Abstract

Microplastics are ubiquitous in global marine systems and may have negative impacts on a vast range of species. Recently, microplastics were shown to impair shell selection assessments in hermit crabs, an essential behaviour for their survival. Hermit crabs also engage in ‘rapping’ contests over shells, based on cognitive assessments of shell quality and opponent fighting ability and, hence, are a useful model species for examining the effects of microplastics on fitness-relevant behaviour in marine systems. Here, we investigated how a 5-day microplastic exposure (25 microplastics/litre) affected the dynamics and outcome of 120 staged hermit crab contests. Using a 2 × 2 factorial design, we examined how microplastics (i.e. presence or absence) and contestant role (i.e. attacker or defender) affected various behavioural variables. Significantly higher raps per bout were needed to evict microplastic-treated defenders when attackers were pre-exposed to control conditions (i.e. no plastic). Also, significantly longer durations of rapping bouts were needed to evict control-treated defenders when attackers were pre-exposed to microplastics. We suggest that microplastics impaired defenders' ability to identify resource holding potential and also affected attackers’ rapping strength and intensity during contests. These impacts on animal contests indicate that microplastics have broader deleterious effects on marine biota than currently recognized.

Highlights

  • Microplastics are ubiquitous in global marine systems and may have negative impacts on a vast range of species

  • The overall outcome of hermit crab contests was not affected by the presence or absence of microplastics or the contestant role, as there was no significant difference in the percentage of contests that ended in shell fights (x42 1⁄4 1:18, p = 0.88), shell evictions (x42 1⁄4 0:77, p = 0.94) or shell swaps (x42 1⁄4 3:41, p = 0.90; table 1)

  • A significant microplastic exposure × contestant role interaction was found for the ‘number of raps within a bout’ (F1 = 5.88, p = 0.02)

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Summary

Introduction

Microplastics are ubiquitous in global marine systems and may have negative impacts on a vast range of species. We suggest that microplastics impaired defenders’ ability to identify resource holding potential and affected attackers’ rapping strength and intensity during contests These impacts on animal contests indicate that microplastics have broader deleterious effects on marine biota than currently recognized. Hermit crabs gather information by investigating a new shell, which allows them to decide whether it is of a higher or lower quality than the shell they currently inhabit [30,31,32] In this way, they offer a useful system for studying cognition, comprising the acquisition, processing, retention and use of information from the environment [33], and the effects of microplastics on key survival behaviours reliant on such information processing. This ‘rapping’ behaviour occurs in distinct bouts and will continue until either the defender evacuates the shell, termed an eviction, or the attacker retreats from the contest

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