A new study of hermit crab behavior in the United Kingdom showed that short-term exposure to microplastics impairs their assessments and decision-making processes. Hermit crabs that were exposed to microplastics were less likely and slower than controls to move to a better shell when they were offered one, instead preferring a worse shell. The study was published in the scientific journal Animals.
Since the mid-20th century, the production of plastic has been increasing exponentially. Used plastic typically ends up as plastic waste. Around 10% of that waste is estimated to end up in the oceans. The most often used types of plastic do not truly decompose in nature, like organic matter does, but only break down into ever smaller pieces, becoming microplastics – plastic particles below 5 millimeters in length or diameter.
Microplastics are currently one of the most common water pollutants appearing in seas worldwide, from surface waters to the deep sea. They represent a major threat to marine life. Both direct exposure to and ingestion of microplastics can negatively affect animal health and physiology. Recent studies have shown that it also adversely impacts animal behavior.
One of the novel ways to study the effects of microplastics on behavior of marine animals is by examining European hermit crabs. These crabs have soft abdomens and, to protect them, they live in empty shells of gastropods (e.g., sea snails and slugs). Normally, they gather information about potential shells they could inhabit through visual and tactile assessments, comparing the new shell to their current shell. They then move to the best shell available.
Previous studies have shown that exposure to microplastics made these crabs less able to assess and switch to new shells. However, it remained unknown how much exposure to microplastic is required for this impairment to develop. Study author Andrew Crump and his colleagues wanted to explore whether short-term exposure to microplastics affects hermit crab shell selection. They devised a study in which they offered hermit crabs a better and a worse shell than the one they were currently using and observed the reactions.
The study was conducted on 51 common European hermit crabs (Pagurus bernhardus). Twenty-five of the crabs were male and 26 were non-gravid females. They were collected from the Ballywalter Beach in Northern Ireland and transported to Queen’s University in Belfast. Once there, crabs were kept in a glass stock tank with 4 liters of sand filtered aerated seawater at 11C until the beginning of the study.
For the experiment, crabs were randomly allocated into two groups and put in different tanks. Researchers exposed one group to microplastics by adding 50 grams of virgin polyethylene spheres to their tank. Polyethylene is the most produced type of plastic in Europe and the type most commonly found in marine organisms.
These spheres were 4 millimeters in diameter and weighed .02 grams each. The study authors chose this size and quantity of plastics to insert into the tank of the first group because it best reflected the concentration and quality of microplastics found on European coastlines.
After five days of exposure to microplastics, study authors cracked open the shells hermit crabs in both groups were using, dried the crabs and weighed them. They then gave each crab a shall of Littorina obtusata, also known as flat periwinkle, a species of sea snail commonly found around the British Isles. These shells were selected to be exactly 50% of the shell weight European hermit crabs prefer. The crabs were then allowed 2 hours to recover from the experience and acclimate to their new shells.
After the two-hour period, crabs were individually moved to a circular glass testing tank. There were two glass vials in the tank. One contained a shell that was a 100% of the weight optimal for the crab, thus better than the shell the crab currently used. The other vial contained a shell that was 25% the weight optimal for the crab and thus worse than the crab’s existing shell.
The crab was placed 10 cm from both vials. Study authors measured the time each crab spent within 5 cm of each of the two vials and for how long it emitted rapping sounds. Rapping refers to the behavior where crabs produce a rapid tapping or drumming sound using their claws or legs. They produce these repetitive tapping sounds to communicate with potential mates, establish their presence, or defend their territory. It can be an indicator of the crab considering the new shell its own territory. A trial with one crab lasted for 15 minutes. The crabs were returned to their original tank after the trial.
The findings indicated that out of 51 crabs, 49 interacted with at least one shell. Among the crabs exposed to microplastics, there were fewer instances where they touched the 100% shell first compared to the control group.
Furthermore, the crabs exposed to microplastics took longer, on average, to touch the 100% shell. Additionally, these crabs exhibited a longer time to interact with any of the shell vials. Moreover, crabs exposed to microplastics spent less time engaging in rapping and exploring the 100% shell.
“Given the choice between two shell vials—one containing a better shell (100%) and the other containing a worse shell (25%) than their current shell (50%)—hermit crabs that underwent acute microplastic exposure were less likely to first touch the 100% shell vial than control hermit crabs,” the study authors wrote.
“Not only were PLAS hermit crabs [crabs exposed to microplastics] less likely to touch the 100% shell vial than CTRL [control group] individuals, but they actually preferred the 25% shell vial. Both new shells were equidistant from the starting point, so touching either required equivalent motor responses.”
“This ruled out a plastic-induced reduction in general activity, and demonstrated that acute microplastic exposure disrupted some aspect of cognition (assessment and/or decision-making). However, we do not yet know which cognitive process was affected,” the study authors concluded.
The study makes an important contribution to the scientific understanding of the effects of microplastics on the behavior of marine animals. However, the study was solely focused on European hermit crabs. Effects on other species might not be the same. Additionally, all crabs used in the study originated from the same beach and the level of their microplastic exposure before the study was not known.
The study, “Short-Term Microplastic Exposure Impairs Cognition in Hermit Crabs”, was authored by Andrew Crump, Catherine Aiken, Eoghan M. Cunningham. and Gareth Arnott.