Sphaerospora fulvidraco n. sp. (Cnidaria, Myxosporea) is associated with ascites disease in farmed yellow catfish Tachysurus fulvidraco in China

Abstract

Yellow catfish, Tachysurus fulvidraco, is a highly popular freshwater fish in China. Endemic ascites disease has caused mass mortality and significant economic losses in this species. During a series of field investigations, we identified a suspected new causative species of Sphaerospora, based on its consistent association with ascites outbreaks. Infections were detected in the renal tissue of all catfish specimens exhibiting prominent abdominal distension, collected from multiple fish farms. Myxospores are subspherical to spherical, measuring 6.6 ± 0.4 (5.6–7.3) μm in length, 6.5 ± 0.4 (5.6–7.3) μm in width, and 7.2 ± 0.5 (6.2–7.9) μm in thickness. Two subspherical polar capsules are located anteriorly within myxospores and lie in a plane perpendicular to that of the suture line, measuring 3.3 ± 0.3 (2.7–3.9) μm in length and 2.8 ± 0.2 (2.3–3.2) μm in thickness. The obtained SSU rDNA sequence (1620 bp) showed low query coverage (<87 %) and high disparity (>3 %) when compared with available sequences in GenBank. Phylogenetic analysis placed the present species within a well-supported clade together with a Sphaerospora sp. (MG214665). These results suggest that this organism isolated from the kidney of yellow catfish, represents a novel species, herein designated as Sphaerospora fulvidraco n. sp. Histopathological examination revealed extensive tissue disintegration and prominent cellular necrosis in the kidney associated with S. fulvidraco infection, which is presumed to impair renal tubule and glomerulus function. By integrating morphological and molecular data, we describe this novel Sphaerospora species and discuss the relationship between its renal infection and the endemic ascites diseases in the farmed yellow catfish based on histopathological findings.