A contagious, leukemia-like cancer is spreading among several species of cockles, clams, and mussels. It’s known as disseminated neoplasia, and according to a new Nature study, it appears to be directly transmitted between different species of marine bivalves – likely through the water.
Most cancers arise from changes and mutations that accumulate in the genomes of cells over time. And while cancer cells can migrate, or metastasize, they usually remain in that one individual. Until recently, we only knew of two examples of transmissible cancer in the wild: a venereal tumor that affects dogs and the Tasmanian devil facial tumor disease. Last year, researchers studying soft-shell clams (Mya arenaria) along the US east coast found that cancerous tumor cells are being passed from one clam to another. That discovery indicated that transmissible cancers could be far more widespread than we think.
To see if cancers in other bivalves are caused by independent cells, a team led by Columbia University’s Stephen Goff collected mussels (Mytilus trossulus, pictured above), cockles (Cerastoderma edule, below left), and golden carpet shell clams (Polititapes aureus, below right) from several locations in British Columbia, Canada, and the Galician Coast in Spain. The team screened the specimens for neoplasia after taking samples of haemolymph, their blood-like vital fluid, and they conducted genetic analyses of the host and cancer tissues.
They found that, not only is the cancer spreading among individuals of the same species, it’s also being passed between different shellfish species. Neoplasias in all three species were caused by clones of transmissible cancer cells that were genetically distinct from their hosts. But while the cancer lineages in mussels and cockles were derived from their respective species, the cancer cells in the golden carpet shell clams showed no genetic match to the species: The contagious cells originated in a different clam species living in the same area called Venerupis corrugata. The cancer had undergone cross-species transmission.
It remains unclear exactly how the cancer spreads. But because these filter-feeding invertebrates don’t really move, the cancer cells are likely floating in the seawater.
Cockles (left) and golden carpet shell clams (right) collected in the ria of Arousa in Galicia, Spain. David Iglesias
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