Don’t move a mussel: Cambridge researchers warn species relocation could spread pathogens

What can we learn from the tale of the gonad-eating parasitic worm?

More than you might think, according to Dr David Aldridge, from the Department of Zoology at the University of Cambridge.

Dr Aldridge, who won Researcher of the Year at last week’s Cambridge Independent Science and Technology Awards, has issued a warning about the risks of relocating species as part of conservation strategies.

It follows the study of mussels - one of the most threatened animal groups on Earth.

They play a crucial role in cleaning the water of many of the world’s rivers and lakes, and there has been growing interest in moving them to new locations to boost threatened populations or to act as biological filters to improve water quality.

But captive breeding programmes that bring mussels from isolated populations together face a major risk of providing a fertile breeding ground for Rhipidocotyle campanula, the gonad-eating parasitic worm. They can leave mussels completely sterile

“We need to be much more cautious about moving animals to new places for conservation purposes, because the costs may outweigh the benefits,” said Dr Aldridge, senior author of a new report in Conservation Letters.

“We’ve seen that mixing different populations of mussels can allow widespread transmission of gonad-eating worms – it only takes one infected mussel to spread this parasite, which in extreme cases can lead to collapse of an entire population.”

In some cases, the presence of pathogens may pose less of a risk unless combined with other factors, such as a lack of food or high temperatures, that can put a population under stress and lead to a sudden outbreak,

The research focuses on mussels, but its message is applicable more broadly. Relocations to protect endangered species or restore degraded ecosystems are a well-known conservation strategy.

But the authors say the tactic should only be used when absolutely necessary and by using quarantine periods, designed to stop transmission of the most likely pathogens being carried.

Josh Brian, a PhD student in the Department of Zoology and first author of the report. added: “Moving animals to a new location is often used to protect or supplement endangered populations. But we must consider the risk this will spread pathogens that we don’t understand very well at all, which could put these populations in even greater danger.”

Four key factors are identified in the report to determine the risk of spreading pathogens when relocating animals:

• the proportion of infected animals in both source and recipient populations;
• the density of the resulting population;
• host immunity;
• the life-cycle of the pathogen - pathogens that must infect multiple species to complete their life-cycle, such as parasitic mites, will only persist if all of the species are present in a given location.

Adaptations to immune systems are known to impact how different populations respond to infection by the same pathogen.

In Yellowstone National Park in the United States, a pack of endangered wolves that had been moved in died because they had no immunity to parasites carried by the local canines.

And it is not just conservation efforts that risk providing a piggyback for pathogens.

Stocking rivers with fish for anglers or sourcing exotic plants for home gardens could likewise move around parasites or diseases.

Isobel Ollard, a PhD student in the Department of Zoology, who was also involved in the study, said: “Being aware of the risks of spreading diseases between populations is a vital first step towards making sure we avoid unintentional harm in future conservation work.”

The research was funded by the Woolf Fisher Trust.

Dr Aldridge was selected as Researcher of the Year for his work on inventing microencapsulated BioBullets - a method of controlling invasive mussels that block pipes in waterworks and power plants, costing the UK water industry alone more than £5m annually.

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