#24-5. Home Sweet (Plastic Debris) Home
In which the E@L highlights a housing crisis among hermit crabs and birds
As a PhD candidate at the University of Washington, I taught a class in Crustacean Biology. Crustaceans are a diverse group of organisms, and within a simple but standard body plan, there are many different adaptations as to shape, size, and function. One of my final exam questions was to design a crustacean that was unlike any known organism, but included all the structural elements of other crustaceans, including body segments, appendages, sensory organs, etc. Most of the designs proposed by students were similar to other crustaceans in many ways, but one stood out. The student who created it was not the best in the class, and up to that point was headed toward a C grade. But his answer was brilliant. He designed a hermit crab, but unlike most hermit crabs that cover their soft abdomens with an abandoned snail shell, his crab had adapted to life in a beer bottle. I gave him an A.
Crab-in-a-bottle-cap
Forty years later, his idea turned out to be prescient. Around the world, a large number of hermit crab species have started using anthropogenic debris for their homes. In a novel study, Jagiello et al. used social media (iEcology) to identify 386 occurrences of hermit crabs (in posted images) using anthropogenic materials as shells, including 10 of the 16 terrestrial species [i]. Plastic caps comprised 85% of the objects used. This should not be too surprising, since plastic items comprise about 85% of marine debris. The proportion of hermit crabs using plastic debris is unknown, however, as controlled sampling of hermit crab populations has not been conducted.
Hermit crabs, and particularly those in the terrestrial family Coenobitidae, and the marine family Paguridae, have soft abdomens that are not covered by a hard shell. In order to protect this vulnerable part of their bodies, they have evolved to use gastropod (snail) shells as cover and housing. Some hermit crabs can withdraw completely inside their spiral home and block the opening with a large claw, preventing entrance or attack by predators. The abundance of mollusks is declining worldwide, due to environmental disturbance, ocean acidification, and other factors, leading to a scarcity of snail shells and high levels of competition for the best shells. In the absence of suitable shells, choosing an artificial substitute may be the best option.
Many different factors are involved in selection of gastropod shells by hermit crabs, including shell size and quality, chemical odors, the presence of predators, and past experience. It seems likely that similar processes are involved in choosing artificial shells as well. Odor cues may be involved, and lightweight plastic shells may be more energetically efficient to carry. The novelty of unusual housing might improve the ability to attract mates or allow the crab to redirect energy resources to reproduction. Where plastic debris is abundant, it may help crabs blend into the background environment, or deter predators that would not normally select for such items.
Some authors have suggested that predation risk is lower for hermit crabs that use plastic debris [ii]. If picked up by a predatory bird, the crab can shed the cover easily, which might allow it to drop back into the undercover and escape predation; in contrast, hermit crabs occupying mollusk shells tended to withdraw into the shell, which could result in their being carried away.
Debris in the trees
Hermit crabs aren’t the only animals adapting to the ubiquity of plastic pollution. While some birds may suffer detrimental impacts of plastic through entanglement or ingestion, others, particularly terrestrial ones, have started incorporating plastic materials into their nests, which may give them some advantage in human environments. Plastics may replace some natural material that is scarce or add strength to the nest, or simply serve as decoration. Birds that incorporate plastic into their nests may enhance sexual signaling by alerting mates to their presence or status. In a study of over 10,000 bird nests (by some of the same authors of the hermit crab study), where anthropogenic nesting materials (ANM) were found, they were incorporated into the structure in 85% of nests and found as regurgitated items in 20% [iii]. Terrestrial bird species incorporate anthropogenic debris into their nests at higher rates than marine bird species. Furthermore, the use of ANM was significantly associated with the human footprint index (HFI), a measure of the density and impacts of humans on the environment. For some bird species, the presence of ANM in nests altered the diversity of arthropods (i.e. insects) in the nests and actually increased the presence of fleas [iv].
An examination of bird nests from historical museum collections in Australia found ANM in nests collected as early as 1956 [v]. By 2018, ANM was found incorporated into 30% of nests. In the Sacramento Valley, California, ANM was found in 85% of 106 nests of American crow Corvus brachyrhynchos, and 5.6 % of nestlings were entangled [vi]. ANM was found in 61 % of nests built by brown boobies Sula leucogaster on islands near the coast of Brazil [vii], and in 20% of nests in the remote St. Peter and St. Paul archipelago islands, about halfway between Brazil and West Africa [viii].
Perhaps the most brazen deployers of ANM are Eurasian magpies Pica pica, which have started incorporating plastic bird-deterrent spikes into their nests. Whether this is simply to protect their eggs, or whether the birds are actually using the spikes for their intended purpose, to deter other birds, is still unknown. If the latter, it would seem as if the magpies are thumbing their beaks at us. Or maybe magpies just decided to join the ranks of goth-punkers and adopt spiky accoutrements.
In light of these recent discoveries the Ecologist@Large risks making a prediction: In the not-too-distant future, we will probably find plastic debris incorporated into structures built by other animals. These might include wasp nests, termite mounds, tubes built by marine worms (polychaetes), and maybe even beaver dams.
Whatever the reason, and whatever the result, the worldwide use of anthropogenic materials, especially plastic, by animals demonstrates how they respond and adapt to human disturbance. Until recently, this might have been seen as supporting the designation of an “Anthropocene” epoch. But since that idea was rejected by scientific consensus, perhaps a previous proposal by your Ecologist at Large (#24-2 The Plasticene Epoch is Upon Us) should be reconsidered.
And for the record (it being Seattle, after all), the preferred beverage of the beer-bottle-bearing hermit crab? Rainier Beer.
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Below are sources for the information in this post:
[i] Jagiello, Z., Dylewski, Ł., & Szulkin, M. (2024). The plastic homes of hermit crabs in the Anthropocene. Science of The Total Environment, 913, 168959. https://doi.org/10.1016/j.scitotenv.2023.168959
[ii] Sasazuka, M., Hamasaki, K., & Dan, S. (2019). Shell utilization and shell-shedding behaviour by the land hermit crab Coenobita spinosus. Ethology Ecology & Evolution, 31(6), 544–556. https://doi.org/10.1080/03949370.2019.1630013
[iii] Jagiello, Z., Dylewski, Ł., Tobolka, M., & Aguirre, J. I. (2019). Life in a polluted world: A global review of anthropogenic materials in bird nests. Environmental Pollution, 251, 717–722. https://doi.org/10.1016/J.ENVPOL.2019.05.028
[iv] Hammer, H. J., R. L. Thomas, G.J.F. Beswick, B. P. Collins, and M. D. E.Fellowes, 2017. Use of anthropogenic material affects bird nest arthropod community structure: influence of urbanisation, and consequences for ectoparasites and fledging success. J Ornithol (2017) 158:1045–1059. https://doi.org/10.1007/s10336-017-1462-7
[v] Potvin, D. A., Fabiola, O., Townsend, K. A., & Knutie, S. A. (2021). Use of anthropogenic-related nest material and nest parasite prevalence have increased over the past two centuries in Australian birds. Oecologia, 196(4), 1207–1217. https://doi.org/10.1007/s00442-021-04982-z
[vi] Townsend, A. K., and C. M. Barker (2014) Plastic and the Nest Entanglement of Urban and Agricultural Crows . PLoSONE 9(1): e88006. https://doi.org/10.1371/ journal.pone.0088006
[vii] Tavares, D. C., da Costa, L. L., Rangel, D. F., de Moura, J. F., Zalmon, I. R., & Siciliano, S. (2016). Nests of the brown booby (Sula leucogaster) as a potential indicator of tropical ocean pollution by marine debris. Ecological Indicators, 70, 10–14. https://doi.org/https://doi.org/10.1016/j.ecolind.2016.06.005
[viii] Brentano, R., de Brum, A. C., Montone, R. C., & Petry, M. V. (2020). Incidence of anthropogenic material in Sula leucogaster nests in a distant archipelago of Brazil. Marine Pollution Bulletin, 151, 110815. https://doi.org/https://doi.org/10.1016/j.marpolbul.2019.110815
As soon as your essay shifted to birds, I immediately thought of a crow’s interest in all things shiny, so as I read on I’m not surprised by the large number of crow nests in the Sacramento Valley with plastic.
I second your opinion on Reiner Beer caps, but as that fine brew is difficult to fine outside of the Northwest, I suggest this could be an excellent subject for a scientific grant money request to research what other brands of beer caps are used by hermit crabs in other locations and of course, sampling of the crab preferred brand would be required as a part of the study.
I enjoyed reading this interesting essay. The spiky magpie nest was my favorite. This made me think of an old ethnographic essay, the Empire's Old Clothes, which featured an indigenous person sporting a full polyester McDonald's uniform.