Broodstock genetics, more than ploidy alone, explains oyster resilience at farm sites impacted by sudden unusual mortality syndrome

Oyster (Crassostrea virginica) aquaculture expansion has been met with frequent mortality, particularly in the southeastern United States during warm months from the spring through early fall. This phenomenon has come to be known as Sudden Unusual Mortality Syndrome (SUMS), and observations have implicated triploid oysters as particularly susceptible to SUMS. To better understand the relative contribution of broodstock genetics and ploidy on the susceptibility of oysters to both SUMS-associated pathology and mortality, we produced diploid and triploid oysters from North Carolina and Virginia broodstock and deployed these oysters to seven field sites across North Carolina and Virginia. Dramatic mortality across commercial oyster farms in North Carolina was reported while this field deployment was underway, with farmer-reported mortalities exceeding 90 % of near market-sized oysters. We observed oyster mortality in line with these grower-reported observations, particularly in Virginia diploids deployed to sites in North Carolina coastal sounds. We found the effects of ploidy to depend on the genetic background of deployed oysters, with North Carolina triploid lines experiencing greater mortality than North Carolina diploids, but with Virginia triploids experiencing less mortality than Virginia diploids. Impacted oysters presented with characteristic digestive diverticula pathology that has been consistently associated with SUMS in North Carolina. These results provide unequivocal evidence that pathology and mortality attributed to triploid oysters must be evaluated within the broader context of the genetic background deployed oysters, rather than ploidy alone. Expanded research into the growing areas impacted by SUMS and the commercial oyster stocks widely grown in these areas should be a foremost priority for future research, as is ongoing understanding of variation in SUMS resistance across commercially available oyster lines.