Sydney A. Weinandt

Bivalve transmissible neoplasias (BTNs) are transmissible cancers that have arisen in multiple bivalve species, including Eastern soft-shell clam ( Mya arenaria ) populations on the East Coast of North America. Two sublineages of a single clone (termed MarBTN) currently circulate at low enzootic levels (1 to 5%) in New England, the United States, and Prince Edward Island, Canada, but MarBTN has not previously been observed in soft-shell clams on the West Coast. In 2022, we collected soft-shell clams in Puget Sound, Washington, and unexpectedly found MarBTN in two sites. Prevalence increased in subsequent years, surpassing 75% at both sites in 2024, while remaining undetectable in other populations, suggesting the early stages of a severe disease outbreak. Based on the presence of somatic transposon insertion sites found only in the USA-sublineage, the cancer was likely recently transplanted from New England. We also found that soft-shell clams in multiple Puget Sound locations are hybridizing populations of M. arenaria and Mya japonica . M. japonica may have decreased susceptibility to this cancer, though further sampling will be needed to confirm this hypothesis. We further developed a sensitive environmental DNA (eDNA) assay, targeting somatic mutations in the MarBTN mitogenome. Using this assay, we surveyed 51 sites throughout Puget Sound, detecting cancer at high levels at sites where MarBTN-positive clams were observed and in the surrounding area. These results identify a severe outbreak of transmissible cancer in a hybridizing population, due to transfer of disease from another ocean, and demonstrate the utility of eDNA methods to track BTN through the environment.

Bivalve transmissible neoplasias (BTNs) are leukemia-like cancers found in at least 10 bivalve species, in which the cancer cells themselves transfer from one individual to another, spreading as an unusual form of infectious disease. Before the infectious etiology was known, there were reports of lethality and outbreaks of cancer in the soft-shell clam ( Mya arenaria ) on the east coast of North America. Using sensitive and specific qPCR assays, we followed the progression of BTN in naturally-infected soft-shell clams from Maine, USA. We observed variable outcomes, with about half of clams (9/21) progressing to high levels of cancer and death, about half exhibiting long-term non-progression (11/21), and a single animal showing regression of cancer. We also observe a significant decrease in survival in animals that progress to >10% cancer in their hemolymph, while we see no effect on survival in clams with BTN that are long-term non-progressors. As most bivalves do not physically contact each other, and BTN cells can survive in seawater, it has been proposed that BTN is spread through release of cancer cells into the water. We used qPCR to detect BTN-specific DNA in environmental DNA (eDNA) in the tanks of animals throughout this experiment. We show that cancer cell release can be detected in tank water of most clams with >24% cancer in their hemolymph, but not below this level. Cancer cell release is variable and occurs in bursts, but above 24% detection in eDNA correlates with progression of cancer in the hemolymph. This study demonstrates both the lethality of BTN and the presence of a block to the progression of BTN in a large portion of clams in a population with enzootic disease. This also further supports the hypothesis that BTN cells transmit through seawater and provides insights into the mechanisms of the transmission dynamics.

Transmissible cancers are clonal lineages of neoplastic cells able to infect multiple hosts, spreading through populations in the environment as an infectious disease. Transmissible cancers have been identified in Tasmanian devils, dogs, and bivalves. Several lineages of bivalve transmissible neoplasias (BTN) have been identified in multiple bivalve species. In 2019 in Puget Sound, Washington, USA, disseminated neoplasia was observed in basket cockles ( Clinocardium nuttallii ), a species that is important to the culture and diet of the Suquamish Tribe as well as other tribes with traditional access to the species. To test whether disseminated neoplasia in cockles is a previously unknown lineage of BTN, a nuclear locus was amplified from cockles from Agate Pass, Washington, and sequences revealed evidence of transmissible cancer in several individuals. We used a combination of cytology and quantitative PCR to screen collections of cockles from 11 locations in Puget Sound and along the Washington coastline to identify the extent of contagious cancer spread in this species. Two BTN lineages were identified in these cockles, with one of those lineages (CnuBTN1) being the most prevalent and geographically widespread. Within the CnuBTN1 lineage, multiple nuclear loci support the conclusion that all cancer samples form a single clonal lineage. However, the mitochondrial alleles in each cockle with CnuBTN1 are different from each other, suggesting mitochondrial genomes of this cancer have been replaced multiple times during its evolution, through horizontal transmission. The identification and analysis of these BTNs are critical for broodstock selection, management practices, and repopulation of declining cockle populations, which will enable continued cultural connection and dietary use of the cockles by Coast Salish Tribes.

Bivalve transmissible neoplasia (BTN) is one of three known types of naturally transmissible cancer— cancers in which the whole cancer cells move from individual to individual, spreading through natural populations. BTN is a lethal leukemia-like cancer that has been observed throughout soft-shell clam (Mya arenaria) populations on the east coast of North America, with two distinct sublineages circulating at low enzootic levels in New England, USA, and Prince Edward Island, Canada. Major cancer outbreaks likely due toMya arenariaBTN (MarBTN) were reported in 1980s and the 2000s and the disease has been observed since the 1970s, but it has not been observed in populations of this clam species on the US west coast. In 2022, we collected soft-shell clams from several sites in Puget Sound, Washington, USA, and unexpectedly found high prevalence of BTN in two sites (Triangle Cove on Camano Island and near Stanwood in South Skagit Bay). Prevalence of BTN increased in subsequent years, reaching >75% in both sites in 2024, while it was not observed in other sites, suggesting the early stages of a severe disease outbreak following recent introduction. We observed that these cancer cells contain several somatic transposing insertion sites found only in the USA-sublineage of MarBTN, showing that it likely was recently transplanted from New England to this location. We then developed a sensitive environmental DNA (eDNA) assay, using qPCR to target somatic mutations in the MarBTN mitogenome, and showed that MarBTN can be detected in seawater at Triangle Cove, as well as several kilometers outside of the cove. We then used this assay to survey 50 sites throughout Puget Sound, confirming that the disease can be detected at high levels at Triangle Cove and South Skagit Bay, and showing that it extends beyond these known sites. However, while normal soft-shell clam mtDNA was widely detected, MarBTN was undetectable throughout most of Puget Sound and currently remains limited to the South Skagit Bay area and north Port Susan. These results identify a previously unknown severe outbreak of a transmissible cancer due to long-distance transplantation of disease from another ocean, and they demonstrate the utility of eDNA methods to track the spread of BTN through the environment.

Transmissible cancers are clonal lineages of neoplastic cells able to infect multiple hosts, spreading through populations in the environment as an infectious disease. Transmissible cancers have been identified in Tasmanian devils, dogs, and bivalves. Several lineages of bivalve transmissible neoplasias (BTN) have been identified in multiple bivalve species. In 2019 in Puget Sound, Washington, USA, disseminated neoplasia was observed in basket cockles ( Clinocardium nuttallii ), a species that is important to the culture and diet of the Suquamish Tribe as well as other tribes with traditional access to the species. To test whether disseminated neoplasia in cockles is a previously unknown lineage of BTN, a nuclear locus was amplified from cockles from Agate Pass, Washington, and sequences revealed evidence of transmissible cancer in several individuals. We used a combination of cytology and quantitative PCR to screen collections of cockles from eleven locations in Puget Sound and along the Washington coastline to identify the extent of contagious cancer spread in this species. Two BTN lineages were identified in these cockles, with one of those lineages (CnuBTN1) being the most prevalent and geographically widespread. Within the CnuBTN1 lineage, multiple nuclear loci support the conclusion that all cancer samples form a single clonal lineage. However, the mitochondrial alleles in each cockle with CnuBTN1 are different from each other, suggesting mitochondrial genomes of this cancer have been replaced multiple times during its evolution, through horizontal transmission. The identification and analysis of these BTNs are critical for broodstock selection, management practices, and repopulation of declining cockle populations, which will enable continued cultural connection and dietary use of the cockles by Coast Salish Tribes.