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Marine resources provide a growing world population with the opportunity to increase food security, particularly through aquaculture. Oysters are prominent aquacultured species, but marine diseases are a threat to the growth and sustainability of oyster culture. Perkinsus marinus is the most important oyster pathogen of the Atlantic and Gulf coasts of the U.S., where it causes substantial mortality. Despite being one of the most well-studied protozoan parasites of bivalve molluscs, much is still unknown about P. marinus. This research investigated multiple characteristics of P. marinus.In Chapter Two, the environmental ecology of P. marinus was explored through tidal sampling and year-long seasonal sampling of eDNA in both the water column and the benthos. Additionally, visualization of cell forms was undertaken. Perkinsus marinus was detectable across the tidal cycle, with levels generally highest at low tide. Notably, environmental abundance of P. marinus was highly variable on short time scales. Seasonally, P. marinus abundance generally increased with temperature, as expected, but it also exhibited marked variability. While P. marinus abundance fell below the level of detection in surface water in late winter, it was detectable year-round in the sediment, indicating that the benthos may be an underappreciated reservoir for P. marinus transmission in marine systems. Visualized life history stages from environmental samples were mainly P. marinus hypnospores. In Chapter Three, shifting baselines in P. marinus histological presentation were examined across a 21-year time period using environmental parameters and generalized mixed-effects models. The results indicated a phenotypic shift to smaller cell sizes had occurred during 1992 to 2012. The change was not fully explained by environmental influences in the models. This shift in size has also been associated with an increase in weighted prevalence in the late 1990s/early 2000s, hinting at a relationship between decreased cell size and increased severity. In Chapter Four, two unusual histopathological presentations of putative P. marinus infections, which could challenge diagnosticians, were characterized. Aberrant infections were analyzed in oyster histology samples from 2011 to 2021. One aberrant presentation was associated with the gonad while the other was within oocytes. Both types of infections were confirmed to be P. marinus through molecular work. Additionally, aberrant gonadal infections were found to be geographically widespread and mainly infecting males with cell characteristics that deviated from contemporary P. marinus. Oysters with infected oocytes were rarer but did span several years. Overall, the research accomplished here advanced understanding of multiple aspects of a significant marine pathogen. Insights into the seasonal environmental ecology of the parasite were uncovered, a past phenotypic change was investigated using statistical models, and unusual putative P. marinus infections were molecularly confirmed and described for this protozoan. These advances help support management of the oyster aquaculture industry and highlight the usefulness of monitoring programs and archival material.

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