Microplastics disrupt hermit crabs’ ability to choose shell, study suggests

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When it comes to moving home, hermit crabs are experts, often swapping shells for the optimal abode.

But now researchers have found that exposure to microplastics disrupts this key behaviour. The finds are the latest to suggest such pollution could be having an impact on the world’s marine creatures.

“Usually a so-called ‘normal’ hermit crab will always want to go for the better shell,” said Dr Gareth Arnott, co-author of the new research from Queen’s University Belfast, adding such shells are typically those of sea snails.

“The striking thing in this study was when [we offered them a better shell], lots of the crabs that had been exposed to the microplastics didn’t make the optimal decision to take [it],” he said.

Microplastics – pieces of plastic 5mm or smaller – are a growing subject of research, with previous studies showing they are present even in the depths of the ocean and are ending up in the bodies of living organisms, from seals to crabs and seabirds.

However, while there is some evidence that exposure to such pollution has affected growth and reproduction in some animals, research into specific effects on animal behaviour and cognition remains scarce.

Arnott and his colleagues placed 29 female hermit crabs in a tank containing seawater, seaweed and 4mm-diameter polyethylene beads, at a concentration akin to levels found in the environment. Another 35 female hermit crabs were placed in a similar tank, but without the polyethylene beads.

Writing in the journal Biology Letters, the team report how they kept each group in the tanks for five days, before removing each crab from its shell and giving it a new shell – crucially, these shells were about half the ideal weight for each crab.

After two hours in their new shell, each crab was then put into a deep dish of seawater and the team presented it with another shell, this time of an ideal weight.

The team found that 25 of the crabs who had not been exposed to the microplastics explored the optimal-sized shells, with 21 of the crabs – 60% – taking up residence in them.

By contrast, crabs that had been exposed to microplastics took longer to begin such exploration and far fewer did so: just 10 made contact with the optimal-sized shells and only nine – 31% of the group – moved home.

This suggests exposure to microplastics affects the shell selection behaviour of hermit crabs, the team say, indicating that pollution could be affecting cognition.

“This shell selection behaviour is an example of a cognitive process – the animal has to gather information about the shell and it has to then decide how it is going to use that information,” said Arnott, adding that he was surprised by the results.

However, the team say there is more to be done, including looking at whether different types of microplastic have a similar effect, exploring whether the microplastics actually enter the crab, and unpicking the mechanism for the effect.

“We hypothesize that either some aspect of the polyethylene is getting into them to affect their decision making, or else it is an indirect effect that the presence of the plastic in the tank might be influencing their feeding behaviour, for example,” Arnott said.

He added that the study only looked at hermit crabs, and it will be important to explore whether the behavioural changes observed in the laboratory are seen in the wild and have real-world implications.

“Based on the striking finding [in this study] this would suggest that there could be a long-term impact in the natural world, but we need to do more work on that,” he said.

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