Tissue Distribution and Abundance of the Parasitic Dinoflagellate Hematodinium perezi in Naturally Infected Portunus trituberculatus

The parasitic dinoflagellate <i>Hematodinium</i> is an infectious pathogen that causes severe enzootic in numerous economically important marine crustaceans worldwide. Previous research has focused on investigating the identification and life stages of <i>Hematodinium</i> parasites, while the parasite abundance and tissue proliferation process of <i>Hematodinium</i> in naturally infected crustacean hosts need to be further studied. In the present study, the tissue tropisms and intensity of <i>H. perezi</i> were investigated in the naturally infected Chinese swimming crabs <i>Portunus trituberculatus</i> by both the qualitative (hemolymph assay, histology) and quantitative analysis (cell count, quantitative PCR). The results showed that in <i>P. trituberculatus</i> with infection level I (4 ± 2 parasites in 200× microscopic field), filamentous trophonts were observed in the hemolymph and stomach tissues, with the average parasite number and ITS 1 copy number of <i>H. perezi</i> quantitatively detected in hemolymph (1.0 × 10² parasites/mL) and stomach tissues (1.7 × 10³ cells/g), respectively. <i>H. perezi</i> trophonts were observed in the hemolymph (4.3 × 10⁴ parasites/mL) and exhibited broad distribution in multiple tissues with its highest abundance of <i>H. perezi</i> in pereiopod muscles (1.1 × 10⁴ cells/g) followed by that in stomach (4.8 × 10³ cells/g) in <i>P. trituberculatus</i> with infection level II (80 ± 10 parasites in 200× microscopic field). In <i>P. trituberculatus</i> with infection level III (200 ± 35 parasites in 200× microscopic field), a high abundance of <i>H. perezi</i> sporoblasts was found in the hemolymph (3.1 × 10⁷ parasites/mL) and all of the other examined tissues, with its highest abundance detected in pereiopod muscles (3.5 × 10⁴ cells/g). In addition, the number of host's hemocytes was significantly decreased during the <i>Hematodinium</i> infection. This study provides a comprehensive quantitative characterization of the tissue distribution and abundance of <i>H. perezi</i> in its natural crab host which will contribute to better understanding of the crustacean host-<i>Hematodinium</i> interactions.