Seasonal patterns of Hematodinium perezi differ between blue crab host and environmental DNA in a South Carolina estuary.

The parasitic dinoflagellate Hematodinium perezi is known to harm and kill marine crustaceans worldwide. It is unclear, however, how the phenology of host infections differs from that of in-water life stages of H. perezi, and how these cycles may vary regionally. Across the US East Coast, including in South Carolina (SC), H. perezi is a parasite of the economically important Atlantic blue crab Callinectes sapidus. The aim of this study was to determine and compare the seasonal presence of H. perezi in C. sapidus and benthic water of the Charleston Harbor estuary, SC, USA, using real-time PCR (RT-PCR). Hemolymph of 322 blue crabs was sampled over 21 mo beginning in September 2022, with concurrent benthic water eDNA sampling beginning in April 2023. DNA from H. perezi was detected in a total of 11 blue crab samples collected in the spring, fall, and winter of 2023. Positive environmental detections of DNA occurred every sampling season, including when H. perezi was reported absent in C. sapidus. Although other unknown factors likely contributed to the observed prevalence, our results suggest a seasonal pattern of the presence of H. perezi in the estuary largely driven by both water temperature and salinity. Altogether, our findings provide foundational information on the phenology of this parasite, critical for understanding disease dynamics in blue crabs in the southeast USA.