Morphological and Molecular Evidence of Fish Parasite Gyrodactylus kobayashi and Argulus japonicus Isolated from Ornamental Fish (Carassius auratus)

Ectoparasites, including Gyrodactylus and Argulus, belong to the monogenean and crustacean groups and are essential pathogenic parasites infecting diverse freshwater fish species. High parasitic loads affect growth and immune parameters, and severe infections often cause high mortality in fish. The present study identified the emerging parasites, including Gyrodactylus kobayashi and Argulus japonicus, in infected Carassius auratus, using morphological characterization and molecular tools. The morphological results highlighted that the Gyrodactylus parasite had an opisthohaptor consisting of 16 marginal hooks, dorsal bar and hamulus point curve angle (HPCA). Moreover, Argulus contained the proboscis, antennule, compound eye, sucker, maxilla, antennae, thorax, and abdomen. Molecular characterization, including DNA extraction, 18S rRNA amplification, and Sanger sequencing, demonstrated that the parasite specimens (PP922389 and OQ660496) have higher sequence similarity with other reported Gyrodactylus and Argulus species. A prevalence study was investigated, and month-by-month analysis revealed that Gyrodactylus kobayashi infection was highest in July and December in male and female goldfish, respectively. Similarly, for both male and female goldfish, Argulus japonicus infection was higher in June and July. Furthermore, the sex-wise comparative study showed that the mean intensity and mean abundance rates of Gyrodactylus and Argulus were prone in the female fish. Taking together, the study highlights the prevalence of two economically essential parasite groups, Gyrodactylus and Argulus. Further studies are needed to develop suitable management strategies to control the infection of parasites in fish species.