A news article on the BBC website introduced a paper measuring the use of our rubbish by these little decapods. The authors used iNaturalist as a source of images to find those using pieces of plastic, metal or glass for homes instead of snail shells. Of the nearly 30,000 images they reviewed 81 were found.
It may be ‘cute’ to see one of these crabs using a discarded light bulb as a home but the authors suggest the availability of such artefacts could have profound impacts one their behaviour.
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That is a troubling prospect.
These statements from section from section 3.1.1. Environmental availability of the paper are also foreboding:
Given the decline of mollusc populations worldwide (UNEP 2019), gastropod shells are likely becoming increasingly scarce, especially in human-altered environments. Thus, in order to find the optimal shell, it may be less costly energy-wise to find an artificial shell than a natural one.
The following, from section 3.2. Individual choice of the paper, also underscores the profound degree to which anthropogenic debris is changing the overall character of some environments:
… artificial shells may act as efficient camouflage in polluted environments, as crabs tend to prefer shells which match its environmental background (Wilding et al., 2008).
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I know that corrosion from increasingly acidic oceans is part of the reason shells are less available… but I do wonder how hard it would really be to mass-produce artificial “snail shells” out of chalk / analogous materials, which would allow hermit crabs in areas low in shells to use similar, but man-made objects.
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I’ve become a bit fascinated with Carrier Shells (Xenophora) recently. For those that don’t know they’re a type of sea snail that attach debris like shells, rocks and coral to their own shell to save energy creating their own shell material. There have been reports of those using marine pollution like glass and coins instead of the usual natural stuff. It’s been hard to find exact details though.
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Is that inherently a bad thing, though?
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On the positive side, it serves as an effective adaptation to the developing conditions in the environment of those hermit crabs, which does aid in their survival. On the other hand, it is a symptom of pollution and a marked decline in gastropod populations. So we could regard it as a fortunate reaction to an unfortunate situation.
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The shells they use have been scaped over a long time so that none of the hermit crabs on land (to rounding error) are using new shells, unless they are very young and have to.
They excavate the shells to make them lighter, but much prefer one already fixed. They also exchange shells to move up in size as they grow, and eventually, die. The sculpted shells on land are therefore a limited resource, for which there is fierce competition. Sculpted shells are a different pool / resource than newly washed up shells, and not as quickly renewed.
I can’t find the time scale over which remodeled shells became the primary source of shells, or how old some of them are.
Sources:
https://www.naturalhistorymag.com/features/122719/the-social-lives-of-hermits
https://news.berkeley.edu/2012/10/26/hermit-crabs-socialize-to-evict-their-neighbors/?utm_source=pocket_saves
https://www.scientificamerican.com/article/life-shell-game-hermit-crabs-exchange-shells/
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That makes sense. Didn’t know that. A flimsy plastic bottle cap - has the benefits of being light, and camouflage.
I wonder if the shells provided for captive hermit crabs allow for lighter, or target human eye candy?
Then this bubbled up on my FB feed - rescuing hermit crabs from plastic
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I have seen carrier shells that have used metal beer bottle caps as attachments.
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Strangely the same thing just surfaced in my Youtube recommendations. I think The Man is watching us https://www.youtube.com/watch?v=uGRkYmxFrD8
People must be taking home buckets full of shells for this to be such a problem though. It’s probably good that awareness of plastic and marine life is being raised.
On beach cleans I remove lots of plastic bottle caps that arrive from all over the world. Now in the EU the law is that caps must have a tab that attaches them to the bottle so they stay in place to be recycled. I’m seeing lots of posts across the internet of people saying they just rip them off because they cause a mild inconvenience. Another case where a law has been brought in to make things better but people haven’t been made aware of the scale of the problem and why this had to be done.
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