The 480-million-year-old parasite still plagues shellfish today

A new study has unexpectedly discovered that a common parasite of modern shellfish began infecting biota hundreds of millions of years before the dinosaurs went extinct.

Research, published in iscimentused high-resolution 3D scans to look inside 480-million-year-old shells from a Moroccan site known for exceptionally well-preserved marine life. The scans revealed a series of distinctive patterns both on the surface of the fossils and hidden within them.

“The marks were not random scratches,” said UC Riverside pathologist Karma Nanglu, who led the study. “We saw seven or eight of these question mark shapes on each shell fossil. That’s a pattern.”

“It took us a while to figure out the mystery behind these strange markings. It was as if they were giving us their question mark-like appearance.”

“But as is often the case, we answered deep in the obscure literature before our eureka moment,” he said.

The research team determined that the marks were the work of a soft-bodied marine bristle worm, which is common in today’s oceans. The worms, which belong to a group called speonids, live and feed on mussels and oysters without killing them, although they are still destructive.

“They parasitize bivalve shells like shellfish, not the meat of the animals themselves,” Nanglu said. “But damaging their shells can increase oyster mortality rates.”

The shells examined in the study belonged to an early relative of modern reptiles that flourished during the Ordovician, a period of rapid environmental change.

“This is when marine ecosystems became more intense,” Nanglu said. “You see movement, predictability and, frankly, the rise of parasites.”

The researchers considered the possibility that the question marks on the fossils were made by the shellfish themselves or by some other organism. But the evidence was strongest for the espionage explanation.

“There is one image in particular, from modern insect studies, that shows exactly the same shape inside the shell,” Nanglu said. “That was the smoking gun.”

Beyond the thrill of identification, the discovery offers an extraordinary evolutionary insight.

“This group of insects hasn’t changed their lifestyle in about half a billion years,” Nanglu said. “We tend to think of evolution as constant change, but here’s an example of a behavior that worked so well, it stayed the same through multiple mass extinction events.”

To get a look inside these question mark-sized markings, the researchers used a method similar to a medical CT scan but much more detailed, called microCT scanning. This revealed another discovery, that more biolivas with more parasites were hidden from view inside the rock, where the layers of fossils were stacked up like a multi-layered cake.

“We would never have seen it without the scanner,” Nanglu said.

The parasite’s life cycle also provided an important clue to its identity. It seems to have followed a consistent pattern: beginning life as a larva, settling on a host shell at a particular time and place, then dissolving into a small area to anchor itself. As it grew, it moved further into the shell, creating the distinctive question mark shape.

No other known animal produces this exact pattern.

“If it’s not a spynade, then it’s something we’ve never seen before,” Nanglu said. “But it has to be developed in the same place, in the same way.”

The same shell-boring behavior seen in fossils still affects oysters today. Although spiny worms do not feed directly on animals, the structural damage they cause can lead to high mortality in commercial fisheries.

“This parasite didn’t just survive the cutthroat Ordovician period, it thrived,” Nanglu said. “It’s still interfering with the shellfish we want to eat, just like it did millions of years ago.”

A fossil site in Morocco is famous for offering snapshots of long-lost behavior. Other finds include animals on the remains of squid-like creatures, providing rare evidence of ancient interspecies interactions frozen in time.

“You’re lucky to have a record of an animal that long ago,” Nanglu said. “But to see evidence of two animals interacting? That’s gold.”