Pathogen Identification and Histopathological Observation of Polylabri-sosis in Mottled Spinefoot, Siganus Fuscesce

Abstract

From October to November 2019, a disease characterized by the appearance of pale gills as the main symptom was recorded in mottled spinefoot (Siganus fuscescens) cultured in a pond at the Pilot Research Base for Marine Fish Seed Breeding in Fujian Province, China. The aim of this study was to investigate the pathogens that caused this disease in spinefoot through clinical, anatomical, histopathological, and morphological observations, as well as 28S rDNA sequence analysis. External clinical signs included slow and impaired swimming, along with intermittent quick swimming, and subsequent death. The gill filaments of the diseased fish were whitish, and nu-merous worms were observed on the gill filaments. Gross signs of anemia were observed in the gills, liver, heart, spleen, kidneys, and muscles. Histopathological analysis found that very few red blood cells were found in the gills, liver, heart, spleen, kidney, and other tissues of the diseased fish. The worms isolated from the diseased fish were nearly spindle-shaped and lanceolate, with a highly specialized haptor of two parallel rows, each comprised of 36–46 microcotylid clamps, and the copulatory organ was a conical chitin tube structure with an average height of 0.055 mm (range: 0.045 to 0.062) and an average base diameter of 0.032 (range: 0.026 to 0.034). The results of the 28S rDNA (C1-D2 region) sequencing analysis showed a high similarity (100% identity) to other available Polylabris cf. mamaevi sequences in the GenBank database. Based on the morphological characteristics and ge-nomic DNA sequencing, the worms were identified as P. cf. mamaevi, a parasite that causes white-gill disease in spinefoot. Our results provided a reference for the diagnosis, prevention, and control of diseases in cultured mot-tled spinefoot.