Epizootiology of the parasitic dinoflagellate Hematodinium sp in the American blue crab Callinectes sapidus

Hematodinium sp. is a parasitic dinoflagellate that infects and kills blue crabs Callinectes sapidus. Periodic outbreaks of dinoflagellate infections with subsequent high host mortalities prompted a study of the epizootiology and distribution of the crab pathogen. Hemolymph samples from over 13 000 crabs were assessed for infections over 8 yr. Moderate to high prevalences were found at several locations along the Atlantic and Gulf coasts of the U nited States. In the coastal bays of Maryland and Virginia, prevalence foll owed a seasonal pattern, with a sharp peak in late autumn. Infections were significantly more prevalent in crabs measuring less than 30 mm carapace wi dth; host sex did not influence prevalence. Prevalences were highest in cra bs collected from salinities of 26 to 30 parts per thousand; no infected cr abs were found in salinities below 11 parts per thousand. Intensity of infe ction did not vary among crab sizes, molt stages, or sexes. Naturally and e xperimentally infected crabs died over 35 and 55 d in captivity, with a mea n time to death of approximately 13 and 42 d, respectively. Several other c rustaceans, including gammaridean amphipods, xanthid (mud) crabs, and the g reen crab Carcinus maenus, were found with Hematodinium-like infections. Co nsidering its widespread distribution and high pathogenicity, we suggest th at Hematodinium sp. represents a significant threat to blue crab population s in high salinity estuaries along the Atlantic and Gulf coasts of the USA.