These tube-shaped creatures may be the earliest known parasites

Tube-dwelling creatures that spent
their lives cemented to the shells of clamlike brachiopods over 500 million
years ago may be the earliest known parasites.

“Parasitism is an integral part of
life on Earth, but it’s been hard to determine when it emerged,” says Tommy
Leung, a parasitologist at the University of New England in Armidale,
Australia. But, he says, it likely arose early, in part because today
“practically every living thing has some kind of parasitic thing living on or
in them, even down to parasites themselves.”

Sometimes, scientists get lucky and
find parasites preserved
with their hosts in amber (SN: 12/10/19). But usually parasites don’t fossilize well because their
bodies are often small and soft, Leung says. And even if two organisms happen
to be entombed in the same fossil, it can be difficult to discern whether their
relationship was parasitic, mutualistic or somewhere in between. Fossils of tongue worms from 425 million years ago represent a clear early example
of parasitism, but previously found older fossils from the Cambrian only hint
at possibly parasitic relationships.

Now, a 512-million-year-old
bed of tube-encrusted brachiopods in Yunnan, China offers compelling
evidence of a parasite-host relationship, Zhifei Zhang, a paleontologist at
Northwest University in Xi’an, China and his colleagues report June 2 in Nature
Communications. 

In a tan-colored outcropping in southern
China, researchers discovered thousands of brachiopods clustered together.
Hundreds of them had numerous tubelike, tapered structures affixed to the
exterior of the shells. Those structures were arrayed like the spines of a fan
with the mouthlike parts positioned along the open edge of a shell. The tubes
appeared only on brachiopods, never alone or associated with other fossils,
suggesting that the organism couldn’t survive on its own.

The brachiopods were likely filter
feeders, catching whatever food happened to drift into their open shells. Zhang
and his colleagues hypothesized that these tubes might have snatched food from
the edge of the shell before the brachiopod could eat it, making them
kleptoparasites.

If that were true, tube-covered
brachiopods should be lighter than their tube-free brethren, since they’re
getting less food. The researchers estimated the mass of brachiopods with and
without tubes, finding that the tube-free brachiopods were almost always
heavier than their tube-laden brethren, though the number of tubes didn’t have
any effect. 

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The study “demonstrates these
organisms had an intimate association,” says Leung, who wasn’t involved in the study. But he
isn’t so sure that the relationship was antagonistic. If the relationship were
truly parasitic, brachiopods with more tubes should be worse off, he says, but
that wasn’t the case. While brachiopods with tubes were smaller, Leung says
this might not reflect a cost of parasitism. Instead, the tube creatures might just prefer to
affix to smaller shells.

Whether a relationship is parasitic or not can depend on the ecological context. Tube-laden clams might become stressed by tubes only if food becomes scarce. Or, perhaps tubes catch food too small for the brachiopods anyway. “With these kinds of relationships, the answer isn’t always that this is good or bad,” Leung says. “Interactions are usually more complicated than that.”