Two new species of Myxodavisia (Myxosporea: Sinuolineidae) from the gallbladder of two marine fishes from the South China Sea.

Myxodavisia species are myxosporean parasites that primarily infect the urinary system or gallbladder of marine fishes, with only 32 species described previously. During an investigation of myxosporean species diversity in China, two new Myxodavisia species, Myxodavisia nehereus n. sp. and Myxodavisia indicus n. sp., infecting the gallbladders of Harpadon nehereus and Platycephalus indicus, respectively were found. Mature myxospores of M. nehereus n. sp. were hemispherical, measuring 9.3 ± 0.6 (8.2-10.4) μm in length, 12.9 ± 0.5 (12.0-13.8) μm in width, and 8.5 ± 0.3 (8.2-8.7) μm in thickness, with lateral appendages of 20.5-34.3 μm long. Two polar capsules of 3.6 ± 0.2 (3.1-3.9) μm in diameter were spherical and possessed coiled 2-3 turns polar filaments. Mature myxospores of M. indicus n. sp. were flattened ellipsoidal, measuring 5.6 ± 0.4 (5.1-6.5) μm long, 9.0 ± 0.5 (8.0-9.6) μm wide, and 4.5 ± 0.3 (4.0-5.1) μm thick, with lateral appendages of 16.0-30.0 μm long. Polar capsules of 3.0 ± 0.3 (2.3-3.5) μm in diameter were spherical and possessed 1-2 coils polar filaments. These two new species can be easily morphologically distinguished from each other, as well as those Myxodavisia species reported previously. Phylogenetic analysis indicated that these two new Myxodavisia species did not cluster with each other and other Myxodavisia species described previously, but dispersed within different Ceratomyxa species clades. This finding demonstrated that the genus of Myxodavisia was not monophyletic and showed a close relationship with Ceratomyxa spp. This study expands the known diversity of marine myxosporeans and underscores the importance of integrating morphological, molecular, and ecological data for resolving the taxonomy and evolutionary relationships within this group.