1. Microplastic Pollution: a
potential threat to the intertidal
invertebrate food web
Literature Review
All work present in this
report is my own and
was carried out during
the course of Applied
Bioscience and Zoology
during the session
2016-2017.This report
has been carried out to
fulfil the requirements
of the Bioscience
Research Project within
the School of Science at
the University of the
West of Scotland.
Zoe Sloan B00266133
Supervisor – Dr Brian Quinn

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Table of Contents
1. Introduction........................................................................................................................2
2. Microplastics Pollution......................................................................................................3
2.1 Microplastics in the Marine Environment...................................................................3
2.1.1 Primary Microplastics..........................................................................................4
2.1.2 Secondary Microplastics......................................................................................4
2.2 Distribution of Microplastics ......................................................................................5
2.2.1 Geographical Distribution....................................................................................5
2.2.2 Distribution in the Water Column........................................................................7
2.3 Interactions with Marine Organisms...........................................................................8
2.3.1 Ingestion...............................................................................................................8
2.3.2 Translocation........................................................................................................9
2.3.3 Absorption of Pollutants ......................................................................................9
3. Trophic Transfer of Microplastics ...................................................................................11
3.1 Intertidal Food Web ..................................................................................................12
3.1.1 The Blue Mussel (Mytilus edulis)......................................................................12
3.1.2 The Common Starfish (Asterias rubens)............................................................13
3.1.3 The Edible Crab (Cancer pagurus).....................................................................13
3.2 Microplastic Uptake by the Blue Mussel (Mytilus edulis) .......................................14
3.3 The Transfer of Microplastics to the Crab ................................................................15
3.4 Microplastics and Echinoderms ................................................................................15
4. Conclusion .......................................................................................................................16
5. References........................................................................................................................17

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1. Introduction
Plastics are synthetic materials, made from organic polymers that are produced from the
polymerisation of monomers obtained from gas and oil sources (Cole et al., 2011). As
plastics are inexpensive to produce, as well as lightweight, durable and do not easily degrade,
their production has drastically increased (Wang et al., 2016). In the 1950s, 1.5 million metric
tonnes of plastic were produced globally, by 2014 the annual production had increased to 311
million tonnes (Avio et al., 2016; Cole et al., 2011) (See figure 1). Europe is one of the
largest producers of plastic in the world, producing approximately 20% of the global annual
production, they are second only to China, who produce 26%.
As a result of plastics being durable and unsusceptible to degradation, disposal can prove
difficult and can become harmful to the environment when they are not recycled or disposed
of correctly. Of the plastic produced, approximately 10% ends up the oceans (Avio et al.,
2016; Cole et al., 2011). When plastics are in the ocean they degrade at a much slower rate
and this process can take hundreds of years (Thompson et al., 2004). With the annual
production increasing and a long degradation time, plastics are becoming a serious
environmental threat to marine life. The accumulation of macroplastics has a serious effect on
Figure 1: A comparison of the global plastic production and the plastic produce in Europe
from 1950 to 2014 (Statista, 2016). Since 1950 the global production of plastic hasrapidly
increased from1.5 million metric tonnes to 311 million tonnes in 2014. The production of plastic
in Europe, the second largest produced, has also increased from0.35 million tonnes in 1950 to a
peak of 60 million tonnesin 2008. Since 2008,Europe’s plastic production has remained at an
average of 57 million tonnes per year.

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many marine vertebrates, including injury and death of sea birds, marine mammals, and fish
as a result of becoming entangled or by ingesting such plastics. The accumulation of plastics
in the environment also has negative impacts of the economy. From the costs of beach
cleaning to the financial damage caused to marine ecosystems, it is estimated that such
pollution costs $13 billion per year.
Macroplatics are not the only plastic pollutant that is causing damaging effects to the marine
environment. An emerging threat in marine pollution is microplastic pollution (Wang et al.,
2016). Microplastics are defined as any small plastic particle with a diameter of 5mm or less
and have recently been identified as a threat due to ubiquity in the oceans and their potential
to have harmful interactions with marine biota. The presence of microplastics has been
recorded on beaches, in the surface waters, at various points in the water column and in the
benthos (Wright et al., 2013), and in various marine environments from the poles to the
equator (Thompson et al., 2004). Due to their small size and their wide distribution,
microplastics are considered bioavailable to a variety of marine animals. They are of similar
size to plankton and so have the potential to be ingested by filter- and suspension-feeding
animals such as the blue mussel (Mytilus edulis). Invertebrates such as the blue mussel are the
basis for many food webs and so it is possible that microplastics can be transferred to high-
trophic level animals through ingestion (Wang et al., 2016).
The aim of this study is to determine whether trophic transfer of microplastics is possible
between the blue mussel (Mytilus edulis), the common starfish (Asterias rubens) and the
edible crab (Cancer pagurus).
2. Microplastics Pollution
2.1 Microplastics in the Marine Environment
Microplastics are generally described as plastic particles with a diameter of <5mm (Wang et
al., 2016; Avio et al., 2016). The presence of microplastics in oceans was first recorded in the
1970s but received very little attention (Cole et al., 2011). In recent years, scientists have
developed a new interest in the source of these plastics, the distribution in the environment
and their potential to cause harm to marine biota. Microplastics enter the marine environment
in several ways: (1) from beaches and other land-based sources. Such as rivers, wastewater,
runoff of storm water, and can be transported by the wind (Avio et al., 2016); (2) from

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materials lost during nautical activates including commercial and recreational fishing, and (3)
from rubbish being dumped into the sea. Common microplastics that have been recorded in
the oceans are polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinylchloride
(PVC), polyamide (PA), polyethylene terephthalate (PET) and polyvinyl alcohol (PVA)
(Andrady, 2011; Wright et al., 2013; Avio et al., 2016). Depending on the origin of such
microplastics, they can be classified as primary or secondary microplastics (Avio et al., 2016;
Cole et al., 2011).
2.1.1 Primary Microplastics
Primary microplastics are plastics that are specifically manufactured to be microscopic in size
for various purposes (Cole et al., 2011). Typically, these plastics are used in cosmetic
products, and face and body scrubs. They are also found in air-blast media, textiles, and in
synthetic clothing materials (Avio et al., 2016). In the 1980s, the use of microplastics in
exfoliating scrubs and other cosmetic products was patented and their use has drastically
risen ever since. Polyethylene particles of <5mm and spherical polystyrene plastics of <2mm
and both commonly reported as being present in cosmetics (Cole et al., 2011). As well as
their use in cosmetics, microplastics are commonly used in air-blasting. This is a technique
for removing rust from boat hulls, engines and other machinery by blasting it with
microplastic scrubbers such as acrylic melamine and polyester. These “scrubbers” are used
constantly until they become too small to remove rust and paint so can no longer fulfil their
purpose and as a result of repetitive usage, they can be subjected to heavy metal
contamination. Heavy metal contaminated microplastics have the potential to cause serious
environmental damage. Primary microplastics enter the marine environment directly by
runoff.
2.1.2 Secondary Microplastics
Secondary microplastics form the majority of the microplastics present in the ocean (Avio et
al., 2016). They arise from the degradation of meso- and macro-plastics. Degradation is a
process by which chemical, biological and physical changes reduces the overall molecular
weight and the structural integrity of macroplastics, causing the polymers to become so brittle
that they begin to fragment into smaller microplastics (Cole et al., 2011; Avio et al., 2016).
This process is mostly controlled by reactions such as photo- and thermal-oxidation,
biodegradation (controlled by microbes), and hydrolysis. Photo-oxidation is the result of

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plastics being exposed to UV radiation over a prolonged period of time, this causes the
polymer matrix to become oxidised and the bonds being broken.
The rate at which degradation occurs is dependent on the type of plastic, the availability of
oxygen, and the temperature of the surrounding environment. Plastic litter present on beaches
are exposed to high oxygen levels, high temperatures and direct sunlight, so degradation is
rapid. However, in the significantly colder saltwater environment, where photo-oxidation is
limited, degradation will occur much slower.
The degradation process is continuous and plastic fragments will become smaller the longer
they are exposed to these conditions. It has been suggested that microplastics may eventually
degrade to nanoscopic size, however the smallest recorded plastic was just 1.6µm in diameter
(Cole et al., 2011). A review by Andrady (2011) investigates the potential threat of
nanoplastics in the environment. In this review Andrady (2011) suggests that plastic
nanoparticles in the marine environment are particularly a threat to nano- and pico-plankton
as they are within a similar size range. He highlights that although there is limited data on the
effects of nanoplastics, they may have the potential to absorb heavy metal contaminants and
persistent organic pollutants (POPs).
2.2 Distribution of Microplastics
2.2.1 Geographical Distribution
In order to understand how much of a threat microplastic pollution is to the marine
environment, it is important to understand their distribution – both geographically and in the
water column (Lusher, 2015). Recent studies have been carried out to determine the
abundance of microplastic litter present in the oceans and suggest that it is possible there is
between 7000 and 35,000 tonnes of plastic in the open oceans.
Thompson et al. conducted a study in 2004 to determine the accumulation of microplastics in
pelagic zones and sedimentary habitats in the U.K. The research team collected sediment
samples from beaches, estuaries and from subtidal sediments in Plymouth. They separated the
less dense particles by floatation and removed particles that appeared to be of natural origin.
The particles were identified using Fouier Transform infrared (FT-IR) spectroscopy and one
third of the plastics were known synthetic polymers including polyethylene, polypropylene,
polyamide and polyester. These plastics are all commonly used in clothing and packaging,

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suggesting that these are secondary microplastics as a result of degradation. Thompson et al.
also noted that there was a greater abundance in microplastics in the subtidal sediment
samples (Figure 2).
Thompson et al. examined a further 17 beaches in order to determine the extent of
microplastic pollution and found that the results were similar to those demonstrated in the
Plymouth samples, indicating that microplastics are a common occurrence in sediments. As
well as sampling sediments, Thompson et al. sampled plankton to investigate the long-term
abundance of microplastics in the marine habitat. Samples were collected regular from the
1960s from between Aberdeen and the Shetlands, totalling in 315km, and from Sule Skerry to
Iceland, totalling in 850km. In 1960, they found microplastics in the plankton samples but
recorded an increased in abundance as time progressed (Figure 2). The findings this study
demonstrate the wide geographical range of these plastics and their ability to accumulate in
the environment.
A similar study was carried out by Browne et al. in 2011 to investigate the spatial distribution
and accumulation of microplastics on shorelines in six continents, and to determine if their
distribution relates to sources and sinks. They collected sediment samples from eighteen
shores across the six continents including Australia, Chile, USA, the UK, Oman, the
Philippines, Portugal, South Africa and Mozambique. They discovered that all eighteen
shores had microplastic particles present, the abundance of which ranged from 2 (Australia)
Figure 2: (D) a comparison of microplastic abundance in sandy, estuarine,and subtidal
sediments, (E) microplastics present in samplesfrom 1960s to 1990s (Thompson et al.,
2004). Microplastics are present in sandy, estuarine, and subtidal environments, with
subtidal zones have a greater abundance. Microplastics were also recorded in plankton
samples. It was found that overtime, microplastic concentrations increased.

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to 31 (UK and Portugal) fibres per 250 ml of sediment (Figure 3). Using FT-IR they
identified these microplastics as polyester, acrylic, polypropylene, polyethylene, and
polyamide. Their research also demonstrated that microplastic abundance was greater in
highly populated areas and less abundant in areas where there was a smaller population
density. The research carried out by Browne et al. provides evidence that microplastic
pollution is a global issue.
2.2.2 Distribution in the Water Column
The position of microplastics in the water column depends on the polymer density (Wright et
al., 2013). Plastics with a lower density than seawater will float in the upper levels of the
water column and will be present in the surface water, whereas high-density plastics will sink
to the bottom of the water column and contaminate the benthos (table 1).
However, the density of microplastics can be altered by a process known as biofouling (Cole
et al., 2011; Wright et al., 2013). Plastics in aquatic environments develop a layer of
microbes, known as a biofilm, which allows algae to colonise and invertebrates to attach on
to the plastics surface. This increases the density and allows the plastic to sink at a faster rate
(Avio et al., 2016). The rate at which biofouling occurs is dependent on the type of polymer,
surface energy and the conditions of water. The position of microplastics in the water column
determines their bioavailability. Plastics with a lower density are more likely to be ingested
Figure 3: The global distribution of microplastics and their abundance (Browne et al., 2011).
Microplastic were recorded in all of the eighteen shores that were sampled. The abundance of
microplastics varied from2 fibres per 250ml of sediment in Australia,to 31 fibres per 250ml of sediment
in the UK.

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by filter feeders and planktivores that generally inhibit the upper water column (Wright et al.,
2013), whereas higher density polymers that sink to the benthos will be ingested benthic
suspension- and filter-feeding organisms, as well as by detritivores and deposit feeders.
Table 1: Plastic Density and Origin
2.3 Interactions with Marine Organisms
2.3.1 Ingestion
Ingestion of microplastics is likely the most common way that they will interact with marine
biota. Especially organisms that do not have the ability to differentiate between particles of a
similar size (Avio et al., 2016). However, because of their small size and their presence in the
Matrix Density (g/cm3
) Source Reference
Sea Water 1.025 (Avio et al., 2016)
Low-density polyethylene
(LDPE)
0.91 – 0.93 Plastic bags, wire cables,
bottles.
(Wang et al., 2016;
Andrady. 2011).
High-density
polyethylene (HDPE)
0.94 Milk jugs, household
cleaner bottles, butter
containers
(Wang et al., 2016;
Andrady, 2011).
Polystyrene (PS) 1.04 – 1.11 Plastic utensils,
disposable cups, egg
cartons, food containers.
(Andrady, 2011; Avio et
al., 2016; Wang et al.,
2016).
Polypropylene (PP) 0.89 – 0.91 Bottle caps,netting, rope (Andrady, 2011; Avio et
al., 2016; Wang et al.,
2016).
Polyamide (PA) 1.13 – 1.5 (Avio et al., 2016)
Polyvinyl chloride (PVC) 1.20 – 1.45 Plastic film, medical
equiptment, shampoo
bottle
(Andrady, 2011; Avio et
al., 2016; Wang et al.,
2016).
Polyvinyl alcohol (PVA) 1.19 – 1.35 (Avio et al., 2016)
Polyethylene
terephthalate (PET)
1.38 – 1.39 Water and other beverage
bottles.
(Andrady, 2011; Avio et
al., 2016; Wang et al.,
2016).

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water column and the bethos, ingestion of microplastics poses a threat to many marine
species (Cole et al., 2011). Various studies indicate that primary- and low-trophic level
organisms are particularly susceptible to microplastics ingestion (Cole et al., 2011; Wang et
al., 2016). These organisms include zooplankton, mussels, barnacles, echinoderms and shore
crabs (Carcinus maenas). It is yet to be understood if microplastic ingestion has any negative
effects on the health of marine biota such as morbidity, mortality or on the success of
reproduction, but it has been suggested that it may cause similar mechanical effects that large
plastics would cause. They can potentially effect feeding by blocking appendages or prevent
food from passing through the intestinal tract, they could affect enzyme production, reduce
the rate of growth, or they could also leach toxins into an organism after ingestion has
occurred (Wright et al., 2013). The ingestion of microplastics not only poses a threat to the
organisms that have ingested them, but also have the potential to be passed through the food-
web and cause damage to higher-trophic level organisms as well.
2.3.2 Translocation
After ingestion, microplastics have several possible fates. They might remain in the digestive
tract, be egested in the faeces, be taken up by the epithelial lining in the gut through
phagocytisis, or be translocated to other tissues such as the circulatory system (Wang et al.,
2016). The exact mechanisms for translocation is currently unknown (Wang et al., 2016;
Wright et al., 2016) but it has been demonstrated in blue mussels (Mytilus edulis) and crabs.
Studies have shown that after translocation occurs, microplastics are retained for longer
periods of time. Microplastics were observed in the haemolymph and haemocytes up to three
days after exposure to microplastics (Wright et al., 2016). However, despite microplastics
being present in the circulatory system of M.edulis, there was no adverse effects observed.
This implies that further research should be done to determine if negative effects can occur as
a result of translocation. The process of translocation also poses a risk for higher-trophic level
organisms because the longer microplastics remain in an organism, the more likely a predator
species will uptake those plastics through the ingestion of prey items.
2.3.3 Absorption of Pollutants
Microplastics, particularly those with a large surface area to volume ratio, are highly
susceptible to contamination by various pollutants, such as heavy metals, endocrine
disruptors, persistent organic pollutants (POPs), and various other chemicals (Cole et al.,

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2011; Avio et al., 2016; Wang et al., 2016). However, the potential effects of these pollutants
on marine biota is not very well studied (Browne et al., 2013).
Typically, plastics are considered to be biochemically inert however, plastic additives are
commonly added into polymers during the manufacturing process to increase their resistance
to heat and improve other desirable properties (Avio et al., 2016). Plastics can also
accumulate pollutants from the environment, such as nonlphenol and phenanthrene, which
could then potential desorb and be released after ingestion.
A study by Brown et al (2013) aimed to determine the impacts of contaminated microplastics
on the lungworm (Arenicola marina). In this study, they exposed the lungworm to sand that
contained 5% microplatics, the microplastics have previously be exposed to nonlphenol and
phenanthrene. They found that the microplastics were able to transfer the pollutants to the gut
tissues of the worms. These pollutants reduced the survival rate of the worms and had
negative impacts of feeding, immunity, antioxidant capacity.
Microplastics that are contaminated with POPs can be transported throughout oceans and are
found from coastal regions to remote subtropical gyres (Cole et al., 2011; Wang et al., 2016).
Several POPs are considered as toxic and include a variety of endocrine disruptors, and
carcinogenic and mutagenic chemicals. Polyethylene and polypropylene are both common
microplastics that can absorb POPs such as DDTs, polycyclic aromatic hydrocarbons
(PAHs), and polychlorinated biphenyls (PCBs). Since both polyethylene and polypropylene
are bioavailable to low-trophic invertebrates, it is likely that microplastics can act as a vector
for POPs and result in bioaccumulation in higher-trophic vertebrates (Frias et al., 2010).
In 2014, Bakir et al. investigated the potential for microplastics to absorb a variety of POPs
and to determine their potential to be bioavailable to marine biota after ingestion. They
investigated the potential for PVC and PE to desorb DDT and other POPs under simulated
gut conditions, and the role of gut surfactants in this process. Their results showed that the
desorption rate of POPs was significantly higher, almost 30 times, than in seawater alone.
They also determined that the pH and temperature influenced the rate of desorption.
Desorption rate was significantly increased in gut condition simulating warm-blooded
organisms.

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3. Trophic Transfer of Microplastics
The ingestion of microplastics by primary trophic-level organisms such as phytoplankton and
zooplankton presents a potential for the transfer of these plastics into the food web (Lusher,
2015). The migration of zooplankton through the water column could allow microplastics to
be transferred to organisms that feed at the water’s surface as well as organisms that live and
feed on the seabed.
A recent study was carried out in 2014 by Setälä et al. to investigate how microplastics may
be transferred through planktonic food webs. They first aimed to determine if various
mesozooplankton would ingest fluorescent polystyrene spheres and then tested the possible
transfer of these microspheres to mysid shrimps. They found that all of the zooplankton
species tested contained microplastics but at varying concentrations. When they exposed
these contaminated zooplankton to mysid shrimps (Mysis spp.) they discovered that all
exposed individuals of the Mysis relicta species had the polystyrene spheres present in their
intestines.
This study demonstrated that microplastics can be trophically transferred into food webs. As
zooplankton are the key food source for the blue mussel (Mytilus edulis) there is a possibility
that the trophic transfer of microplastics may have a negative impact on the intertidal food
web.
As well as impacting invertebrates, microplastics have the potential to enter the digestive
system of vertebrates through trophic transfer, although this has not been greatly researched.
Lusher et al., (2015) investigated the presence of microplastics in the cetacean the True’s
beaked whale (Mesoplodon mirus). In May of 2013, three True’s beaked whales stranded on
the north and west coasts of Ireland. The stomach of each whale was examined and analysed
for the presence of man-made debris. One of these whales was screened for microplastics,
and it was found that they were present throughout the whole digestive tract. Along with
microplastics, partially digested mesopelagic fish were also recovered from the stomach of
the whales. However, the source of the microplastics was not determined in the study, but it
is possible that they were ingested via trophic transfer from their prey. In a previous study by
Lusher et al. (2012) it was shown that pelagic fish can ingest microplastics from the
environment. Five species of pelagic fish were investigated, as well as five species of
demersal fish. Microplastics were found in the digestive tract of each of the ten species.

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Considering that microplastics are found in a variety of fish species, there is a likelihood that
these can be transferred to predatory species. Although, further research is required to
determine the potential, population-level impacts.
3.1 Intertidal Food Web
3.1.1 The Blue Mussel (Mytilus edulis)
Mussels are a bivalve species that common in low and mid intertidal zones in circumpolar
regions (Little and Kitching, 1996). They are an essential prey item in several benthic
communities due to their wide geographical range (figure 4) and ability to form mussel beds
(Little and Kitching, 1996; Browne et al., 2008). Mussel beds form on rocky surfaces and soft
seabed’s, where they act as “economical-engineers, providing a habitat and food source for
large marine organisms.
Despite the mussels’ strong shell, they have a variety of predators. The common starfish
(Asterias rubens) is one such predator. It uses its appendages to pry apart the mussel shell
and, when a small gap is opened in the shell, the starfish will push its stomach into the mussel
to begin digestion (Hickman et al., 2013, p.472). Crabs, including Carcinus maenas (the
shore crab) and Cancer pagurus (the edible crab), also feed on mussels, using their claws to
crush the shell (Little and Kitching, 1996).
Figure 4: The geographical distribution of Mytilus edulis and other marine mussels.
(Gaiitan-Espitia et al., 2016).

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3.1.2 The Common Starfish (Asterias rubens)
Starfish are an echinoderm species that can be found in the subtidal and intertidal zones
(Calderwood et al., 2016). Due to them being a major predator in these habitats, starfish are
often referred to as a keystone species. In Europe, the most common starfish species is
Asterias rubens. It is an opportunistic predator that, due to its tube feet and the ability to evert
its stomach, can feed on a large array of prey including molluscs, crustaceans and other
echinoderms, they particularly prefer the blue mussel M. edulis. Starfish actively hunt their
prey using chemoreception and are often known to aggregate in high populations where there
is a rich abundance of food sources (figure 5).
3.1.3 The Edible Crab (Cancer pagurus)
The edible, or brown, crab (Cancer pagurus) is a widely distributed species found on shores
from the English Channel and the North Sea to the Portugal coast (MarLIN, 2005). It can be
found in rocky shores and in the lower and sublittoral zones. It is an active hunter that preys
on a variety of species such as dog whelks (Nucella lapillus), periwinkles (Littorina littorea),
blue mussels (Mytilus edulis), and will also eat other crabs including the shore crab (Carcinus
maenas). As well as actively hunting their mobile prey, C.pagurus can also burrow into the
sediment in order to ambush their prey (figure 6).
Brown crabs often dig large pits in the sediment in the hunt for bivalves such as razor clams
and mussels. These pits can also be used for hiding during the day. This is a form of predator
avoidance in order to avoid being eaten by wolf fish, seals and cod.
Figure 5: A) Asterias rubens aggregate on a bed of blue mussels (Mytilus edulis) (North
Wales Wildlife Trust, 2016). B) Starfish feeding on mussels.
A) B)

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3.2 Microplastic Uptake by the Blue Mussel (Mytilus edulis)
Mussels are suspension feeding bivalves that have specific mechanisms for suspension
feeding (De Witte et al., 2014). As microplastics have a small size range that is similar to the
size of planktonic organisms they are bioavailable to small invertebrates, including mussels,
that would not normally be affected by large plastic debris (Van Cauwenberghe et al., 2015).
Von Moos et al. carried out a study in 2012 to determine if the blue mussel (Mytilus edulis) is
able to uptake microplastics and if these plastics would have an effect on their cells and
tissues. To carry out this study they used a high-density polyethylene (HDPE) fluff, with
sizes ranging from 0-80µm, as the model plastic. They set up an experiment of six glass
beakers each containing three mussels. Three of the beakers were exposed to the HDPE and
the other three were a negative control with no exposure to the microplastics. The results of
this experiment demonstrated that the mussels were able to uptake the microplastics through
two pathways. The first pathway was via the gills. They found that microplastics were
transferred to gills through endocytosis. This pathway also transported microplastics to the
blood lacunae in the gills. The second pathway by which microplastics can enter the mussels’
tissues occurs through ciliae movement. This pathway transported the microplastics into the
stomach and intestines, and other digestive organs. This study clearly demonstrated that is
possible for microplastics to directly enter the mussels’ digestive system.
Figure 6: Edible Crab (Cancer pagurus) feeding on dead, stranded starfish (Asterias
rubens).

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Further research is required to determine if this uptake of microplastics has a negative impact
to the health of the mussel. Research is also required to investigate the accumulation of
microplastics in mussels due to direct uptake and through transfer from zooplankton. Mussels
are a food source to humans and as such, the bioaccumulation of microplastics in mussels
may have an effect on humans as well.
As mussels are an important prey item to various intertidal invertebrates, it is important to
identify the threats that trophic transfer may pose to the intertidal population.
3.3 The Transfer of Microplastics to the Crab
Studies involving the transfer of microplastics to the edible crab have not been carried out as
of yet. However, there are studies that show that the common shore crab has the potential to
uptake microplastics and to ingest them by eating mussels.
In 2004, Watts et al. designed an experiment to test the uptake of microplastics by shore crabs
(Carcinus maenas). During this experiment the crabs were fed mussels that had been exposed
to polystyrene microspheres (10µm). After 24 hours of exposure to the contaminated mussels,
all of the crabs that were sampled had microplastics present in the foregut. This, therefore,
demonstrates that the trophic transfer of microplastics from mussels to a predatory species is
possible.
A similar study was carried out by Farrell and Nelson in 2013. For their experiment, they
exposed mussels to 50µl of 0.5µg fluorescent microspheres for one hour. They opened the
shell of the mussels to determine if the microplastics had been ingested. The mussels that had
ingested the microplastics were then fed to the shore crabs. Samples of the crab haemolymph
were taken at one hour, two hours and four hours after the feeding. They found that
microplastics were present in the tissue samples of the stomach, hepatopancreas and the gills.
Microplastics were also found in all haemolymph samples that were taken throughout the
experiment. This further proves that trophic transfer of microplastics will occur in nature
3.4 Microplastics and Echinoderms
The impacts, or potential transfer, of microplastics to starfish have not yet be investigated.
However, the ingestion of microplastics in other echinoderm species has been studied.
Graham and Thompson (2009) produced a study to determine if four species of sea cucumber

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would uptake microplastic fragments along with sediment. Their model organisms were two
deposit-feeding holothurians (Holothuria floridana and Holothuria grisea), and two
suspension-feeders (Cucumaria frondosa and Thyonella gemmate). The four echinoderm
species were exposed to three different plastics: PVC fragements (0.25mm – 5mm), nylon
(0.25mm – 1.5mm), and PVC pellets (4mm).
The results of this study were that all four species ingested microplastic fragments. It was
also shown that not only were microplastics ingested, but they were actively selected over
sand grains and other sedimentary particles. This could have serious negative effects on the
feeding habits of holothurian species. During the course of this study, it was found that PCBs
were present on some of the microplastics that were sampled. Since this study showed that
the microplastics were ingested, it can be assumed that the PCBs were also taken up by the
organisms. As well as causing damaging effects to the health of the organism, these
contaminates may accumulate and be transferred to predatory species.
4. Conclusion
Microplastic pollution is a potential threat in the marine environment. Due to their small size
and bioavailability, marine invertebrates are particularly at risk due to ingestion.
Microplastics that are ingested by marine invertebrates can then be transferred through the
food web and have negative impacts on larger marine organisms. Evidence shows that the
blue mussel is able to uptake microplastics directly from the water and indirectly through
ingestion of microplastic contaminated zooplankton. These microplastics can then be
transferred to predatory species. Although previous studies indicate the transfer of
microplastics to crabs, it may be possible for microplastics to be transferred to other predators
such as starfish (Asterias rubens). Starfish may then also be a source of trophic transfer to
their predators such as the edible crab. This study may show that microplastic pollution is a
potential threat to the intertidal food web, and therefore could be a threat to many other
marine organisms.

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