The myxozoan genus Davisia Laird 1953 is preoccupied by Davisia Del Guercio 1909 (Insecta: Hemiptera). Here, Myxodavisia nomen novum is proposed to replace the preoccupied name, and a new species is described. Myxodavisia sebastisca n. sp. was found in the urinary bladder of Sebastiscus marmoratus, collected from coastal waters off Xiamen in the East China Sea. The parasite is characterized by a disporous trophozoite; spherical to subspherical spore, 13.1 ± 0.3 (12.7–13.6) by 12.3 ± 0.9 (10.9–13.5) μm in size; curved sutural line; 2 shell valves each with a long lateral appendage 119.4–335.2 μm in length; and 2 spherical or subspherical polar capsules, equal in size, 4.6 ± 0.6 (3.2–4.6) μm in diameter. Traditionally, Myxodavisia is distinguished from Ceratomyxa, Sinuolinea, and Sphaerospora spp. by having spores that possess a distinct central chamber and lateral appendages. A review of the literature reveals that the presence or absence of a clear septum between these spore components is open to interpretation. Indeed, in immature spores of M. sebastisca n. sp., there was an indication of a demarcated appendage, but in some mature spores, no clear separation was apparent. Our findings suggest that future revision of this genus is warranted, particularly once DNA sequence data become available.