Citation: Crump, A.; Aiken, C.; Cunningham, E.M.; Arnott, G. Short-Term Microplastic Exposure Impairs Cognition in Hermit Crabs. Animals 2023, 13, 1055. https:// doi.org/10.3390/ani13061055 Academic Editor: Sarah J. Dalesman Received: 11 January 2023 Revised: 23 February 2023 Accepted: 10 March 2023 Published: 14 March 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). animals Article Short-Term Microplastic Exposure Impairs Cognition in Hermit Crabs Andrew Crump 1,2, * , Catherine Aiken 1 , Eoghan M. Cunningham 1,3 and Gareth Arnott 1 1 Institute for Global Food Security, School of Biological Sciences, Queen’s University Belfast, Belfast BT7 1NN, UK 2 Animal Welfare Science and Ethics, Pathobiology and Population Science, Royal Veterinary College, Hatfield AL9 7TA, UK 3 Queen’s University Marine Laboratory, School of Biological Sciences, Queen’s University Belfast, Portaferry BT22 1PF, UK * Correspondence: andrewcrump94@gmail.com Simple Summary: Little is known about how microplastics impact animal cognition—the way animals gather and process information. We investigated whether microplastic exposure impaired hermit crab shell selection, which relies on cognitive assessments and decision-making. After short- term microplastic exposure in a laboratory, hermit crabs were worse at choosing a shell. Microplastics, therefore, disrupted cognition. If wild hermit crabs are also susceptible to this effect, microplastic pollution may hinder shell selection, a crucial survival behaviour. Abstract: We tested whether acute microplastic exposure impacts information gathering and pro- cessing (cognition) in hermit crabs (Pagurus bernhardus). For five days, we kept 51 hermit crabs in tanks containing either polyethylene microspheres (n = 27) or no plastic (n = 24). We then transferred individuals into an intermediate-quality shell and presented them with two vials containing either a better or worse shell. Because touching both shell vials required an equivalent behavioural response, this design controlled for general activity. Plastic-exposed hermit crabs were less likely and slower than controls to touch the better shell vial, instead preferring the worse shell vial. Microplastics, therefore, impaired assessments and decision-making, providing direct evidence of acute microplastic exposure disrupting hermit crab cognition. Keywords: animal cognition; hermit crab; microplastic pollution; shell selection 1. Introduction Global plastic production has grown exponentially since the 1950s [1] and now exceeds 360 million tonnes per year [2]. Up to 10% of plastics ultimately enter the ocean [3,4]. Plastics < 5 mm in length or diameter are termed microplastics [5–7]. These are formed either as industry-made particles (primary microplastics [8]) or when larger plastics break down (secondary microplastics [9]). Among the most common ocean pollutants [10], microplastics occur from the tropics to the poles [11], and from surface waters [10] to the deep sea [12]. Microplastic pollution is a major threat to marine biodiversity [1,13]. Both direct exposure and ingestion of microplastics can negatively affect animal health and physiol- ogy [14–16] (but see [17]). There is also growing concern about microplastic impacts on animal behaviour. For example, microplastic exposure disrupts locomotion (zebrafish, Danio rerio [18]; oysters, Crassostrea gigas [19,20]; amphipods, Platorchestia smithi [21]; cope- pods, Temora turbinata [22]), feeding (amphipods, Orchestoidea tuberculata [23]; copepods, Calanus helgolandicus [24]), and shoaling behaviour (carp, Carassius carassius [25]). European hermit crabs (Pagurus bernhardus) are an emerging model for the effects of microplastic exposure on animal behaviour [26,27]. To protect their soft abdomens from predators, these marine crustaceans live in empty gastropod shells [28,29]. Hermit crabs gather information about potential shells through visual and tactile assessments, which Animals 2023, 13, 1055. https://doi.org/10.3390/ani13061055 https://www.mdpi.com/journal/animals