Besides hermit crabs, a few modern-day species of crustaceans and worms inhabit the cast-off shells of other marine creatures, mostly for protection against predators, says Martin Smith, a paleontologist at Durham University in England. Until recently, the oldest known fossils suggesting hermiting behavior were about 170 million years old, he says.
Now, Smith and his colleagues say that they have unearthed fossils of hermiting creatures almost three times that age, from a geologic period dubbed the Cambrian.Australia’s oceans are home to a startling array of biodiversity — whales, dolphins, dugongs, and more. But not all components of Aussie marine life are the charismatic sort of animal that can feature in a tourism promotion, documentary, or conservation campaign.
The echiuran, or spoon worm, is one such animal. It is also called the penis worm.
There is no “Save the Echiuran Foundation” and no influencers selling merchandise to help save them. But these phallic invertebrates are certainly worth your time as integral and fascinating members — of Australia’s marine ecosystems.
What makes them so interesting?
Taxonomists have classified echiurans in various different ways over the years, including as their own group of unique animals. Today, they’re considered a group of annelid worms that lost their segmentation. There is uncertainty about the exact number of species, but an estimate is 236.
The largest echiuran species reach over two meters in length! They have a sausage-shaped muscular trunk and an extensible proboscis (or tongue) at their front end. The trunk moves by wave like contractions.
Most echiurans live in marine sand and mud in long, U-shaped burrows, but some species also live between rocks. And they’re widespread, living up to 6,000 meters deep in the ocean all the way to the seashore, worldwide.
Remains of the ancient squatters were preserved in rocks laid down as seafloor sediments about 500 million years ago in what is now southern China. The cone-shaped shells that seem to hold the occupants probably had belonged to hyoliths, a once-common group of ancient marine invertebrates that died out more than 250 million years ago (SN: 1/11/17).
The marine creatures that then took shelter in those vacant shells, the researchers say, belong to a group called priapulid worms — commonly known as penis worms, thanks to their suggestive body shape. The Chinese rocks contain dozens of empty shells, Smith says. But four of those shells appear to have been inhabited by penis worms, he and his colleagues report November 8 in Current Biology. Because there were no free-ranging priapulids preserved in the ancient sediments, the researchers propose that the worms were living inside the shells.A relatively consistent ratio between the size of a worm and the shell it was preserved within suggests that the animals picked a shell based on its size and then moved to another when they outgrew their adopted home, Smith says. Modern-day hermit crabs use the same strategy, though none of the 20 species of penis worms around today have this hermiting behavior.
The researchers “have made some good observations to support their claims” that the association between the shells and the penis worms isn’t merely fortuitous, says Jakob Vinther, a paleontologist at the University of Bristol in England who wasn’t involved in the research. It’s not clear, however, whether the priapulids carried the shells from place to place, like hermit crabs do, or whether the animals merely lived inside them, he says.
A great variety of creatures — including most of the major groups of animals alive today and a proliferation of predators — evolved rapidly during the Cambrian Period, which began about 542 million years ago (SN: 3/21/19). As a result, many researchers refer to that explosion of diversity as “life’s Big Bang.”
“Perhaps it’s not a surprise that some priapulids became hermits when you think about what this predatory arms race was all about: eating, ducking and hiding,” Vinther says.
You must be logged in to post a comment.