These oddly named worms may have been the first hermits

Hermit crabs' sheltering strategy — borrowing shells left by other animals and trading them for bigger models as they grow — was believed to have emerged 180 million years ago in the Jurassic period, when their ancestors appeared in oceans, said Martin Smith, a palaeontologist at the University of Durham in England. But in a study published Monday in Current Biology, Smith and colleagues suggest the practice of hiding out in borrowed shells dates back hundreds of millions of years earlier, to the dawn of complex ecosystems.

Those early hermits weren’t crabs, but priapulan worms — carnivorous, ocean mud-dwellers known as penis worms. “They’re a neat group of animals with an unfortunate name,” Smith said. “They’ve got this really strange morphology: a trunk that looks a bit like a finger, and a throat lined with little teeth that turns inside out.”

Priapulan worms appeared more than 500 million years ago during the Cambrian period, when animal life evolved rapidly in sophistication, complexity and size. The worms played an important role in the Cambrian seas, Smith said: The start of the Cambrian period is formally marked by the first appearance of their burrows, which helped mix oxygen into the sea floor, making it more hospitable to other forms of life.

Previously, no priapulan worms were known to live in the discarded shells of other animals, Smith said. Over the past decade, however, a team of paleontologists led by Zhang Xi-guang at Yunnan University in China has collected detailed Cambrian fossils from approximately 510 million-year-old deposits in China’s Yunnan province. Among these deposits, discovered in 1999, the researchers found the imprints of four priapulan worms, coiled up in the empty, conical shells of small tentacled creatures called hyoliths.

“The shells are almost exactly the same size as the worms within them, and the worms were always in the same orientation — housed in them with the mouth sticking out,” Smith said. “This made us think this wasn’t just chance, but a biological relationship.”

“It makes for a convincing case,” said Javier Ortega-Hernandez, a paleontologist at Harvard who specialises in invertebrates and was not involved in the study. Fossils that show two species appearing to interact can be deceptive, he said, as bodies can be swept together by water currents or collapsing mud dunes. But the repeated presence of worms tucked neatly into the shells is hard to argue with, he said.

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