Plastic or natural? The impact of marine debris on the shell preference of a terrestrial hermit crab

Plastic pollution is one of the main human-induced rapid environmental changes threatening marine organisms. Terrestrial hermit crabs, which inhabit coastal areas where litter accumulates, are increasingly documented using plastic debris as shells, and a recent study hypothesized that this shift may be an adaptive response. They suggested that ‘plastic shells’ may reduce transport costs, provide camouflage in a polluted environment, and may even be favoured by sexual selection. Our study is the first to quantify plastic shell use in a population, to examine shell preference and to test the hypothesis that plastic shells optimize the ratio of weight to volume. We investigated the prevalence of plastic shell use by Coenobita compressus at Playa Nancite, Costa Rica. We conducted field surveys to assess shell use among hermit crabs, evaluated the environmental availability of natural and plastic shells, and performed a choice trial experiment to determine their shell preference. Of the 714 crabs collected, all were found in natural shells, despite the shoreline survey revealing a far greater abundance of available artificial shells, primarily plastic. Choice trials showed a strong preference for natural shells, with crabs only occasionally entering, and never retaining, plastic shells. Despite the widespread availability of plastic debris at Playa Nancite, hermit crabs strongly prefer natural shells, suggesting limited current impacts on this population's shell selection behaviour and evolution. However, the pervasive presence of plastic on beaches necessitates urgent measures to mitigate its environmental impact. Furthermore, research across different species and locations is essential to understand the broader implications of marine debris on hermit crabs. • Most terrestrial hermit crab species are reported to use plastic litter as shells. • We studied a crab population at a site with high macroplastic pollution. • No crabs used plastic debris as shells in our shoreline field surveys. • In laboratory trials, most crabs preferred natural shells over plastic ones. • Plastic pollution probably harms hermit crabs in many ways beyond shell selection.