Hermit crabs snap up bright litter in hope of finding a mate
Hermit crabs are using litter as makeshift shells in an apparent attempt to find a mate, scientists have discovered.
Plastic caps are the most popular form of rubbish adopted by terrestrial hermit crabs, data show, with the preponderance of plastic waste in nature offering other options to the usual discarded snail shell.
Two thirds of the 16 known terrestrial hermit crab species were found to have used litter as a shell by scientists from Poznań University in Poland.
Photographs posted online by members of the public were analysed and a total of 386 crabs were seen using waste as a shell, covering 10 of the 16 known terrestrial species.
It is thought that the crabs are using scraps of rubbish instead of shells for a variety of reasons, including the sheer amount of litter that there is, which makes it a readily available resource.
Dimethyl sulphide
Novel shells made of plastic are also more colourful than some natural options, the scientists say, which may be useful in attracting a mate.
“Artificial shells may be attractive for females, as novelty per se is a premium in the evolution of sexual display and sexual selection,” the authors write in their study, published in Science of the Total Environment.
Crabs may be choosing plastic over snail shells because they are lighter and therefore easier to carry, they add. A third possible reason for plastic ornaments being worn by the crustaceans is that the creatures are drawn to a chemical that the plastic emits called dimethyl sulphide (DMS).
This chemical is also produced by the decomposing bodies of dead crabs and hermit crabs that detect it are drawn to the odour as they know it will present an opportunity to steal their shell.
DMS from plastic, therefore, gives the same smell signal as a natural shell.
Another reason for plastic shell use is that it could be strong and camouflaged, helping the animals avoid predation.
The scientists do not yet know whether the switch to anthropogenic shells is harmful to the crabs, and call for more research to be conducted to analyse impacts on fitness.
“When I first saw these pictures, I felt it was heartbreaking,” study co-author Prof Marta Szulkin, from the University of Warsaw, told BBC Radio 4’s Inside Science.
“Instead of being adorned with a beautiful snail shell, which is what we’re used to seeing, they would have a red plastic bottle cap on their back or piece of light bulb.”
Plastic caps were the most common form of plastic used as fake shells (85 per cent), often preferred to other types of pollution as well as old snail shells.
“We harnessed the power of iEcology – an emerging, technology-driven naturalist data source to describe for the first time the use of anthropogenic waste (largely plastic) as artificial shells in two thirds of the world’s terrestrial hermit crab species,” the scientists write.
“To date, this behaviour was largely unnoticed by scientists.
Global occurrence
“Materials such as plastic or metal acting as artificial shells for hermit crabs was found to be a global occurrence, likely to impact natural and sexual selection and individual fitness, with the potential of ultimately impacting the evolution of hermit crabs in the long run.”
The scientists say that the ramifications of the rubbish shells remain unknown, as they could have some pros and some cons.
More research will be needed to determine if the phenomenon is altering the evolution of the crabs or if it is simply a trap of the Anthropocene epoch.
“Relatively straightforward field surveys, combined with field and lab experiments, are needed to understand the prevalence, future spread and fitness consequences of artificial shell use in hermit crab biology,” the researchers say.
