Ancient Penis Worms Pioneered The Hermit Crab Lifestyle


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer


Artist's reconstruction of an Eximipriapulus (a type of "penis worm") occupying a hyolith shell 530 million years ago. Image Credit: Prof Zhang Xiguang

It's a good thing animals don't know the names we give them, never more so than penis worms (Priapulida). After all, they might argue, they were here 530 million years before us and our genitalia. Now it turns out penis worms weren't just hanging around in the sea looking phallic-like, they came up with a method for avoiding predators since made famous by hermit crabs.

Like so many other strange-looking animals, penis worms (not to be confused with penis fish) appeared during the Cambrian era, but unlike most, they survived, and haven't changed all that much since. While related invertebrates such as arthropods have diversified spectacularly, penis worms have just stuck with what works, unbothered by what humans think.


Having had a “no bones day” for half a billion years, penis worms don't fossilize very well. Nevertheless, they have been so abundant and often lived in places so well suited to making fossils that we have a record of their existence in places like the Guanshan deposits in China. Now, a new paper in Current Biology has revealed four specimens from Guanshan and the shells in which they were found, suggesting they had adopted the hermit crab lifestyle millions of years before hermit crabs.

Dr Martin Smith of Durham University and co-authors report the presence of four penis worms of the genus Eximipriapulus inside hyolith (bottom-dwelling conical shell builders) shells. One might have been a coincidence, but four couldn't be explained that way, particularly since no Eximipriapulus have been found outside shells in the area.

“The worms are always sitting snugly within these same types of shells, in the same position and orientation” Smith noted in a statement. “The only explanation that made sense was that these shells were their homes – something that came as a real surprise.". 

A hyolith shell with a penis worm showing how it got its name. Image Credit: Prof Zhang Xiguang

Hermit crabs prove occupying shells made by others can be a good way to escape predators without having to go through all the trouble of building a home yourself. We know of other examples stretching back to the mid-Jurassic, but Smith and co-authors were surprised by the penis worms' need for protection so early.


“Not long before these organisms existed, there was nothing alive more complex than seaweeds or jellyfish,” Smith said. “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.”

Penis worms were predators in their own right, but at less than an inch long the authors found there were plenty further up the food chain in the Guanshan ecosystem to threaten them. The intense predation pressure shaped the behavior of those under threat. The paper notes this is a rare case of fossils preserving evidence of Cambrian behavior. Along with the recent discovery of communal tube-building in the Burgess Shale, the finding suggests the Cambrian was more like modern times than previously acknowledged.

It looks like a bad imitation of Banksy, but it's actually a penis worm inside a hyolith shell preserved for 530 million years. Image Credit: Prof Zhang Xiguang



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