Microplastics are a growing concern in the marine environment. Studies have found microplastics to be in areas as remote as Antarctica. Marine species have been documented to ingest microplastics, however little research has gone into whether they can discriminate against microplastics and thus adapt their feeding behaviour. I looked at whether fan worms (Branchiomma sp.; n=20) change the amount of particles they consume when in an algal or microplastic environment and at differing concentrations. I found a significant effect of particle type whereby the fan worms consumed more microplastics than algae. There was no significant effect of concentration. This has potential implications on fan worms being a biofilter of microplastics, however further research is needed to determine any long term negative survival and population effects.

Microplastic pollution is a growing concern for the marine environment. Global demand for plastics is reaching around 245 million tonnes per year, many of which are single use plastics (Andrady, 2011). Microplastics are defined as plastics that are 5mm in size or smaller (Andrady, 2011). Microplastics originate from products such as cosmetics (Fendall and Sewell, 2009) or from degradation of larger plastics (Gregory & Andrady, 2003). Microplastic pollution is a widespread issue, occurring in China (Zhang et al., 2017), Canada (Mathalon & Hill, 2014) and even Antarctica (Andrady, 2011).  

Along with being widespread, microplastics are also affecting marine wildlife. This is mainly due to ingestion of microplastics. Many studies have found microplastics to be ingested by a variety of organisms such as fish (Zhang et al., 2017), bivalves (Mathalon & Hill, 2014), amphipods (Thompson et al., 2004) and sponges (Jonsson, 2015). Microplastics can then be transferred through the trophic web and through the marine environment. Marine organisms may ingest microplastics indirectly through feeding on organisms that have already ingested microplastics themselves (Wright, Thompson & Galloway, 2013). Microplastics can also move through the environment by aggregating in the pelagic environment due to microplastic-contaminated faecal matter sinking to the benthic environment (Wright, Thompson & Galloway, 2013). This exposes benthic species to microplastics. The increasing reports in microplastic ingestion and spread through the environment is alarming.

This body of literature demonstrates that marine organisms do ingest microplastics, however little research has shown on whether they can identify microplastics and attempt not to consume it. Murray and Cowie (2011) suggested amphipods are unable to distinguish microplastics from food items and thus might be a primary consumer of microplastics. Other studies have looked into this selectivity in other organisms, however not with microplastics. Theisen (1977) found that the blue mussel (Mytilus edulis) would decrease their feeding rates in environments of high turbidity. This suggests that for blue mussels, they are able to distinguish food items from non-food items and adapt their behaviour accordingly. This was supported by Jonsson (2015) who looked at sponges (Amphimedon sp). Jonsson found that sponges fed on microplastics at a slower rate than with algae. Again, this suggests an ability to distinguish non-food items. It is difficult to draw any hard conclusions form this small body of research. Therefore, it is important to investigate further into this field to shine light on how marine organisms may behave in microplastics polluted environments.

Few studies have looked into selectivity against microplastics in fan worms. Fan worms (family: Sabellidae) are highly efficient filter feeders (Licciano et al., 2007; Dame, Bushek & Prins, 2001) that create and live in tubes made of sand particles (Dame, Bushek & Prins, 2001). They are such efficient filter feeders that Licciano et al. suggested utilising fan worms to help filter pollution out of the marine environment. They are robust biofoulers and can survive in poor quality water, such as those found in mariners. Because of this, they are likely to be exposed to microplastics from the degradation of plastics due to runoff from populated areas. This makes fan worms ideal organisms to test how they react to microplastic environments.

For this study, I used Branchiomma sp., a fan worm commonly found in Australia. Harriet (2016) found that particles sized 1µm were fed on the most by Branchioma sp. This is supported by Zebe and Schiedek (1996) who found that marine annelids retain small particles more so than large particles. As such, I decided to use 1µm sized algae, Nannochloropsis sp., and microspheres. My aim was to determine whether Branchiomma sp. changed the amount they consumed when in a food particle environment versus a microplastic environment. I also wished to determine if different concentrations of these particles would have any effect. Based on previous literature, I hypothesised that individuals exposed to microplastics would feed less than individuals exposed to algae. I also hypothesised that individuals would feed less in the high concentration condition. 

Test Organism

Twenty individuals of Brachiomma sp. were used for my experiment. Of which, five individuals were places into each condition, thus having five replicates for each condition. These individuals were collected off of settlement plates that were placed at Manly Harbour, Australia. The settlement plates were kept in the Degnan Lab Aquaria of University of Queensland, St Lucia when the fan worms were not being used.

Design

I conducted a 2x2 factorial study looking at the effects of particle type (algae and microplastic) and particle concentration (low and high) on feeding rates of Brachiomma sp.

Set-Up

Individuals were each placed into wells of a 6 well cell culture plate. Artificial sea water was used to reduce the amount of particles that were not algae or microplastics. I produced the artificial sea water by mixing Tropic Marin Pro-reef sea salt with reverse osmosis water until a concentration of 35ppt was reached. For the algae condition, I used Nannochloropsis sp. (1-2µm) fromReed Mariculture’s Nanno 3600. For microplastics, I used Fluoresbrite® BB Carboxylate Microspheres 1.00µm.

Procedure

For the low concentration condition, I used 1µl and then added 10ml of artificial seawater. For the high concentration, I used 2µl and again added 10ml of artificial sea water. I then took 2ml of the solution from each well and placed it into a cuvette. These cuvettes were used in a spectrophotometer to find the optical density of both algae and microplastics. Algae was measured with an OD of 550nm whereas microplastics were measured with an OD of 600nm. This was done because the chlorophyll in algae least absorbs a wavelength of 550nm (Griffiths et al., 2011). Microplastics was measured at an OD of 600nm because Jonsson (2015) used this wavelength. Individuals were removed from the settlement plates and then placed into five of the available six wells on each culture plate. The sixth plate was used as a control. After 1.5 hours, the fan worms were taken out of the wells and a final spectrophotometer reading was made. The algae and microplastic treatments were performed a week apart due to unforeseen circumstances.

I combined the spectrophotometer reading and the number of particles found in 1ml of algae and microplastics (stated on their product) to estimate particle count. I measured the amount of particles consumed as the decreased percentage from before and after spectrophotometer readings. I analysed this data using RStudio v. 1.0.136.

A t-test was used to check that that the change in optical density was due to consumption of particles by the fan worms. Of all four treatments, only the high microplastic condition was significantly different from the control (t_4­ = 2.13, p = 0.03). The low algae (t_4­ = 1.34, p = 0.25), high algae (t_4­ = -0.73, p = 0.51) and low microplastic (t_4­ = -1.42, p = 0.23) conditions were non-significant.

A two-way ANOVA was used to analyse the effects of particle type and concentration. A significant effect of particle type was found (F_{1, 16} = 17.32, p < 0.001). The mean percentage of particles consumed for the 1µl algae, 2µl algae, 1µl microplastic and 2µl microplastic conditions were 28%, 25%, 65% and 86% respectively (Fig. 3). This shows that the fan worms had a higher feeding rate in the microplastic environment than in the algae environment (effect size = -0.49).

A non-significant effect of concentration was found (F_{1, 16} = 0.57, p = 0.46). However, there was a trend in the data whereby the feeding rate decreased at higher concentrations of both algae and microplastics. 

The aim of my study was to determine how fan worms (Brachiomma sp.) change their feeding behaviour when exposed to microplastics and algae and at different concentrations of both. I hypothesised that the fan worms would feed less in the microplastic environment and would also feed less at higher concentrations. My study has found that fan worms do change their feeding rates. However, I found that they fed more in the microplastic condition than in the algae condition. This goes against my first hypothesis. I also found that there was no change in feeding rate under different concentrations. As such, my second hypothesis was not supported.

My finding opposes both Jonsson (2015) and Theisen (1977). They found their test organisms to decrease feeding rates in a microplastic and high turbidity environment (respectively), however I found the opposite. This instead somewhat supports Murray and Cowie’s (2011) finding where amphipods were unable to distinguish microplastics from food items. However, my study does not fully align with Murray and Cowie’s finding. If the fan worms were unable to distinguish microplastics from food items, I would expect to see no difference between the algae and microplastic conditions. Instead, I found that the fan worms consumed more in the microplastic condition than in the algae condition. My finding better aligns with Bayne et al.’s (1993) study where they found blue mussels to consume more in a low organic particle environment. They suggested that this could be a compensatory behaviour to extract as much nutrients from the environment as they could. Likewise, microplastics do not provide fan worms with nutrition. As such, they might have consumed more microplastics to acquire they nutrients they need.

This explanation should be taken lightly though. While this compensatory behaviour could factor into my findings, there are a couple of alternate explanations. The first explanation is an issue with the methodology. Due to unforeseen constraints, I had to perform the algae and the microplastic treatments a week apart. This led to the treatments being placed in different parts of the lab which may have affected the fan worms’ feeding behaviour. Another reason to take my findings lightly is due to the controls showing a similar amount of decrease in optical density in comparison to the treatments. Only the high microplastic condition was found to be different. This suggests that the decrease in optical density was not due to consumption by the fan worms, but due to either natural change, an unknown variable, error or any combination of these factors. As I only had one control for each condition, this finding could due to error. As such, I recommend for future studies to perform multiple replicates of the control to get a more accurate reading.

There are a couple of large implications, assuming that my finding of fan worms consuming more microplastics than algae does reflect reality to some degree. High consumption of microplastics by fan worms could lead to increased microplastic transfer throughout the trophic levels and the environment. Microplastics could be further integrated into the sediment due to faecal matter and higher trophic animals may indirectly consume microplastics through the worms. However, there may be a potential up-side to my finding. Louvard (2016) found that microplastics did not negatively harm deposit-feeding Cirriforma sp when exposed to microplastics for one week. In conjunction with Licciano et al.’s (2007) finding, this could potentially mean that fan worms may be able to help filter some microplastics out of the environment. However, Louvard (2016) only studied surface levels variables to indicate health of the Cirriformia sp. (e.g. ability to dig and stay buried). Neither Louvard (2016) or I looked into how microplastics might affect the digestive tract and the long-term survivability of the worms. So while fan worms could potentially be a biofilter for microplastics, further research would need to be done in order to test any long term survival and population effects on fan worms.

An issue with my study is that it does not reflect real conditions of the environment. It is highly unlikely for the marine environment to contain purely microplastics. As such, my findings are probably not widely applicable. This can be improved by exposing fan worms to both algae and microplastics and change the concentrations of both particles in the one environment. Another reason to test microplastics and algae together is because we might see clearer selectivity of particles as Shumway, Bogdanowicz and Dean (1988) found that preferential selection was seen when Myxicola infundibulum (family: Sabellidae)
were exposed to different algal species at the same time. 

In conclusion, I found that Branchiomma sp. fed more in the microplastic conditions than in the algae conditions. This may have potential implications for this genus to be utilised as a potential biofilter of microplastics however further research would need to be conducted to determine the long-term survival and population effects. Due to error within my study, this is only a potential idea. Future research should attempt to improve the methodology first. 

I would like to thank Bernard Degnan and Sandie Degnan for their help in the conception of this research. I would also like to thank the Degnan Lab of the University of Queensland, St Lucia for providing the equipment and the test organisms.

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