Morphological and molecular characterization of Myxobolus aculeatus n. sp. (Myxozoa: Myxosporea) from the ovary of Macrognathus aculeatus, Bloch, 1786 (Synbranchiformes: Mastacembelidae) in China.

Macrognathus aculeatus is a commercially significant cultured fish in China. However, prior to this study, there had been minimal attention directed towards its myxosporean infection. The present study has identified a previously unrecorded species of Myxobolus found in the ovary of M. aculeatus. The mature spores were elliptical in the frontal view, and fusiform in the sutural view, measuring 13.0 ± 0.2 (11.5-14) μm in length, 6.4 ± 0.3 (6.0-7.2) μm in width, and 5.3 ± 0.2 (4.9-5.6) μm in thickness. The two equal polar capsules were pyriform, with a length of 4.5 ± 0.2 (3.5-4.7) μm and a width of 2.4 ± 0.1 (2.3-2.6) μm, and contained 5-6 polar filament coils. A rigorous morphological comparison revealed that the current species exhibited notable distinctions from the previously documented congeners. Histopathological analysis indicated that the parasite resided between oocytes, yet no discernible host inflammatory response was observed. A comparison of the SSU rDNA gene sequences revealed that the present species exhibited the highest degree of similarity to Myxobolus parasites found in the gills of Tachysurus fulvidraco, with a sequence similarity of 86.54 %. Further phylogenetic analysis demonstrated that the current species exhibited a clustering pattern with several myxobolids infecting Tachysurus (Siluriformes), yet displayed a distinct positioning. The Myxobolus species in question represents a new scientific discovery and has been designated as Myxobolus aculeatus n. sp. on the basis of an integrated analysis of morphological, histopathological and molecular data.