Study into infested fossil worms shows ancient examples of symbiosis

One of the earliest examples of two invertebrate species living together in a symbiotic relationship has been found in 520-million-year-old fossils from China.

The fossils, discovered by a team including researchers from our University, show two species of marine worms with other, smaller worm-like animals attached to the outer surface of their body. 

The study, published in Nature Ecology and Evolution, was produced by an international group of scientists including The Natural History Museum, London, the University of Leicester and Yunnan University in China.

Symbiotic relationships, which involve two different kinds of organism interacting with close physical contact, are common in nature. However, few prehistoric examples involve soft-bodied animals because they are normally not fossilised.

Although fossils of the two species of marine worm, Cricocosmia jinnigensis and Mafangscolex sinensi, have been found before, these are the first reported examples to show other animals attached to them.

The smaller worm-like guests, a new species named Inquicus fellatus, are up to 3mm long and attached at their bottom ends to the stiff skin of their hosts, with their feeding ends pointing away.

Despite the fact that Inquicus fellatus are attached to their host worms, there is little indication they were feeding by penetrating the skin of their hosts, causing the authors to conclude it was unlikely the relationship was directly parasitic.

Sarah Gabbott, Professor of Palaeobiology from our School of Geography, Geology and the Environment, said: “When we first saw the large worm curled around, almost hugging, lots of tiny worms, we suspected that we had uncovered an adult with offspring. But, careful inspection with a high-powered microscope, revealed that the large and small worms were different species - so that theory was completely blown away, and we realized that a symbiotic relationship was most likely.

“We then asked ourselves - was it parasitic or not - were the small worms feeding on the large one? We could tell that it was always the posterior of the small worms, and not the mouth, that was attached so this was altogether a more ‘neighborly’ relationship.”