Disseminated neoplasias in three species of bivalve mollusc are attributed to transmissible clonal lines, and neoplasias in one species are caused by cross-species transmission of cancer, suggesting that transmissible neoplasia is common in marine species. Transmissible neoplasia is rare, but it has been found to occur in the Tasmanian devil (a marsupial), dogs, and recently in soft-shell clams. Here Stephen Goff and colleagues identify three marine bivalve species — cockles, mussels and shell clams — that also exhibit disseminated neoplasia. In shell clams, the cancer lineages are transmitted from a closely related species. The findings suggest that transmissible neoplasia is more common than was previously thought, and that cross-species transmission of cancer can occur, albeit rarely. Most cancers arise from oncogenic changes in the genomes of somatic cells, and while the cells may migrate by metastasis, they remain within that single individual. Natural transmission of cancer cells from one individual to another has been observed in two distinct cases in mammals (Tasmanian devils1 and dogs2,3), but these are generally considered to be rare exceptions in nature. The discovery of transmissible cancer in soft-shell clams (Mya arenaria)4 suggested that this phenomenon might be more widespread. Here we analyse disseminated neoplasia in mussels (Mytilus trossulus), cockles (Cerastoderma edule), and golden carpet shell clams (Polititapes aureus) and find that neoplasias in all three species are attributable to independent transmissible cancer lineages. In mussels and cockles, the cancer lineages are derived from their respective host species; however, unexpectedly, cancer cells in P. aureus are all derived from Venerupis corrugata, a different species living in the same geographical area. No cases of disseminated neoplasia have thus far been found in V. corrugata from the same region. These findings show that transmission of cancer cells in the marine environment is common in multiple species, that it has originated many times, and that while most transmissible cancers are found spreading within the species of origin, cross-species transmission of cancer cells can occur.

The highest shellfishery catch in Galicia (NW Spain) has traditionally been cockle Cerastoderma edule. The shellfish bed located in Lombos do Ulla (Ría de Arousa) used to be among those with the highest cockle production; however, cockle mortality rate increased sharply in this bed in April 2012, reaching 100% in May 2012. Salinity and temperature were discounted as potential causes of the mortality. Marteiliosis, which was first detected in February 2012 and reached 100% prevalence in April 2012, was identified as the most probable cause. Marteiliosis had never been detected in Galician cockles, but extensive surveillance of the Galician coast in May to July 2012 detected marteiliosis in most cockle beds of the Ría de Arousa, whereas it was not found in other rías; 2 mo later, the cockle catch in the Ría de Arousa became negligible. Examination of the aetiological agent of marteiliosis with light and transmission electron microscopy supported its assignation to the genus Marteilia; morphological features showed similarity, but not complete identity, with the recently described species M. cochillia Carrasco et al., 2013. Regarding its molecular characterisation, a consensus sequence of 4433 bp containing a partial sequence of the intergenic spacer region, the complete 18S rRNA gene and a partial sequence of the first internal transcribed spacer region was obtained. The obtained sequences were compared with those available for Marteilia spp. and other Paramyxida. Molecular data support that this parasite corresponds to the species M. cochillia, and a PCR assay was designed for its specific diagnosis. The association of huge cockle mortality with M. cochillia infection urges extreme caution to avoid spreading this disease.