The northern bay scallop is vital to the American scallop industry, but since 2019, summer mortality events have caused a decline in their commercial landings in New York. Pathological studies linked these mortality events to an apicomplexan parasite named Bay Scallop Marosporida (BSM), with older scallops being more heavily infected than younger individuals. This raises the question of whether the increased infections are due to a longer exposure time to the parasite or a decline in immunity with age.
To explore this question, scallops from three age groups were collected from an embayment in eastern Long Island, NY, and their immune responses were analyzed in three experiments. In Fall 2023, juveniles (5 months old) and young adults (1 year old) were compared to measure their baseline immune parameters. In Winter 2024, young adults (1 year old) and older adults (2 years old) were exposed to a bacterial cocktail, and their immune responses were tracked over five days. In Summer 2024, young and old adults were assessed for baseline immunity. Immune metrics such as total hemocyte count, percentage of phagocytic cells, percentage of dead cells, and reactive oxygen species production (ROS) were measured in hemolymph samples, while BSM infection levels were assessed in scallop tissues. Some metrics were experiment-specific: percentage of aldehyde dehydrogenase (ALDH)-positive cells (Fall 2023, Winter 2024), bacterial counts (Winter 2024, Summer 2024), and lysosome contents (Summer 2024).
Results showed that young adult scallops had higher immune performances than juveniles and older adults, displayed as higher hemocyte counts, percentages of phagocytic and ALDH-positive cells, and ROS levels, along with lower bacterial counts. The only metric where young adults appear to have underperformed was in having a higher percentage of dead cells compared to older adults. Most results were statistically significant (P < 0.05) and even the non-significant results favored young adults.