A tumour affecting mussels in Chile and France is traced to a single Northern Hemisphere mussel of a separate species.

A contagious cancer affecting shellfish has crossed oceans — and species barriers — to threaten mussels in Europe and South America.

Transmissible tumours are rogue cell lineages that spread between individuals. Such tumours have been found in Tasmanian devils (Sarcophilus harrisii) and bivalves, including the bay mussel (Mytilus trossulus), which lives in the Northern Hemisphere.

Since 2014, a related species, the blue mussel (Mytilus edulis), has experienced mass die-offs in France. These events show signs of being caused by a transmissible tumour. A team led by Michael Metzger at the Pacific Northwest Research Institute in Seattle, Washington, sequenced DNA found in blue mussels from France and the Netherlands, and identified tumour cells with genetic markers characteristic of bay mussels.

The researchers also found that Chilean mussels (Mytilus chilensis), a species from Argentina and Chile, had tumours that were almost genetically identical to those afflicting the European blue mussels. The tumours originated in a single bay mussel and then spread to the South American and European mussel species — probably via international shipping vessels, the researchers suggest.