How Microplastics Affect Hermit Crabs

Microplastics, defined as pieces of plastic that are less than five millimeters in diameter or length, are produced by industry or from the breakdown of larger plastics, such as bottles or bags. Many microplastics eventually find their way into oceans and other bodies of water.

Close to 2.3 million tonnes of microplastic pollution float in the world’s seas, and yet we don’t know how microplastic exposure alters the behavior of marine animals. For example, does it disrupt their feeding and locomotion? Because cognitive processes in European hermit crabs are sensitive to human-driven changes in the oceans, researchers are starting to use them to study the effects of microplastic exposure on marine animals.

Hermit crabs seek out empty shells abandoned by gastropods for protection against predators. They compare shells using both sight and touch before making a decision. To see if hermit crabs make worse decisions after exposure to microplastics, this study exposed them to polyethylene microspheres for five days and studied their subsequent response to shell selection.

Researchers gave the hermit crabs a shell that weighed only half as much as hermit crabs prefer. Once they were acclimated to the 50% shell, researchers offered them a choice between two shells, one that weighed as much as they prefer and one that weighed only a quarter of what they prefer. The researchers put each shell in a vial. 

Not only were the microplastic-exposed hermit crabs much slower to touch either shell than the control crabs, but 17 out of 25 in this group touched the worse shell vial first, compared to only 8 out of 24 in the control group. Microplastic-exposed hermit crabs spent less time exploring the better shell and spent less time rapping at its vial. Conversely, they spent more time rapping at the vial of the worse shell. 

Male hermit crabs that were exposed to microplastics seemed less likely to touch the better shell when compared to females, indicating that males were more vulnerable to the effects of microplastics. While the reason for this difference is not known, other studies have shown that microplastic exposure increased the expression of certain genes among males in freshwater crustaceans, making them more sensitive to such exposure.

Overall, the results of this study indicate that microplastic exposure negatively impacted the cognitive and decision-making abilities of hermit crabs, leading to suboptimal shell selection. As the hermit crabs don’t seem to have ingested the microplastics, the authors believe that the materials secreted from the plastics may have worsened the hermit crabs’ vision and sense of smell. Weaker senses may have affected the crabs’ ability to figure out which shell was the best. The authors also speculate that the materials leached from microplastics may have affected their brains to make them take more risks. The study was conducted over only five days of microplastic exposure, and therefore may not generalize to longer periods of exposure.

No meaningful legislation monitors and prevents the flow of plastic waste into the oceans. However, progress may be happening: in 2022, 175 countries under the U.N. Environmental Assembly signed an agreement to tackle the plastic pollution crisis starting in 2024. As industry stakeholders are likely to oppose any attempts to curb plastic production, advocates should raise awareness of the studies that show the enormity of the effect of plastic pollution on animals.