To accommodate their growing bodies, hermit crabs search for larger shells. It’s more than curb appeal. When they spot an ideal home, they carefully check the outside and inside to see if it’s a good fit. Unfortunately, microplastics in our oceans affect the behavior of hermit crabs to locate suitable quarters.
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“Hermit crabs reside in shells as they have a soft abdomen, unlike many other crab species,” Mánus Cunningham, BSc, MSc, one of the lead researchers on a new study from Queens University Belfast, explains. “So, finding and entering new shells as the hermit crabs grow and progress through their life is essential to their survival.”
Why we should care
Hermit crabs are scavengers and play an important role in rebalancing the ecosystem. By eating decomposed sea life and bacteria, they recycle energy back into their habitats. They also serve as prey for otters, octopuses, and other sea creatures.
Plastic waste makes up 80 percent of marine debris
At least 8 million tons of plastic end up in our oceans every year. It’s a staggering amount. It harms humans, fish, sea creatures, and even hermit crabs.
“Previous studies have shown how microplastic exposure can negatively affect the likelihood of hermit crabs to find and enter new shells,” Cunningham says. “We took this study a step further and assessed the impact of microplastic exposure on shell contests—a common behavior where hermit crabs compete for shells to reside in.”
How hermit crabs find a new home
Hermit crabs choose their new shells based on shape, shell opening, and weight. “They gather information on shell quality, or resource value, through touch,” Cunningham says. “When making contact with a shell, they can determine whether the new shell is of greater value than their own. At this stage, a hermit crab will enter the new shell and discard their old one.”
Sometimes, hermit crabs find empty shells for their own use. Other times, they engage in a behavior called “rapping” where they fight with other hermit crabs for a bigger and more useful shell. “Rapping” is when they violently shake the shells inhabited by other hermit crabs and forcefully evict them; they claim their opponent’s shell as their own.
How their fighting works
“The attacking crab will assess the fighting ability and size of the opposing hermit crab as well as the value of the shell,” Cunningham says. “If the attacker believes the value of the shell outweighs the damage that can be inflicted by a fight, it will try to evict the opposing crab by rapping its shell against it in quick bursts. The defender can also assess the strength of the attacker through the shell rapping bouts and decide to flee for safety.”
“We found microplastic exposure affected both the attacking intensity of hermit crabs and the ability of defending hermit crabs to access the fighting power of their attackers,” Cunningham says.
The research involved keeping hermit crabs in two tanks: one contained a microplastic pollutant and the other was a clean tank without plastic. The hermit crabs lived in these tanks for five days. The team of scientists created hermit crab contests by placing pairs of hermit crabs in an arena and gave the larger crab a shell that was too small and a smaller crab a shell that was too big.
Plastic-exposed hermit crabs displayed weaker attacking behaviors during shell fights than the hermit crabs not exposed to the plastics. Scientists also found microplastics reduced the ability of the defending crab to access their attackers during these fights. It also impaired their decision to give up their shells.
“These findings are hugely significant,” Cunningham explains. “They illustrate how both the information-gathering and shell evaluations were impaired when exposed to microplastics.”
“Although 10 percent of global plastic production ends up in the ocean, there is very limited research on how this can disrupt animal behavior and cognition. This study shows how the microplastic pollution crisis is threatening biodiversity more than is currently recognized.”
Gareth Arnott, Ph.D., a principal investigator of the project, agrees. “This study provides an insight into the potential for microplastics to alter important aspects of animal behavior that are critical for survival and reproduction. We need to further investigate how microplastics affect their behavior and the consequences, armed with this knowledge to advocate for change to protect our ecosystem.”
The next step
According to Cunningham, “The next step is to determine how the chemicals that are leached from microplastic particles into the water column affect hermit crab behavior. During our study, no ingestion of microplastic particles was found; hence we believe that the chemical leachate is what affected the hermit crab behavior and possibly physical condition.”
Fun hermit crab facts
- Hermit crabs are not born with shells. They’re acquired from other crabs.
- The name “hermit” is a misnomer. They’re not solitary creatures. They like the company of other hermit crabs.
- While they can exist on land, they start their life in the sea.
- Hermit crabs can grow lost and damaged tissue.
- Worldwide, there are more than 800 species of hermit crabs.
- Hermit crabs live between 5 and 15 years. Some have even reached age 25.
If you’d like to watch a short video from Queens University Belfast of two hermit crabs acquiring shells, click here.
- Hermit crabs are not solitary creatures. (Photo courtesy of Queens University Belfast.)
- Plastics disrupt hermit crab behavior. (Photo courtesy of Queens University Belfast.)