The microspores of the parasitic dinoflagellate Hematodinium perezi infect crustacean hosts via waterborne transmission.

The parasitic dinoflagellate in Hematodinium genus is an important pathogen of economically important wild and cultured marine crustaceans worldwide. Numerous studies have been conducted to investigate its epidemics, life cycle history, and transmission route. Recent in-field and lab studies have validated the effectiveness of the waterborne transmission route and hypothesized that dinospores are the key life stage to infect the crustacean hosts via waterborne transmission. However, until now, there is still no direct evidence to establish the infectious roles of Hematodinium dinospores for crustaceans via waterborne transmission. In the present study, Hematodinium microspores naturally released from Helice tientsinensis were used to immerse healthy juvenile Portunus trituberculatus with different development stages (C2-C4 instars, C6-C7 instars) and adult Helice tientsinensis, respectively. Hematodinium presence and prevalence in the two crustacean species were detected by hemolymph smear assay, H&E staining histopathology, and PCR analysis. The results showed that Hematodinium microspores successfully infected the two susceptible crustacean hosts, causing an overall Hematodinium prevalence of 40.4 % in P. trituberculatus (C2-C4 instars), 27.3 % in P. trituberculatus (C6-C7 instars), and 36.7 % in adult Helice tientsinensis, based on PCR analysis. The major findings in this study first provided direct and solid evidences to demonstrate that Hematodinium microspores are the key life stage to infect susceptible crustacean hosts via waterborne transmission. The physiological molting stage of crustacean hosts was found to be not associated with the infectious process of this parasite, while the growth stage of hosts was likely an important factor affecting the invasive and infectious process of Hematodinium microspores during waterborne transmission. The present study first established the waterborne transmission life stage of Hematodinium parasites, which will provide important information for effective prevention and control of Hematodinium epidemics in importantly economic marine crustaceans in future.