How Plastic Bottles Become Death Traps for Hermit Crabs

We’ve all seen the images: plastic bags choking whales, six-pack rings strangling sea turtles, and plastic caps and other bits fatally filling up the bellies of seabirds. (By some estimates, each year another 14 million tons of plastic end up in the oceans.) And now, a new study brings our attention to how plastic drinking bottles are a particularly insidious threat to hermit crabs.

Hermit crabs are like tiny vacuum cleaners. By the thousands, these crustaceans scurry over beaches when the tide is out in search of scraps of rotting plants and even tinier animals to eat. The crabs also explore just about everything else that washes ashore, and unfortunately, that is all too often plastic debris. This gets them into trouble when the plastic comes in the form of a bottle, especially when it’s turned at such an angle that the little hermies can get in but not back out due to the smooth slipperiness of the container’s sides.

In less than a week, the trapped crabs die from dehydration and relentless sun exposure. Then things get even worse.

When the hermit crab finally expires, it releases a very particular scent that other hermit crabs find irresistible. Because hermit crabs can’t create their own shells, they use the discarded shells of other animals. Hermies are constantly seeking new, better-fitting armor, and when they sniff out the death of a comrade they come running—not so they can pay their respects, but so they can yank out the other crab’s corpse and steal its home.

Normally, all of this helps the animals survive. But when a hermit crab detects the death scent at the bottom of a Coke bottle, it can lure the creature to its doom.

Researchers took stock of these death traps in 2017 by sampling the beaches of two remote coral atolls, one known as Henderson Island and the other a member of the Cocos Islands. Each had shores smothered with garbage. Mind you, these aren’t islands off the coast of New York City or downstream of Kolkata. Cocos is located 1,700 miles northwest of Australia in the Indian Ocean. And Henderson is in the South Pacific roughly halfway between New Zealand and Peru. Which means these islands are out there. Henderson is uninhabited, and Cocos, comprising two atolls and a bunch of smaller islands, has a resident population of just 600. And the sea dumps our flotsam and jetsam on them all the same. Currently, the researchers estimate there are nearly half a billion pieces of human-made debris, much of it plastic, sitting on and buried within these shores. And everywhere there are bottles, there are hermit crab graveyards.

On average, the researchers found that each bottle contained about nine dead crabs, though one wide-mouthed container was found to contain shells from 526 hermit crabs. How long that bottle had been sitting there murdering crabs, no one can say.

“When we found it, we were initially quite stunned and had to pause for a moment to reflect on what we were seeing,” says Jennifer Lavers, a marine scientist at the University of Tasmania and lead author of the study detailing the team’s findings in the Journal of Hazardous Materials. Lavers and a colleague, Alexander Bond, spent several hours counting, measuring, and recording each crab from this single bottle. They did so in the shade at the base of a cliff, surrounded by a sea of washed-up, sun-bleached trash.

All in all, the scientists estimate that 569,000 strawberry hermit crabs (Coenobita perlatus) die in plastic bottles each year on just these two islands. But putting the loss into numbers seems secondary to Lavers, who struggled with her duty as a scientist to objectively document what she was seeing and her need, as a person, to grieve it. “There are no words that can truly describe how this kind of work feels, what it does to a person. It’s traumatic,” she says.

Even though most of us will never see what Lavers has seen, we’ve surely all gulped from a bottle at some point and should take a moment to consider what’s being lost out there every day as our plastic pollution continues to collect but never truly breaks down.

While pet hermit crabs do little more than sit in a sour-smelling terrarium, wild hermit crabs scuttle to and fro collecting and shredding leaf litter and other organic materials. They are the recycling trucks of the shorelines, breaking down what they find and converting it into edible bits that keep the nutrient cycle humming. Hermit crabs are also fond of seeds, snatching the tiny morsels up and dragging them all over the place, like millions of tiny Johnny Appleseeds. Finally, hermit crabs burrow into the sand during the day as a way to avoid extreme heat. This churns up the sand and soil, ensuring that it doesn’t remain compacted and unwelcoming for seedlings looking to take hold.

And that’s just the stuff we know about. Some studies suggest these crabs may be able to live upwards of 30 years in the wild. And they have what Lavers calls “a remarkable capacity to learn.”

“Each time I arrived on Cocos, the crabs would hear the sound of my kayak and emerge from the forest in their hundreds, like a welcoming committee,” she recalls. “It’s the most incredible experience to be surrounded by so many crabs.”

Some were big, others small. Some sported pointy shells, others purple and red ones. “All [were] marching in the same direction,” she says. “In those moments you admire them for what they are—intelligent little beach vacuum cleaners that are fundamental to the island ecosystem. This is what we stand to lose.”

Perhaps the new study will help people see these creatures in a new light, one not produced by a fluorescent bulb in a dinky terrarium. “Every species deserves a champion,” says Lavers. “It’s my hope that people rally around the crabs and see them not as pets but as wildlife, no different from a whale or a seabird.”

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