It is estimated that over 35,000 tons of microplastics (<4.75 mm) is currently floating in the world’s oceans today (Eriksen et al. 2014). These are the tiny plastic particles used in cosmetics, toiletries and paints that enter the oceans via domestic and industrial drainage systems (Cole et al. 2011). For the past few decades concerns have been raised on the environmental impacts of microplastics on marine communities (Cole et al. 2011; Browne et al. 2007; Moore 2008). Various studies have been conducted to see if marine invertebrates can cope with microplastic exposure. Whilst there is evidence that microplastic ingestion for some invertebrates shows little impact (Kaposi et al. 2014; Browne et al. 2008), others suffer poor health and physiological complications (Wright et al. 2013; Browne et al. 2013; Cole 2014).

Research mainly focuses on indiscriminate feeders (Cole et al. 2011; Andrady et al. 2011), because they are the most susceptible to ingesting microplastics as they have no means of distinguishing plastic particles from food particles (Moore 2008). But discriminate feeders can also ingest microplastics by either confusing them with their food or eating microplastic-contaminated prey (Shaw and Day 1994; Eriksson and Burton 2003). The taxa used in these studies are mainly higher-trophic animals, though. Here I wish to investigate whether lower-trophic discriminate feeders will ingest microplastics. To test this I will use the prawn, Palaemon serrifer, an algae-grazer.

Four tanks of P. serrifer (with each tank containing three members) were exposed to different concentrations of 40 µm diameter microbeads (spherical microplastics) either with or without algae covering them. Two tanks had a low concentration (0.5 g) and high concentration (2 g) of microbeads not covered with algae (sterile microbeads or SMBs). The remaining tanks were the same two concentrations, except with algae-covered microbeads (AMBs). These are merely microbeads that were left in saltwater tanks with exposure to sunlight for a week so algae could coalesce around​ them. They were then transferred to the two AMB tanks. A control tank with no microbeads was also set up, making a total of five groups. All P. serrifer were left for a week in their respective tanks.

At the end of the week two sets of data were collected: the survival rate of P. serrifer in the tanks and the microbead number in their foreguts. To measure microbead number the prawns were dissected so that their foreguts could be removed and placed on microscope slides. Under the microscope, the foreguts were scanned by eye in an up-down fashion at five random points along each foregut and all microbeads spotted were tallied.

All P. serrifer in the all five groups survived to end the week. Although, the control, high AMB and high SMB tanks each lost one prawn as they jumped out and perished whilst being transferred to their tanks.

The microbead number in the foreguts of P. serrifer in the microbead-filled tanks showed no appreciable difference between those exposed to SMBs and AMBs (Welch t test: t = 1.229, df = 4, p = 0.286) (Table 1). The difference in microbead number between those left in the high and low concentration tanks was also insignificant (Welch t test: t = 1.294, df = 5, p = 0.252) (Table 1).

It was hypothesized that AMBs would be more appetizing than SMBs seeing as P. serrifer graze on algae. Given that algae freely grows on ocean plastics, AMBs, as opposed to SMBs, would essentially be the only type of microbead found in the oceans as any SMBs entering oceans are rapidly covered with algae. It was predicted from this hypothesis that AMBs would be ingested more than SMBs and therefore more microbeads would be found in the guts of prawns in the AMB-filled tanks. This hypothesis is false, though, seeing as there was no appreciable difference in microbead number in the foreguts of P. serrifer left in SMB- and AMB-filled tanks (Table 1).

Decapod crustaceans are selective feeders with the ability to reject apparent palatable food even up until entering the oesophagus (Aggio et al. 2012). This means that microplastics must pass many groups of sensory receptors before arriving at the foregut. Plastics are generally considered chemically inert and as such should be able to pass the chemosensory systems found on both the mandible and in the oesophagus. However, during manufacture toxic additives can be incorporated into the plastics (Browne et al. 2007). The polymerization process is also generally incomplete, leaving unbonded monomers from the polymers which are referred to as residual monomers (Araujo et al. 2002). In this case, the microbeads used in this experiment are polymethyl methacrylate (PMMA), meaning that the residual monomers includes the highly toxic methyl methacrylate (Lu 2013), among other chemicals. The residual monomers of the microbeads used in this experiment make up <1% of their total mass (Spheromers AS), but they are still nonetheless present. In addition to this, ocean plastics have been shown to absorb metals (Ashton et al. 2010), persistent organic pollutants (Betts 2008) and endocrine disrupting chemicals (Ng and Obbard 2006).

It most likely the case that the chemosensory systems of the prawns reject microbeads because of the toxins that seep from them, irrespective of whether or not they are covered with algae. In fact, AMBs may even be less palatable given that they most likely seep more toxins. PMMA is a photodegradable plastic (Aboulezz and Waters 1978) and in order to get algae to cover them they were left in sunlight for a week in shallow water. This may have lead to the breakdown of some PMMA, resulting in even more toxic monomers and the accelerated leaching of other toxins. This reflects the natural condition of ocean microplastics, given that many plastics also degrade due to sunlight exposure and other factors whilst floating in the oceans (Cole et al. 2011).

Identifying the toxin or toxins that trigger the chemoreceptors to reject the plastic particles may be worthwhile investigating. Some toxins mentioned can be dismissed as possible triggers. For instance, whilst it is common for microplastics in the oceans to absorb environmental toxins over long periods of time, this may not apply to the microbeads in this experiment seeing as they remained in seawater for a only up to two weeks, giving them very little time to absorb any chemicals from their surroundings. Some of the incorporated toxins also might not trigger the chemoreceptors, especially synthetic toxins as the chemosensory systems of the prawns may not be adapted to recognize them.

If P. serrifer finds microbeads unpalatable, how did any get into their digestive systems? Crustaceans pump water into the guts anally (Fox 1952), meaning their are two possible entrances into the digestive system for microplastics. The anal opening is generally clenched unless otherwise stimulated open, allowing water to flood into the guts. This may have been how some microbeads reached the foreguts of the prawns. To test this hypothesis, future experiments should focus on examining in the hindguts of prawns for microbeads. If this is true, more microbeads should be found in the hindgut than the foregut.

There was also no discernible difference between high and low concentration exposure of microbeads (Table 1). However, it should be noted that the microplastic concentrations that the P. serrifer were exposed to was astronomically high, much more so than what is found anywhere in the oceans. Even the tanks with so-called “low” microplastic concentrations had more than 40000 times the concentration of ocean habitats adjacent to plastic factories (Lozano and Mouat 2009). The microbead concentrations were not meant to replicate the current polluted conditions that marine wildlife face, but merely meant to test if microbeads can enter the digestive systems of the low-trophic discriminate feeders. So given the prawns were subjected to extreme microbead concentrations with so few being found in their foreguts, it is unlikely that microplastic ingestion in P. serrifer and similar low-trophic discriminate feeders is a problem.

All the prawns exposed to the microbeads survived to the end of the week, showing that extreme short-term microbead exposure is not lethal. No effort was made to examine their health, so to the question as to whether this level of microbead exposure causes non-fatal physiological complications in the short term cannot yet be definitively answered. Questions regarding long-term exposure are also still just as unanswerable. Although, it can be speculated that given such small numbers of microbeads were found in their foreguts, that their health was not affected. This assumes; however, that microbeads were only taken up into the guts and no other organs or tissues. Microplastic uptake has been observed to occur in the gills of other marine invertebrates (Watts et al. 2014; Darmody et al. 2015; von Moos et al. 2012). Other organs should be examined for the presence of significant microbead numbers and, if so, they should be studied to see if their health deteriorates as a result.

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