In this study, Scaphanocephalus parasite (Platyhelminthes: Heterophyidae) metacercariae were found in Siganus argenteus (forktail rabbitfish or streamlined spinefoot) in the Arabian Gulf of Jubail province, Saudi Arabia. The findings may constitute new host and locality records for this parasite. Based on the number of black spots containing parasite cysts per fish, our study indicated that Siganus argenteus had high infection intensities of encysted metacercariae belonging to the genus Scaphanocephalus. Of the 3500 S. argenteus specimens examined, 800 (22.9%) showed multiple black cysts over the entire body surface, including the membranous parts of fins, while none were seen on the internal organs. The prevalence of infection was highest in summer (June-August) (8.8%). The excysted metacercariae differed morphologically from previously identified Scaphanocephalus spp. Molecular analysis of rDNA showed 100% identity with an unnamed Scaphanocephalus sp. reported in Caribbean fish. Therefore, our findings may indicate a new species of this previously rarely recorded fish parasite. The histopathological examination revealed that the encysted parasites were restricted to the dermal layer of the skin and surrounded by melanophores and a fibrous connective tissue capsule, with focal myositis and Zenker's necrosis in the underlying muscle tissue. The characteristically 'winged' parasite was clearly observed within the cysts. The high prevalence of Scaphanocephalus infection in siganid fish we detected requires further epidemiological study.
Keywords: Jubail; Saudi Arabia; Siganus argenteus · Scaphanocephalus · Pathology.