule are
also very abundant on the shores. These are a plentiful source of food (3). The
density of the mollusks is directly related to the movement of the shore crabs.
The abundance of the mollusks has however been decreasing in the region, it is
possible that they are moving deeper into the sediment to avoid the present
predation (fig 4). The shore crabs are affected by temperature, moving further
out to sea the warmer the ocean gets above 8 degrees centigrade. This would
increase the percentage of shore crabs that live on the shore.

 

Conclusion,

 

Predation appears to be the main factor in
mollusk distribution in the region. The shore population is affected by
numerous predators, such as oystercatchers and shore crabs. The mid shore is
the safest region, as demonstrated by their increased abundance. This study
also occurred during the height of the oystercatcher season, in October.
However, other factors would need to be investigated in order to understand the densities of mollusk distributions.

 

References

 

Hunt, Heather L., and Lauren S. Mullineaux. ‘The Roles
Of Predation And Postlarval Transport In

Recruitment Of The Soft Shell Clam ( Mya Arenaria )1.
Limnol. Oceanogr. 47.1 (2002): 151-164. Web.

 

Urrutia, M.
B., J. I. P. Iglesias, and E. Navarro. Hydrobiologia 355.1/3 (1997): 173-180.
Web

 

Sanchez-Salazar,
M.E., C.L. Griffiths, and R. Seed. ‘The Interactive Roles Of Predation And
Tidal

Elevation
In Structuring Populations Of The Edible Cockle, Cerastoderma Edule’.
Estuarine, Coastal

and Shelf
Science 25.2 (1987): 245-260. Web.

 

Sutherland,
William J. ‘Spatial Variation In The Predation Of Cockles By Oystercatchers At
Traeth

Melynog,
Anglesey. I. The Cockle Population’. The Journal of Animal Ecology 51.2 (1982):
481. Web.

 

Bangor.ac.uk,.
‘Fieldwork Locations I College Of Natural Sciences I Bangor University’. N.p.,

2015. Web.
30 Nov. 2015.

 

Glaucus.org.uk,.
‘Menai Strait (BMLSS Information)’. N.p., 2015. Web. 1 Dec. 2015

 

Sutherland,
William J. ‘Spatial Variation In The Predation Of Cockles By Oystercatchers At
Traeth

Melynog,
Anglesey. II. The Pattern Of Mortality’. The Journal of Animal Ecology 51.2
(1982): 491.

Web.

 

Knox, G. A.
The Ecology Of Seashores. Boca Raton, Fla.: CRC Press, 2001.

 

Stanley,
Steven M. Relation Of Shell Form To Life Habits Of The Bivalvia (Mollusca).
(Boulder,

Colo.:
Geological Society of America, 1970

 

K. Thomas Jensen, Dynamics and growth of the cockle,
Cerastoderma edule, on an intertidal mud-flat in the Danish Wadden sea: Effects
of submersion time and density, In Netherlands Journal of Sea Research, Volume
28, Issue 4, 1992, Pages 335-345, ISSN 0077-7579

 

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