A new study has revealed that a parasite still plaguing oysters today has been around for nearly half a billion years—making it one of the longest-surviving troublemakers in Earth’s history.
Researchers from the University of California, Riverside, and Harvard University discovered fossil evidence showing that tiny marine worms known as spionids were already infesting shellfish 480 million years ago, during the Ordovician Period, long before the rise of the dinosaurs.
The findings, published in iScience, came from high-resolution 3D scans of fossilized shells collected from a site in Morocco famous for its exceptionally preserved sea life.
When researchers looked inside the ancient shells, they noticed something peculiar—tiny, question mark-shaped patterns etched both on the surface and within the shells.
“The marks weren’t random scratches,” said Dr. Karma Nanglu, a paleobiologist at UC Riverside who led the study. “We saw seven or eight of these perfect question mark shapes on each shell fossil. That’s a pattern.”
At first, the team wasn’t sure what to make of these strange traces.
“It was as if they were taunting us with their shape,” said Dr. Javier Ortega-Hernandez, a co-author and evolutionary biologist at Harvard University. After combing through obscure scientific papers, the researchers finally had their “eureka” moment—the same distinctive marks appear in modern shells infested by spionid worms.
Spionids are small, soft-bodied marine bristle worms that live inside the shells of mussels and oysters. Unlike parasites that feed on their hosts, these worms burrow into the hard shell itself, forming tunnels where they live and feed on food particles drifting by.
Although they don’t eat the animal directly, their burrowing weakens the shell, increasing the risk of death for the host.
The fossil shells studied belonged to an early relative of modern clams that lived in ancient seas teeming with life. During the Ordovician, marine ecosystems were rapidly evolving, with new predators, prey, and parasites emerging.
“This was a time when ocean life got more intense,” Nanglu said. “You see the rise of mobility, predation—and clearly, parasitism.”
To confirm the worms’ identity, the team used micro-CT scanning, a technique similar to a medical CT scan but far more detailed.
The scans not only revealed the signature question mark tunnels but also uncovered hidden layers of fossilized shellfish inside the rock, showing just how widespread the parasitism was.
The shape of the burrows matched those made by modern spionid worms almost perfectly. “There’s one image from a study of living worms that shows exactly the same shape inside a shell,” Nanglu said. “That was the smoking gun.”
The discovery suggests that spionid worms have been living this way for hundreds of millions of years, surviving multiple mass extinctions—including the one that wiped out the dinosaurs. “We tend to think of evolution as constant change,” Nanglu said, “but here’s an example of a behavior that worked so well it stayed the same for almost half a billion years.”
Today, spionid worms still plague oysters and other shellfish, including those raised in aquaculture. While they don’t consume the animal itself, the damage they cause to shells can lead to higher mortality rates, costing commercial fisheries millions of dollars each year.
“This parasite didn’t just survive—it thrived,” Nanglu said. “It’s been interfering with the oysters we want to eat for nearly 500 million years. Some things never change.”
Source: UC Riverside.
