Contagious cancers can jump between sea clam species, study finds


A contagious blood cancer has passed from one species of clam to another and spread among clams living in the Atlantic Ocean and the Mediterranean Sea, shows a study published today in eLife.

The results add to the evidence that cancers can spread among different species of bivalve molluscs and suggest that human activities may inadvertently contribute to the spread of these cancers to new places and species.

Contagious cancers have been identified in dogs, Tasmanian devils and bivalves such as clams and mussels. These diseases usually spread between individuals of the same species. But previous studies have documented at least two cases of contagious cancers spreading among bivalve species.

We set out to confirm whether a leukemia-like blood cancer found in some bivalves also infects warty venus, otherwise known as the warty clams found in the seas of southern Europe.”

Daniel García-Souto, postdoctoral researcher in genetics, University of Santiago de Compostela – USC, Galicia, Spain

Daniel García-Souto is co-first author of the study alongside Alicia Bruzos and Seila Diaz at USC.

The researchers collected 345 warty clams from coastal areas of Spain, Portugal, France, Ireland and Croatia. They found a type of blood cancer called hemic neoplasia in warty clams collected from two different coastal regions of Spain. One group of infected clams was found along the country’s Atlantic coast, while the other group was found more than 1,000 nautical miles away in the Mediterranean Sea.

The team used a technique called whole genome sequencing to reveal that the cancer originated in a single clam, then became infectious and spread among the warty clams. The cancer contained genetic sequences from both the warty clam and another unknown species of clam. By comparing the unknown genetic sequence to a genetic database of bivalve species, the researchers were able to identify the mystery clam as Chamelea gallina, or the striped Venus clam.

Further DNA tests taken from the mitochondria and cell nucleus of the two species of clams confirmed that the cancer had progressed from the striped clam to the warty clam.

“The genetic similarity of cancer cells found in warty clams from the Atlantic Ocean and the Mediterranean Sea suggests that human shipping activities may have transported cancer from one region to another,” says co-first author Alicia Bruzos, who was a researcher. A PhD candidate at USC at the time of the study, he is now at the Francis Crick Institute in London, UK. This idea is supported by a previous study in eLife* which showed that mussels carried contagious cancer across the Atlantic by hitchhiking on ships.

The team now hope to conduct further studies to determine the age of the tumors in their clam specimens and to explore how long cancer may have spread among these species.

“Our work confirms that contagious cancers can cross between clam species,” concludes lead author José Tubío, a genomes and disease researcher at USC. “As this can pose a potential threat to marine ecology, we must continue to study and monitor pathogens, including cancers, to help protect these species.”


Journal reference:

Garcia-Souto, D., et al. (2022) Mitochondrial genome sequencing of marine leukemias reveals cancer contagion between clam species in southern European seas. eLife.


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