Hermit Crabs Are Using Trash as Shells Across the World, Scientists Find

Researchers analyzed photographs of the crustaceans online, identifying nearly 400 examples of artificial shells, which were often plastic bottle caps

A pinkish hermit crab, in profile facing left, wears the top of a glass bottle as a shell -- brown broken glass with a bright red cap still attached.
A hermit crab wears the top of a broken bottle as a shell. Shawn Miller

Pictures and videos of hermit crabs creating new homes from human litter—co-opting anything from Lego pieces to soda cans to laundry detergent caps—have circulated the internet for well over a decade.

Now, researchers from the University of Warsaw and Poznań University in Poland have used these viral sightings to their advantage, conducting an internet image-based analysis to quantify just how often hermit crabs repurpose our waste.

The study, published this month in Science of the Total Environment, identified 386 total instances of the crustaceans wearing artificial shells. Of the world’s 16 terrestrial hermit crab species, ten have been spotted wearing our trash—and the behavior occurred in every tropical region on Earth.

“When I first saw these pictures, I felt it was heart-breaking,” Marta Szulkin, a biologist at the University of Warsaw’s Anthropocene Biology Lab and a co-author of the new study, tells BBC News’ Victoria Gill. “At the same time, I think we really need to understand the fact that we are living in a different era, and animals are making use of what is available to them.”

A hermit crab in profile, wearing what appears to be the metal bottom of a broken lightbulb as a shell.
A hermit crab wears a broken lightbulb as a shell. Shawn Miller

Exactly why the crustaceans are making homes out of trash remains an open question. Maybe hermit crabs, which molt and exchange their shells every 12 to 18 months, are struggling to find natural sources of protection, the authors suggest. Marine snail shells, their main choice for housing, are likely in decline as a result of shrinking gastropod populations, per the paper. Humans might also pick up viable shells as souvenirs. Perhaps, presented with a housing crisis of sorts, the crabs have turned to plastic trash as it becomes more and more prevalent in aquatic habitats.

Still, this idea remains speculation. “We can’t test this hypothesis without more information on the demographics of local snail populations,” Mark Briffa, an animal behavior researcher at the University of Plymouth in England who was not involved in the study, writes in the Conversation. Alternatively, “as the authors point out, plastic might be lighter than the equivalent shells, affording the same amount of protection but at lower energy cost of carrying them.”

For the new study, researchers scoured scientific journal databases and popular image-sharing websites including Flickr, iNaturalist, Google Images, YouTube and Alamy, as they sought to identify which characteristics of the litter most appealed to the crustaceans. When hermit crabs select a natural shell, its sexual signaling, weight, odor and color camouflage are all important considerations. Based on the photographs, the team found that 85 percent of the chosen trash was plastic, often white or black bottle caps. Metal, glass and a combination of the two materials each comprised about 5 percent of the remaining artificial homes.

A map indicating the hermit crab species the researchers identified in images, and the corresponding locations around the globe in which they were photographed with artificial shells.
Terrestrial hermit crab species wearing artificial shells were identified all across the globe. Zuzanna Jagiello, Łukasz Dylewski, Marta Szulkin; Science of the Total Environment, 2024, under CC BY 4.0 DEED

Researchers did not examine how living inside these knockoff shells might affect hermit crab health or evolution. But on the whole, outside researchers are confident that the estimated 171 trillion pieces of plastic currently inside our oceans are wreaking havoc on marine and terrestrial species alike, breaking down into tiny toxic particles and entering organisms’ bodies.

“We have now produced and disposed of so much plastic waste that it has sadly become a component part of almost every ecosystem,” David Santillo, a senior scientist for Greenpeace International’s Science Unit who was not involved in the study, tells the Independent’s Jabed Ahmed. “What the hermit crabs are doing, in selecting plastic and other litter over natural materials, should serve as a very visual warning of what is a much wider but often less visible problem of our own creation.”

A purple hermit crab, facing right, wears a sandy red plastic cap as a shell.
A hermit crab wears a red plastic cap as a shell. Shawn Miller

For as much as they may be using human trash for their benefit, hermit crabs are still incredibly vulnerable to its complex materials and shapes. A 2019 study conducted on the Cocos (Keeling) Islands, an Australian territory in the Indian Ocean, found that more than half a million hermit crabs died each year after getting stuck inside bottles and other debris. In 2021, researchers found discarded tires to have a similar effect.

“Hermit crabs, which famously inhabit discarded shells, climb into abandoned tires seeking shelter or a meal, only to find themselves unable to escape the recurved walls of the tire’s interior—and eventually they starve to death,” wrote Alex Fox for Smithsonian magazine in 2021.

Mark Miodownik, a materials engineer at University College London who was not involved in the study, tells BBC News that the newly analyzed images suggest an important takeaway for humanity: “Just like the hermit crabs… we should be reusing plastics much more, instead of discarding it.”

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