The biosecurity of bivalve seed transfers during different life stages with respect to their pathogens

The expansion of molluscan aquaculture in the United States has induced a rising demand for molluscan seed to sustain it. Out-of-state transfers of seed from hatcheries and nurseries are considered a pathogen risk to both aquacultured and wild animals. Another obstacle to the maintenance of the industry is the small number of laboratories able to conduct molluscan health certification. We hypothesized that larger animals would have higher pathogen prevalences, as they proportionally filter more water. To investigate this, we analyzed health data of three pathogens: Perkinsus spp. (dermo disease) in eastern oysters ( Crassostrea virginica ), Mucochytrium quahogii (QPX disease) in ( Mercenaria mercenaria ), and Haplosporidium nelsoni (MSX disease) in both oysters and clams. Analyses included 1309 samples containing over 75,000 animals, which were categorized by life stage (gametes, larvae, seed, juveniles and adults), as well as the type of water treatment in which animals were reared. We found higher dermo prevalences (> 50 %) only in larger oysters (> 40 mm) that were reared in raw water. Hard clams did not present high prevalences of either dermo or QPX in any life stage, nor did MSX in oysters. There was a marked difference in prevalences between raw and treated water for all three pathogens, highlighting the importance of biosafety practices to ensure both product quality and reliable hatchery operations. • Low prevalences of MSX in oysters and QPX in clams found in all life stages, regardless of water treatment. • Accentuated dermo prevalences in larger oysters (>40 mm) that were reared in raw water. • No pathogens found in oyster larvae samples. • Water treatment is recommended to improve biosafety.