Tube-dwelling sea creatures may be oldest known parasites

More than 500 million years ago, tube-dwelling creatures spent their lives stuck to the shells of clam-like sea animals called brachiopods (BRAK-ee-oh-podz). Scientists now believe those tube dwellers may be the earliest known parasites.

Parasites are organisms that must live in or on other organisms to stay alive. And their host pays a price.

Usually, parasites don’t become fossils, says Tommy Leung. That’s because their bodies are often small and soft, he explains. Leung is a parasite specialist who did not take part in the new study. He works at the University of New England in Armidale, Australia.

Parasites are “an integral part of life on Earth,” he says. But it’s been hard to tell when the parasite lifestyle emerged. It likely was very, very long ago, he notes. Today, he notes, “Practically every living thing has some kind of parasitic thing living on or in them.”

Five years ago, scientists reported finding one early parasite. It was a type of tongue worm. Fossils showed that these organisms lived on sea crustaceans some 425 million years ago. Earlier fossils had only hinted at possible parasites.

Now, a fossil bed of brachiopods in Yunnan, China, offers strong evidence for parasites from almost 100 million years earlier. Zhifei Zhang is a paleontologist at Northwest University in Xi’an, China. He was part of a team that described these parasites June 2 in Nature Communications. The fossilized animals they studied date back to 512 million years ago.

Thousands of brachiopod fossils at the site had been clustered in sediment once covered by the sea. Hundreds of them had tube-shaped structures anchored to the outside of their shells. The mouthlike parts of the tubes fanned out along the shell’s opening edges. These tubes appeared only on brachiopods — never alone or on other animals. This suggests that the tube-dwelling creature needed the brachiopod to survive.

The brachiopods were likely filter feeders. That means they caught whatever food drifted into their open shells. Zhang and his colleagues wondered if the tube-dwellers had been snatching food at the shell’s edge, before the brachiopod could eat it. If true, then tube-covered brachiopods would get less food. And that means they should weigh less than brachiopods without the tube dwellers.

To investigate, Zhang’s group estimated the mass of brachiopods with and without tubes. Tube-free ones almost always were heavier than their tube-covered companions. And that was true regardless of how many tubes were present. 

The study “demonstrates these organisms had an intimate association,” says Leung. But he isn’t convinced their relationship was truly parasitic. Brachiopods with more tubes should be worse off, he says. In fact, that wasn’t true. While brachiopods with tubes were smaller, Leung says, this might not be due to parasitism. Instead, the tube creatures might just have preferred to anchor onto smaller shells.

Brachiopods hosting tube dwellers might become stressed only when food becomes scarce, for instance. Or it could be that the tube dwellers catch food that would be too small for the brachiopods. “With these kinds of relationships, the answer isn’t always that this is good or bad,” Leung says. They tend, he says, to be “more complicated than that.”