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‘Penis worms’ invented the hermit lifestyle some 500 million years ago by moving into discarded shells

The “hermit” lifestyle best known from crabs today was first invented 500 million years ago by worms, new research reveals.

Illustration of a Cambrian penis worm inhabiting a hyolith shell. Image credits Zhang Xiguang.

Hermit crabs today are famous for seeking out and taking residence in discarded snail shells. This offers the animals protection against predators while allowing them to avoid the huge energy and nutrient costs associated with building such a shell from scratch. It’s quite an effective strategy for the crabs.

However, they’re not the ones to first think of it. Researchers at Durham University and Yunnan University report that penis worms (phylum Priapulida) were busy taking up residence in discarded shells during the Cambrian period some 500 million years ago. This is the oldest known evidence of the ‘hermit’ lifestyle that we’ve found so far, the team adds.

Renting a place

“Priapulan-like worms were very common in the Cambrian, sometimes forming shallow burrows but often moving along the surface of the sea bed,” said Dr. Martin R. Smith, an Associate Professor in Palaeontology at Durham University and a co-author of the paper, in an email for ZME Science. “Some had little defensive plates or built their own tubes for protection from predators; others even had little claw-like hooks on their underside to help them move about.”

“Today, priapulans are only found in hard-to-inhabit places, such as those with low oxygen, or miniaturized between sand grains — places where predators find it hard to make a living. We used to think that predators struggled a bit in the Cambrian too, but clearly — at least in the Guanshan [area, where these worm fossils vere recovered] — predators were enough of a threat to make priapulans reach for a more ‘modern’ style of defensive mechanism!”

Fossil deposits in the Guanshan, an urbanized area in southeastern Taiwan. The fossil deposits in this area are famous for often preserving soft tissue alongside harder material such as shells, which is quite rare for fossils. Four specimens of penis worms in the genus Eximipriapulus from this area were recovered inside the conical shells of hyoliths, another group of long-extinct animals.

“The worms are always sitting snugly within these same types of shells, in the same position and orientation. My first thought was ‘can I be sure that this isn’t a coincidence’, so we had to look hard at the fossils to be sure that the worms were in (rather than on or below) the shells, that the relationship was consistent, etc.,” explains Dr. Smith for ZME Science.

The findings cast new light on our understanding of the ecosystem relationships during the Cambrian. This was a geological period that was a hotbed of evolution, with many and exotic forms of life appearing all over the planet. The rapid evolutionary changes during this time have earned it the nickname of the “Cambrian explosion”. And some very peculiar lifeforms appeared during this time; in many ways, life on Earth during the Cambrian was quite alien-looking.

It was definitely a valuable time in history. The trial-and-error that happened during this time led to the emergence of what we’d call ‘the modern’ animal body.

But more to the point, the high rates of diversification during the Cambrian led to the emergence of predators in many environments where they were completely lacking before. This, in turn, led to an arms race between predator and prey that still continues to this day.

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

Hermit crabs today take permanent residence in the shells they find laying on the sea bottom, carrying them around on their backs. The team believes penis worms also carried hyolith shells around, that they “likely dragged the shells with them”, Dr. Smith told me.

“We believe that they dwelt in them permanently (except to up-size when they grew). It’d be lovely to see trace fossils showing the grooves made by dragging the shells, but as the fossils were transported to their resting place, we don’t have definitive evidence,” he told ZME Science.

Hermit crabs today eventually outgrow their shells. Groups of hermit crabs looking for an upgrade will often get together and line up from smallest to largest, then trade shells among themselves. It’s a pretty handy way to make sure that everybody has at least a shell, and that most crabs have a proper shell to live in (some, sadly, are left with shells that are a bit too tiny or defective in some way). I asked Dr. Smith whether there’s any sign that the worms engaged in similar social behavior to find shells to inhabit.

“No evidence, and there seems to have been no shortage of shells in this environment, so it probably didn’t happen in Guanshan – but we don’t have evidence to say either way,” he explaned for ZME Science. “I’d be surprised if they did, as this ‘feels’ like a sophisticated behaviour for this group, and for the Cambrian period  – but then I would have said that about hermiting at all before I saw these fossils!”

Despite their hermiting lifestyle, penis worms were not helpless beasts in their ecosystems. Dr. Smith tells me that shells from brachiopods (lamp shells), trilobites, and hyoliths have been found in the guts of penis worms related to the ones the team investigated here.

“Worth mentioning that ecologically, priapulans were both predators and prey; many fed on organic material within mud, but others scavenged and some may have been active predators themselves,” he told ZME Science.

The findings are valuable as they represent the earliest known case of an animal engaging in a hermit lifestyle. It gives us context to better understand the ecosystem dynamics of the Cambrian period, and of the emergence of wide-scale predation on Earth.

The paper “A ‘hermit’ shell-dwelling lifestyle in a Cambrian priapulan worm” has been published in the journal Current Biology.

Above image: Two individuals of Harrimania planktophilus, a modern enteropneust (harrimaniid) worm. Photo: C.B. Cameron, Université de Montréal.

The tooth-lined ‘penis worm’ now gets a dentist’s handbook

One of the perks of being a writer for ZME Science is that I frequently get to feature some really amazing, yet bizarre creatures. Take for instance Ottoia prolifica (priapulidor the penis-worm as it’s also known, for obvious reasons. This phallic creature actually had a throat full of teeth which it used to munch its meaty prey, and the weirdness doesn’t stop here. It could turn its mouth inside-out and use those teeth for traction to easily move about. Talk about double standards. Now, a team has systematically studied these ancient Cambrian fossils (520 million years old) to compile a dentistry handbook to distinguish between other penis worm species. This proved to be wise, since in their compiling work the researchers at University of Cambridge have already reported what they believe to be new Ottoia species.

Above image: Two individuals of Harrimania planktophilus, a modern enteropneust (harrimaniid) worm. Photo: C.B. Cameron, Université de Montréal.

Above image: Two individuals of Harrimania planktophilus, a modern enteropneust (harrimaniid) worm. Photo: C.B. Cameron, Université de Montréal.

“Taken together, our study has allowed us to compile a ‘dentist’s handbook’ that will help paleontologists recognize a range of early teeth preserved in the fossil record,” lead author Martin Smith of the University of Cambridge said in a statement.

“As teeth are the most hardy and resilient parts of animals, they are much more common as fossils than whole soft-bodied specimens,” he explained. “But when these teeth – which are only about a millimetre long – are found, they are easily misidentified as algal spores, rather than as parts of animals. Now that we understand the structure of these tiny fossils, we are much better placed to a wide suite of enigmatic fossils.”

The authors used high-resolution imaging techniques to identify the penis worm teeth from fossils like those found in the Burgess Shale, a geologic formation in the Canadian Rockies that contains fossils of a myriad of bizarre creatures dating from the Cambrian period. Findings were reported in the journal Paleontology.

A fossil of a newly described penis worm species,  Ottoia tricuspida. Image credit: MARTIN SMITH

A fossil of a newly described penis worm species, Ottoia tricuspida. Image credit: MARTIN SMITH

Penis worms are still alive today, though they’re very hard to find since they only live in extreme environments. Check the video below for some penis worm action.

It’s easy to class the penis worm as somewhat uninteresting, scientifically-speaking, but another study published in 2012 proved this thinking wrong. Basically, a classification that orders animals in the tree of life is based on the order in which they develop a mouth and an anus as embryos. It follows that there are protostomes (from the Greek for ‘mouth first’), in which the mouth formed first, and the anus second, and deuterostomes (‘mouth second’) which describe animals where the mouth formed after the anus. Vertebrates, including humans, are deuterostomes, while invertebrates like jellyfish are protostomes. The penis worm used to be classed as a protostome as well, but researchers at University of Hawaii in Honolulu found that the  priapulid Priapus caudatus (another penis worm) develops like a deuterostome. So, because of the penis worm, the way in which the largest branch of the animal tree of life has been defined now needs revision.

‘Penis worm’ pokes holes in 100 year-old theory



The largest branch of animals in the tree of lifes are the protostomes – historically defined by the order in which they develop a mouth and an anus as embryos. But new gene-expression data conducted on “penis worms” suggests otherwise.

The mouth and the anus

Biologists will have to rename and rethink the protostomes, explains Andreas Hejnol, an evolutionary developmental biologist at the University of Bergen in Norway and lead author on the report: “we need to rethink how our earliest ancestors developed”, he explains.

Even the tiniest differences in embryonic evolution can lead to monumental changes in adults; the best example is when an embryonic ball of cells formed two indentations instead of one, thus leading to the separation of the mouth and the anus, rather than the single, all purposes hole jellyfish and sea anemones have. Back in 1908, more than 100 years ago, animals with both anus and mouth were divided into two different groups: the ones in which the mouth is formed first (protostomes), and the ones in which the anus is formed first.

Today, the protostomes include most invertebrates, while the other group, called deurostomes include most vertebrates, including human – yes, our anus is formed before our mouth. Because analyzing how the embryon is formed and divided is technically challenging and quite costly, researchers have analyzed only a fraction of animals in each group, but this theory has beeon confirmed by other DNA sequencing data and the division between protostomes and deurostomes remained in place.

Shaking things up

Everything seemed to be ok, but this theory was shaken up by the most unlikeliest of creatures: Priapus caudatus, commonly referred to as the “penis worm”. The animal was believed to be a primitive protostome, but Hejnol and his team revelead that it acually develops like a deuterostome.

“Here is an animal that is the poster child for early protostomes, and it develops just like a deuterostome,” says Mark Martindale, a developmental biologist at the University of Hawaii in Honolulu. “We’ve been using the name protostome for 100 years, and now it’s clear that it doesn’t mean anything.”

The problem is that they are not only simply placed on the wrong side of the tree. Other similarities, mostly DNA sequences indicate they very much belong to the protostomes.

“This is a big achievement because for years, people have wanted to do work like this in priapulids, but it’s technically very difficult,” says Detlev Arendt, an evolutionary developmental biologist at the European Molecular Biology Laboratory in Heidelberg, Germany.

So where does this leave us? Obviously, if the research is correct, and there is no reason to doubt this, we need a new classification, and a new class of species; unfortunately… biologists don’t have one ready yet.

“At the turn of the twentieth century, embryologists drew what they saw. But their microscopes stunk and they didn’t know about genes,” Martindale says. “Now we’re finally able to look closer, and we’re finding that they’re often wrong. But if you think about the tools they had, maybe it’s more surprising that anything is right at all.”