Hermit crab lifestyle discovered in 500-million-year-old worms

Hundreds of millions of years before the hermit crab appeared, some of the earliest animals on Earth were already using snail shells as protection from predators. A new study from Durham University has revealed that penis worms invented the hermit lifestyle about 500 million years ago during the Cambrian period. 

The researchers analyzed collections of Guanshan fossil deposits. These fossils preserve soft tissue, such as the bodies of worms, along with other material that is more commonly fossilized.

The experts discovered four specimens of the penis worm Eximipriapulus inside of the conical shells of hyoliths, a group of extinct marine invertebrates. These are some of the first types of shells that appear in the fossil record. 

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

According to the researchers, Cambrian predators were abundant and aggressive, which forced the penis worms to take shelter in empty shells.

“The only explanation that made sense was that these shells were their homes – something that came as a real surprise,” said Dr. Smith.  

“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 illustrates how predators played a critical role in shaping ecology during the earliest stages of animal evolution. The behavior of a hermit crab has never been documented in penis worms or in any other organism that existed so early in history. 

“Grabbing a shell takes a level of behavioral complexity to say, ‘Well, I need to find a shell that I fit in,’” Dr. Smith told New Scientist. 

“And it requires a reasonably sophisticated neural processing level, which isn’t something we’ve associated at all with these worms that just slime around on the seafloor, [nor] with the Cambrian.”

“But fossil records keep throwing us these curve balls, and making us think, ‘Whoa, OK, this was even more of an explosion than we thought.'”

The study is published in the journal Current Biology.

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By Chrissy Sexton, Earth.com Staff Writer