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The dinoflagellate Hematodinium perezi is a highly pathogenic endoparasite that infects blue crabs Callinectes sapidus and other decapods. Annual outbreaks of the parasite along the eastern seaboard of the USA indicate that transmission occurs at high rates; however, the timing and incidence of new infections are unknown. The aim of this study was to investigate the patterns and factors associated with the natural transmission of Hematodinium in juvenile crabs from a highly endemic site. A novel field experiment, using naïve crabs as sentinels, was implemented to measure transmission. Over 965 juvenile crabs (instars C3-C10) collected from a non-endemic site were deployed for set periods of 3, 7, 10, and 14 days at a highly endemic site. Prevalence was 5.1% after a 3-day deployment and 88% after a 10-day deployment. This corresponded to a daily incidence of new infections of 1.7 and 8.8%, in the population, respectively. New infections progressed very quickly; 50% of infected crabs developed heavy infections after only 14 days. The influence of host molting on the transmission of the parasite was investigated by tagging 467 crabs using cyanoacrylate glue and small plastic tags. After 7-, 10-, and 14-day deployments, sentinel crabs were assessed and infections were found not to be associated with host molting. Lastly, two storm events, with high rainfall, strong winds, and lower temperatures, disrupted the natural transmission of H. perezi to sentinel crabs, essentially halting transmission in the coastal system. These sentinel studies indicate that transmission can occur rapidly and at very high levels in endemic sites and that it can be quickly modulated by changes in environmental conditions following storm events. Infections by Hematodinium may increase host mortality and thus may be a threat to blue crab populations in endemic areas. [ABSTRACT FROM AUTHOR]

Hematodinium perezi (genotype III) is a parasitic dinoflagellate that infects blue crabs along the eastern seaboard and Gulf of Mexico, USA. In order to examine the intra-specific genetic variation of this parasite, eleven microsatellite markers from H. perezi (III) were amplified from 227 infected blue crabs collected during 2008-2009 from six sites in Virginia. Simultaneous infections with multiple genetic types in a single-host individual were common and observed in 42 % of the samples. The remaining 58 % of samples had a single allele per locus at all eight polymorphic loci suggesting that the life history stages of the parasite in the host hemolymph are likely haploid. The composition and distribution of multi-locus genotypes (MLG) from samples with infections of a single genetic type indicated high genotypic variation along the Delmarva Peninsula, Virginia, with no evidence of population structure. The lack of linkage disequilibrium combined with the large number of unique MLGs (84 %) is strong evidence for recombination in the life cycle, but the sexual stages remain undetermined. This is the first evidence of ploidy level, infections by multiple genetic types in an individual host animal, high levels of genotypic variation, and sexual reproduction for any species of Hematodinium. [ABSTRACT FROM AUTHOR]