Terrestrial hermit crab (Coenobitidae) trash shells

Back in 2022 I wrote about how octopuses were increasingly turning to human trash for shelter. Sadly, they’re not the only ones dealing with the massive amounts of garbage that our species dumps in the oceans. Today we look at another group making the most of this polluted planet: terrestrial hermit crabs (family Coenobitidae).

A surprise toy

When you imagine the white sand, green palm trees, and turquoise waters of an ideal tropical island, you’re basically picturing Kuramathi. This comma-shaped piece of coral atoll in the Maldives is a magnet for those who can afford its resorts, or to anchor their yacht off its jetties. But even there, in the ocean hundreds of kilometres southwest of India’s southern tip, there’s no escaping the residues of people.

In January 2008, Jorge Avila was on a diving holiday on Kuramathi, when he noticed an odd shuffling thing nearby. He snapped a photo, revealing it to be an unidentified species of hermit crab. But as you might imagine from today’s topic, the protective shell that makes its kind so distinctive wasn’t natural. That’s Jorge’s photo at the top of this post, with the crab tucked neatly into a blue plastic cup.

The discovery was written up in a short paper by two university researchers, Joao Barreiros and Osmar Luiz Jr, based in Portugal and Brazil respectively. They diplomatically described the crab’s home as

half of a plastic box, widely distributed throughout the world, [found] inside chocolate eggs and containing a small toy.

You’ve probably seen the exact kind of thing in a supermarket check-out line. The paper had no further information on what happened to the crab, or how it came to replace the toy in its shelter (the toy itself was most likely made of plastic, of course). This is just one slice in this hermit’s life, glimpsed and gone. But a more recent study, published in 2024, has revealed that the Kuramathi crab is far from alone.

The wrong trousers

Even before the new study, evidence for crab-garbage hybrids was piling up on humanity’s own digital trash heap/gold mine: the internet. For example, this photo from Wikimedia is of a terrestrial (land-living) purple hermit crab (Coenobita brevimanus) taken in 2018 on Pom Pom Island, Malaysia. It’s using a discarded soup can, with its back end wedged firmly into the rusting canister:

Zuzanna Jagiello and her colleagues at the University of Warsaw and Poznan University of Life Sciences, Poland, saw an opportunity in the growing collection of online material like this. They wondered: could crowdsourced images tell us just how widespread this behaviour is? They scoured the most popular image sharing sites—Flickr, iNaturalist, Google Images, YouTube, and Alamy—for any evidence of hermit crabs linked to keywords like ‘waste’, ‘anthropogenic’ and ‘debris’. They place this kind of online survey within the emerging field of iEcology, or internet ecology, in which scientists can gather global data in weeks that would previously have taken years of expeditions or deep delves into library archives.

Their resulting paper, titled ‘The plastic homes of hermit crabs in the Anthropocene’, documents 386 hermit crabs using artificial shells. Thanks to geo-tags or metadata, many of the images could be precisely placed onto a world map, specifically this one:

Obviously the data are biased towards places that people (a) regularly visit, (b) take photos, and (c) upload those photos to the web. But even within those limits, 10 out of the 16 known species of terrestrial hermit crabs were found wearing the wrong trousers. For the record, Flikr turned out to have the most evidence, providing over half the images found, followed by the specialist nature-documentation site iNaturalist. And yes, you can see that the Maldives are well-represented.

Plastic was the favoured material, used by 326 (85%) of the crabs, which is yet another sad indictment of how much of the world’s beaches we have now messed up. But it wasn’t the only option: metal and glass trash, either together or separately, made up the remaining proportion. There are examples of each in Figure 1 from the report by Jagiello and her team:

Instead of snail or gastropod shells, human-made materials are increasingly common fashion choices of the 21st century hermit. These particular images were taken by photographer Shawn Miller (whose work you can find here), whom the scientists credit with prompting their study in the first place. Miller uses his observations in Okinawa, Japan, to push for cleaner environments, including marking and placing empty seashells for hermit crabs to use instead of rubbish.

Below is an example of one of his marked shells, each of which has a number and ‘MTS4N’, for Make The Switch For Nature, engraved on it. If you happen to see one of these on your travels around Okinawa, please take a photo but do not pick it up! The crabs will thank you.

Tool use or shell-ter?

There are many unanswered questions about this behaviour. First, why do the crabs do this? Is it a lack of suitable materials, or might the trash shells give some kind of benefit? Because theirs was an initial survey, Jagiello et al. recognise the need on-the-ground data about whether increasing amounts of plastic at a given site is correlated with its use by hermit crabs. At some point, searching for a new shell—a vulnerable time in a crab’s life as it outgrows its current home and needs a new one—may simply be easier and quicker if human trash is accepted as an option.

There may also be a colour preference. Despite the vivid photos above, most of the surveyed crabs were photographed living in white (28%), black (15%) or transparent (13%) artificial shells. Field tests and data on non-used materials are needed to see whether that is again an availability issue, or something more selective.

Other options yet to be tested include female preference for novel materials, or the relative lighter weight of plastics. Decaying plastics can even give off a similar chemical signature as a dead hermit crab—dimethyl sulphide—which is among the cues that other crabs use to locate a newly vacant home. There’s also a straightforward if grim predator-prey explanation: on increasingly polluted shores, the most garbage-looking crab might have the best camouflage.

Aside from questions of why, however, there remains a more specific issue for us on this blog, which whether or not hermit crab shells should be considered tools. The hard outer carapaces grown by most crabs aren’t tools under any definition, because they are internally manufactured via genetic instructions and form an intrinsic part of the animal’s body. Many crabs do molt, periodically shedding their shell to allow them to grow larger, but unless other crabs then put on those shell-suits and strut around in them, they’re not tools, just as our clipped fingernails are not.

But hermit crabs are different. Despite needing a covering for their soft rear-ends, they still grow too large and must regularly find replacements. When they find a new home they have to grip it tightly to stop it slipping off. Their shells (or plastic eggs or soup cans) are then more like clothing for humans, offering necessary but switchable protection. Hermit crabs don’t switch shells nearly as often as most humans change their clothes, but if we compare them to their namesakes—human hermits—is there really that much difference?

Tools at their heart are objects that enable new or extended abilities. I believe that hermit crab shells are one of the most evolutionarily successful forms of tool use, shared by numerous species and spread across our world. We don’t yet know whether or not the crabby embrace of all the new materials we have shoved into their environment ends up supporting or damaging that technology, but we shouldn’t deny that they may have been tool users far longer than our own ancestors.

We will need many more studies like that of Jagiello and her colleagues to disentangle the changes happening now. If you have access to the internet, plus a bit of time and patience, there is nothing stopping the next iEcology breakthrough from being your own. Good luck!

Further viewing

If you’re looking for two minutes of emotions that might range from fascination to horror to sadness to contemplation of life’s strangeness and resilience (and who isn’t?), this video of a hermit crab walking to the water, using a plastic doll’s head as its shell, is for you. It was filmed in 2019 by Joseph Cronk, on Wake Island in the western Pacific:

Sources: Barreiros, J.P. & O. J. Luiz Jr (2009) Use of plastic debris as shelter by an unidentified species of hermit crab from the Maldives. Marine Biodiversity Records 2: e33. || Jagiello, Z. et al. (2024) The plastic homes of hermit crabs in the Anthropocene. Science of the Total Environment https://doi.org/10.1016/j.scitotenv.2023.168959.

Main image credit: Barreiros & Luiz (2009) || Second image credit: Wikimedia commons; https://en.m.wikipedia.org/wiki/File:Purple_hermit_crab_in_can.jpg || Third image credit: Jagiello et al. (2024) Fig.2 || Fourth image credit: Shawn Miller; Jagiello et al. (2024) Fig.1 || Fifth image credit: Shawn Miller; https://okinawanaturephotography.com || Video credit: Joseph Cronk; https://www.youtube.com/watch?v=cdjs5raf0BE