News Release

Study finds that ancient penis worms invented the “hermit crab” lifestyle

Embargoed until 15:00 GMT on Monday, 8th November 2021

Peer-Reviewed Publication

Durham University

A new study by researchers from Durham University and Yunnan University reveals that penis worms (Priapulida) invented the ‘hermit’ lifestyle, some 500 million years ago, at the rise of the earliest animal ecosystems in the Cambrian period.

Hermit crabs are well known for employing snail shells as shelters against predators, but researchers have now found that penis worms invented the ‘hermit’ lifestyle hundreds of million years before hermit crabs first evolved.

Researchers studied collections of the Guanshan fossil deposits – famous because they preserve soft tissue (such as the bodies of worms) alongside the shelly material that makes up the conventional fossil record. 

Four specimens of the penis worm Eximipriapulus were found inside conical shells of hyoliths, a long-extinct fossil group.

“The worms are always sitting snugly within these same types of shells, in the same position and orientation”, explains Dr Martin Smith, co-author of the study.

The researchers established that Cambrian predators were plentiful and aggressive, forcing the penis worms to take permanent shelter in empty shells.

Dr Smith expands: “The only explanation that made sense was that these shells were their homes – something that came as a real surprise. Not long before these organisms existed, there was nothing alive more complex than seaweeds or jellyfish: so it’s mind-boggling that we start to see the complex and dangerous ecologies usually associated with much younger geological periods so soon after the first complex animals arrive on the scene.”

The research indicates the key role of predators in shaping ecology and behaviour in the very early stages of animal evolution.

The study findings will be published in the journal Current Biology.

A “hermiting” lifestyle has never been documented or observed in living or fossil penis worms; nor has it been directly observed in any organism living earlier than the ‘Mesozoic Marine Revolution’ in the age of dinosaurs.

The fact that it evolved independently in the immediate aftermath of the “Cambrian explosion”, which marked the rapid rise of modern animal body plans, highlights the remarkable speed and flexibility of the evolutionary process.


Media Information

Dr Martin Smith from Durham University is available for interview and can be contacted on, by Twitter at @PalaeoSmith, or +44 (0) 7743537510.

Prof Zhang Xiguang from Yunnan the Key Laboratory for Palaeobiology is available for interview and can be contacted on

Alternatively, please contact Durham University Communications Office for interview requests on


Associated images are available via the following link:

Image caption: The penis worm Eximipriapulus inhabiting a hyolith shell.

Images should be credited to Prof Zhang Xiguang, Yunnan University

Source Information

“A ‘hermit’ shell-dwelling lifestyle in a Cambrian priapulan worm”, Yang, Xiao-yu, Smith, Martin R., Yang, Jie Guo, Qing-hao, Li, Chun-li, Wang, Yu & Zhang, Xi-guang, to be published in Current Biology.

A copy of this paper (embargoed until November 8, 2021) is available from the Durham University Communications Office. Please email Communications Office on

Useful Web Links  

Dr Martin Smith’s staff profile:

Department of Earth Sciences, Durham University:

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END OF MEDIA RELEASE – issued by Durham University Communications Office.

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