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

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

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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 has rapidly increased from 1.5 million metric tonnes to 311 million tonnes in 2014. The production of plastic in Europe, the second largest produced, has also increased from 0.35 million tonnes in 1950 to a peak of 60 million tonnes in 2008. Since 2008, Europe’s plastic production has remained at an average of 57 million tonnes per year.

environmental threat to marine life. The accumulation of macroplastics has a serious effect on

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,

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runoff of storm water, and can be transported by the wind (Avio et al., 2016); (2) from

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,

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biodegradation (controlled by microbes), and hydrolysis. Photo-oxidation is the result of

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,

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polyamide and polyester. These plastics are all commonly used in clothing and packaging,

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

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Figure 2: (D) a comparison of microplastic abundance in sandy, estuarine, and subtidal sediments, (E) microplastics present in samples from 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 over time, microplastic concentrations increased.

shores had microplastic particles present, the abundance of which ranged from 2 (Australia)

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

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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 from 2 fibres per 250ml of sediment in Australia, to 31 fibres per 250ml of sediment in the UK.

determines their bioavailability. Plastics with a lower density are more likely to be ingested

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.

Matrix Density (g/cm 3 ) 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).

Table 1: Plastic Density and Origin

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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

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

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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.,

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

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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.

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,

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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.

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).

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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.

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B)A)

Figure 5: A) Asterias rubens aggregate on a bed of blue mussels (Mytilus edulis) (North Wales Wildlife Trust, 2016). B) Starfish feeding on mussels.

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.

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Figure 6: Edible Crab (Cancer pagurus) feeding on dead, stranded starfish (Asterias rubens).

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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