Every year, the world discards around 30 million metric tons of car and truck tires. The majority of these are reclaimed and used in recycled materials or burned as fuel, but that still leaves millions of tires that end up in landfills or are dumped illegally. Through negligence and unscrupulous intent, some portion of these unused waste tires find their way into the world’s oceans where they can cause serious harm.
The toxic chemicals and microplastic pollution that tires can release into the environment are well documented, but now new research suggests a new way that tires can endanger wildlife: their shape. When tires end up in the world’s oceans their hollowed-out doughnut form can make them deadly for crustaceans, specifically hermit crabs.
The study, published today in the journal Royal Society Open Science, finds that 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. In just one year, the researchers counted more than 1,200 hermit crabs that became imprisoned inside a set of six tires placed on the seafloor.
Atsushi Sogabe, an ecologist at Hirosaki University in Japan and the study’s lead author, writes via email that his inspiration to conduct this study came while studying pipefish in Japan’s Mutsu Bay in 2012. During a research dive, he encountered a tire full of shells on the seafloor. Some of those shells contained hermit crabs, and Sogabe suspected that the tire’s shape created an ecological problem analogous to ghost fishing, in which lost pieces of fishing gear such as nets or crab traps keep capturing sea life but are never retrieved.
To investigate whether hermit crabs couldn’t find their way out of a tire once they’d ventured inside, Sogabe and his collaborators set up a pair of experiments. In the field, Sogabe and his co-author fixed six passenger car tires to the seafloor with tent spikes in about 25 feet of water. The team left the tires to marinate in the brine for about a year and a half to more closely mimic tires that had spent long enough in the marine environment to accumulate algae and barnacles (during this period the researchers made sure to periodically rescue any sea creatures that entered the tires). Then for the next year Sogabe and his co-author swam down to the tires each month and counted the hermit crabs they had captured. After each of these visits, the researchers removed the animals from their rubber-walled prisons and released the critters a good distance away.
All told, the tires racked up a total of 1,278 hermit crabs over the 12 months Sogabe and his co-author spent observing them, with the highest total coming in March when the team found 246 trapped hermit crabs. Presumably, had the researchers not intervened, nearly all of these animals would have perished inside the tires.
The second experiment was conducted in the lab and was designed to test under controlled conditions hermit crabs’ abilities to escape an average car tire. The researchers dropped a tire inside a large aquarium and then released groups of ten hermit crabs at a time either inside or outside of the tire and gave them 18 hours to figure things out. Out of 120 individual hermit crabs from two different species, 19 managed to crawl inside the interior of the tire and none escaped.
Between the two experiments, the researchers showed that hermit crabs have a tough time with submerged tires and that this form of marine pollution has the potential to harm an important part of many ocean ecosystems.
Past research has also shown hermit crabs to be vulnerable to the temptations of crawling inside cozy looking trash. A 2020 study in the Journal of Hazardous Materials found that in one year more than half a million hermit crabs became entrapped by washed-up plastic debris on the beaches of three tropical islands. This study also introduced a sinister wrinkle to the story, pointing out that the stench of death from inside these plastic tombs may actually attract more victims.
Hermit crabs are attracted to the smell of their own dead because it means a perfectly good shell has likely just come back onto the market. So, once a few of their compatriots have starved to death inside a plastic bottle or a rubber tire, a new crop of hermit crabs searching for hot new real estate might be led to their doom.
Ingrid Giskes, who directs the Ocean Conservancy’s ghost gear initiative, says that this creates an unfortunate parallel with what can occur with abandoned crab or lobster traps. “It becomes a vicious cycle where an empty trap becomes a baited trap and keeps going and going,” says Giskes. “And tires are so durable and hard to break down, they could theoretically keep doing this for decades.”
Currently no evidence shows that any of the more than 800 known species of hermit crabs are in trouble because of tires, but if their numbers take a hit in places where tires and other forms of plastic pollution are especially common, it would likely have negative consequences for those ecosystems locally. Hermit crabs are essential scavengers in the places they call home, roaming the sea floor or the intertidal zone looking for morsels other larger species may have missed and generally keeping things tidy. Many of the smaller species of hermit crabs, which are actually more closely related to lobsters than crabs, are also important because of all the other animals that rely on them as a food source.
“The environmental problems identified in this study may be minor compared to global warming and ocean pollution caused by microplastics,” says Sogabe. “However, this is a good example of how our casual behavior can have a negative impact on wildlife in unexpected ways.”
Sogabe says future studies on this topic might seek to refine the scope of the problem by assessing just how many tires end up in the world’s oceans and determining where they might prove dangerous to wildlife such as hermit crabs.
“Tires are another example of a product that human society is producing and discarding that has significant negative impacts,” says Jennifer Lavers, a marine ecotoxicologist at the University of Tasmania and lead author of that 2020 paper that highlighted the dangers of hermit crabs becoming stuck in plastic pollution. “The mortality our paper calculated and the mortality this paper suggests are not small numbers when you extrapolate them to the global scale. Plastic and tire production are likely to keep increasing, which could make those numbers even larger in the future.”