Water Contaminated With Microplastics Leads To Poor Decision Making In Hermit Crabs Moving House

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Rachael Funnell 04 May 2020, 17:59

New research published in the journal Biology Letters has found that the presence of microplastics in water causes hermit crabs to choose poorly when it comes to picking a new shell. The findings give a worrying indication of how the plastic crisis could be impacting the cognitive function of marine creatures, potentially hampering their chances of survival.

If you think moving house is stressful for humans, you should try doing it as a hermit crab. These lovable crustaceans have no choice but to repeatedly switch abodes throughout their lives as they outgrow old digs. Some species practice a behavior where prospective buyers will line up in size order. Once the first hermit crab goes to make the move, a flurry of shuffling occurs as each crab aborts their shell in favor of something more spacious. A convenient trade but a risky one, as without a shell hermit crabs' soft bodies are exposed.

To give themselves the best chance of survival, hermit crabs need to be smart in their shell selection. A good shell is bigger, free from holes and, ideally, organic. Unfortunately, as the ongoing plastic crisis continues to sweep across the globe’s oceans, more and more instances of hermit crabs getting inventive with manmade trash are cropping up (such as this particularly cursed image).

Dr Gareth Arnott, co-author of the new research from Queen’s University Belfast, and his colleagues decided to investigate if microplastics had an impact on the shell selection decisions of female hermit crabs. They placed 29 in a tank containing seawater, seaweed, and 4mm-diameter polyethylene beads, at a concentration akin to levels found in the environment. In a separate tank, they housed 35 female hermit crabs also with seaweed and seawater but without the polyethylene beads.

They let the participants stew in their environment for five days before removing them from their shell and offering them a far less desirable shell, which was half the ideal weight for the crabs. After two hours in the lackluster accommodation, they put the crabs in a deep dish of seawater and offered them another shell, this time of an ideal weight for the crab’s size.

“Usually a so-called ‘normal’ hermit crab will always want to go for the better shell,” said Dr Arnott, co-author of the new research from Queen’s University Belfast in an interview with the Guardian. “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].”

Arnott and his team observed that of the 35 crabs not exposed to plastic, 25 explored the new, favorable shells, 21 of which (60 percent of the total group) moved into them. Of the 29 crabs who were exposed to microplastics, only 10 decided to take a look at the superior shells with nine of these (31 percent of the total group) choosing to upgrade. The results are thought to demonstrate how plastic pollution may be impacting the cognitive function of hermit crabs as they make decisions, which are counterproductive to their survival when exposed to microplastics.

“This shell selection behavior 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. “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 behavior, for example.”

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