Effect of host background and environmental conditions on disease susceptibility and mortality in the northern bay scallop, Argopecten irradians irradians

Since 2019, outbreaks of an apicomplexan parasite caused catastrophic mortalities of adult northern bay scallops (Argopecten irradians irradians) in New York, USA. Anecdotal observations suggested different levels of mortality between different scallop stocks and mortality outbreaks appeared to occur in years displaying positive temperature anomalies. In this study, field and laboratory experiments were designed to evaluate the effect of environmental conditions and host (scallop) background on disease susceptibility and mortality. Wild and aquacultured scallops originally sourced from the same area were deployed in lantern nets (suspended in the water column) or oyster culture bags (on bottom) at two different enzootic sites showing contrasting environmental conditions. Subsets of wild and aquacultured stocks were also used in laboratory experiments exposing scallops to different temperatures and dissolved oxygen levels, mimicking different environmental scenarios. Results showed higher mortality in aquacultured scallops compared to wild scallops, especially in the more stressful conditions. Laboratory experiments showed that the trajectories of disease development and mortality were significantly affected by initial disease levels and environmental factors. For instance, high temperature and low dissolved oxygen were shown to favor disease development when initial disease levels were low (wild scallops) but also appear to lead to an overall reduction in disease via culling of the most heavily infected scallops when initial disease levels and mortality during the experiment were high (aquacultured scallops). Altogether, these findings underline the impact of environmental conditions, in particular temperature, on host-parasite interactions and suggest the potential existence of more resilient scallop stocks.