Microplastics disrupt hermit crab shell selection

Abstract

Microplastics (plastics < 5 mm) are a potential threat to marine biodiversity. However, the effects of microplastic pollution on animal behaviour and cognition are poorly understood. We used shell selection in common European hermit crabs (Pagurus bernhardus) as a model to test whether microplastic exposure impacts the essential survival behaviours of contacting, investigating and entering an optimal shell. We kept 64 female hermit crabs in tanks containing either polyethylene spheres (n = 35) or no plastic (n = 29) for 5 days. We then transferred subjects into suboptimal shells and placed them in an observation tank with an optimal alternative shell. Plastic-exposed hermit crabs showed impaired shell selection: they were less likely than controls to contact optimal shells or enter them. They also took longer to contact and enter the optimal shell. Plastic exposure did not affect time spent investigating the optimal shell. These results indicate that microplastics impair cognition (information-gathering and processing), disrupting an essential survival behaviour in hermit crabs.

1. Introduction

Microplastics (plastics < 5 mm in length [1]) are polluting oceans worldwide, causing substantial scientific and societal concern [2–4]. Waste microplastics enter marine environments either directly, as industry-made particles (primary microplastics [5]), or indirectly, as plastics greater than 5 mm degrade (secondary microplastics [6]). In total, up to 10% of global plastic production ends up in the ocean [2]. Microplastic exposure can reduce growth, reproduction and survival in diverse taxa, from corals to mammals [7–10]. However, the ecological validity and scientific rigour of existing research is questionable, with recent meta-analyses [11–13] and reviews [14–16] finding impacts equivocal and context dependent. As microplastic concentrations are highest along coastlines, littoral species face the greatest potential risks [6].

To date, research into the effects of microplastic pollution on marine organisms has focused on fitness and physiology [17]. A few studies have also investigated behavioural impacts on marine organisms, indicating that microplastics disrupt feeding [18], locomotion [19] and social behaviours [20]. Importantly, behaviour is underpinned by cognition: the mechanisms animals use to acquire, process, store and act on information from their environment [21]. This encompasses information-gathering, resource assessments and decision-making. Crooks et al. [22] identified ingested microplastics in the brains of velvet swimming crabs (Necora puber) and suggested this could impact crucial survival behaviours. Microplastics also transfer from blood to brain in Crucian carp (Carassius carassius), which may disrupt feeding and swimming [23]. However, the effects of microplastic exposure on animal cognition have not been explicitly tested.

Shell selection in common European hermit crabs (Pagurus bernhardus) is an essential survival behaviour, reliant on collecting accurate information about the new shell, assessing its quality and deciding whether to change shells [24]. Hermit crabs inhabit empty gastropod shells to protect their soft abdomens from predators [25], with optimal shell weight determined by body weight [26]. The location and sensory perception of new shells represent aspects of cognition [21]. Hermit crabs then cognitively evaluate shell quality by investigating the interior and exterior with their chelipeds [24]. They decide to swap shells if the new one is assessed as an improvement over the current shell. Accurate assessments are highly adaptive, as lower quality shells reduce growth, fecundity and survival [27]. Because hermit crabs gather information about the new shell, assess its quality compared to their current shell and make a decision manifested in behaviour, shell selection offers a tractable model of cognitive assessments in marine environments.

Here, we investigate whether microplastics affect hermit crab shell selection under controlled conditions. After hermit crabs were kept in tanks either without microplastics (CTRL) or with microplastics (PLAS), we transferred them into a suboptimal shell and offered an optimal alternative. We hypothesized that if plastic pollution impedes cognition, the PLAS treatment crabs would be less likely to find the optimal shell, accurately assess its quality and decide to change shells. Specifically, we predicted that CTRL hermit crabs would be more likely and faster to contact, investigate and enter the optimal shell than PLAS hermit crabs.

2. Material and methods

Hermit crabs were collected from Ballywalter Beach, Northern Ireland, and maintained in Queen's University Belfast's animal behaviour laboratory at 11°C with a 12 : 12 h L : D cycle. We randomly allocated subjects to either CTRL or PLAS treatments. For 5 days, we kept both groups in 0.028 m³ glass tanks (45 cm × 25 cm × 25 cm). All tanks contained 10 L of aerated seawater and 80 g of bladder wrack seaweed (Fucus vesiculosus). The PLAS treatment also included 50 g of polyethylene spheres (Materialix Ltd, London, UK; size: 4 mm, 0.02 g; concentration: 25 particles l⁻¹, 5 g l⁻¹). Lower than most exposure studies, this concentration represented natural conditions more realistically [12]. Polyethylene is the most abundant microplastic found in marine organisms [28].

After 5 days, hermit crabs were removed from their current shell using a small bench-vice to crack the shell [29]. Each subject was then sexed and weighed [24]. We only selected non-gravid females for the study (n = 35 CTRL, 29 PLAS) to control for sex differences in behaviour [25]. Based on their body weight, each hermit crab was provided a suboptimal Littorina obtusata shell 50% of their preferred shell weight [26]. After 2 h acclimating to the suboptimal shell, subjects were individually placed in a 15 cm diameter crystallizing dish 10 cm from an optimum weight L. obtusata shell (i.e. 100% the preferred weight for the weight of the hermit crab). The dish contained aerated seawater to a depth of 7.5 cm. We recorded the latency to contact the optimal shell, time spent investigating the optimal shell and latency to enter the optimal shell. If the hermit crab did not approach and enter a shell within 30 min, the session ended.

Statistical analyses were performed in R (R Core Team, R Foundation for Statistical Computing, Vienna, Austria, version 3.4.4). Data were categorical (1/0) and continuous (latency). Kolmogorov–Smirnov tests revealed our data were not normally distributed, so we used non-parametric tests throughout. We analysed categorical data using Pearson's chi-squared tests and latency data using Mann–Whitney U tests. If subjects did not contact or enter the optimal shell, we assigned a ceiling latency of 30 min. We present data as medians ± inter-quartile range and consider p < 0.05 statistically significant.

3. Results

Compared to CTRL subjects, fewer hermit crabs in the PLAS treatment contacted the optimal shell (${\chi }_1^2=8.736,$ p < 0.005; table 1). The proportion entering the optimal shell was also lower following microplastic exposure (${\chi }_1^2=5.343,$ p = 0.021; table 1). Moreover, the PLAS treatment had longer latencies to contact (W = 290, p < 0.005; CTRL median = 948 s, IQR = 184–1800 s; PLAS median = 1800 s, IQR = 1356–1800 s; figure 1) and enter the optimal shell (W = 349, p = 0.021; CTRL median = 1379 s, IQR = 511–1800; PLAS median = 1800 s, IQR = 1559–1800 s; figure 2). Investigation time did not differ between treatments (W = 142.5, p = 0.406; CTRL median = 129.5 s, IQR = 74.75–195.5 s; PLAS median = 80.5 s, IQR = 70.75–183.5 s).

Table 1.

Number and percentage of hermit crabs that contacted and entered the optimal shell from CTRL and PLAS treatments.

treatment	contact optimal shell (% contacting)	enter optimal shell (% entering)
CTRL (n = 35)	25 (71%)	21 (60%)
PLAS (n = 29)	10 (34%)	9 (31%)

Figure 1.

Latency (s; median, IQR) to contact the optimal shell for CTRL and PLAS treatments.

Figure 2.

Latency (s; median, IQR) to enter the optimal shell for CTRL and PLAS treatments.

4. Discussion

We demonstrated that microplastic exposure impairs shell selection behaviour in hermit crabs. Shell selection requires gathering and processing information about shell quality, so our findings suggest microplastics inhibited aspects of cognition. To our knowledge, this is the first study explicitly testing the cognitive effects of microplastic exposure, and the first microplastic study on common European hermit crabs.

Despite microplastic exposure disrupting shell selection, the mechanism is unclear. Ingested microplastics enter the brain in crabs [22] and carp [23], potentially impeding information-gathering, resource assessments, decision-making and behavioural responses. However, both gut–brain studies used substantially smaller microparticles than the present study (0.5 µm [22] and 53 nm [23]). Smaller microparticles translocate more easily from the gut into other tissues [30]. To establish whether microplastics passed through the gut membrane, researchers could extract subjects' haemolymph after testing (e.g. [31]). More general mechanisms may also be responsible for our results. Ingesting microplastics can induce false satiation in crustaceans [32], reducing food intake, energy budgets and growth [18,32–35]. Lower energy levels could, therefore, explain the PLAS treatment's tendency to avoid changing shells. We hope that further studies address the effects of microplastic exposure on specific cognitive processes.

While contact and entrance latencies were shorter in the CTRL treatment than the PLAS treatment, there was no difference in shell investigation duration. This may indicate that microplastic exposure impaired the ability to assess shells from a distance (i.e. sensory impairment). To some extent, hermit crabs can assess shell quality without contact. Elwood & Stewart [36] observed more approach behaviour when shells were high quality than low quality. Alternatively, the null results for shell investigation time may be due to sample size, as only nine subjects in the PLAS treatment investigated the new shell.

Although this research was laboratory based, our experimental design was more ecologically relevant than previous exposure studies. Microplastic exposure research typically uses unrepresentative concentrations and particle types [16]. Environmental microplastic concentrations range from 39 to 89 particles l⁻¹ in effluent [37] to approximately 13 particles l⁻¹ in the deep sea [38]. Whereas 100 particles l⁻¹ is the highest concentration ever recorded in nature [14,39], 82% of exposure studies test > 100 particles l⁻¹ [11]. Our 25 particles l⁻¹ concentration was, thus, more realistic than most laboratory-based microplastic research. A recent meta-analysis reported more deleterious effects at higher concentrations [11], although others have found little evidence for concentration- or duration-dependent effects [12,13]. Microparticle shape also influences uptake and effects. While fibres and fragments are more abundant in field observations [14,28], we used spheres, because they have more negative impacts on marine life [13]. However, microplastic pollution encompasses various shapes, sizes and polymer types [40]. Future laboratory studies could replicate this heterogeneity.

Our results contribute to previous research demonstrating the adverse effects of microplastics [18,32–35]. Such findings have serious real-world applications: more than 10 countries have banned cosmetic microbeads since 2015, including the United States, UK, France, Italy, New Zealand and South Korea [3,4]. However, the overwhelming majority of microplastic pollution is due to secondary microplastics. Lassen et al. [9] attributed > 99% of Danish microplastic pollution to secondary sources and estimated that cosmetic microbeads account for only 0.1%. At 60%, tyre dust was by far the biggest contributor (see also [41–43]). Secondary microplastics represent an important prospective avenue for research programmes and legislative efforts [14,42].

In conclusion, hermit crabs exposed to polyethylene spheres were less likely to contact and enter a better-quality shell than control animals, and took longer to do so. There was no difference in time spent investigating the new shell. This proof-of-concept study indicates that microplastic exposure impairs information-gathering, resource assessments and decision-making in hermit crabs. However, more research is needed to confirm the aspect of cognition affected. Future studies could also establish the generality of our findings across different species, cognitive processes and microplastic exposures.

Ethics

Crustacean research is not regulated under UK law, but we followed the Association for the Study of Animal Behaviour's Guidelines for the Use of Animals in Research. After the experiment, all hermit crabs were returned to the shore unharmed.

Data accessibility

Data are available in the electronic supplementary material.

Authors' contributions

A.C., C.M. and G.A. designed the study; C.M. conducted the experiments; A.C., E.J.B., E.M.C. and G.A. analysed and interpreted the data; A.C. prepared the manuscript. All authors revised the manuscript, approved the final version, and agreed to be held accountable for every aspect of the work.

Competing interests

We declare we have no competing interests.

Funding

This study was funded by Department for the Economy, Northern Ireland.