First report and morpho-molecular analysis of Ryukyua circularis (Isopoda: Cymothoidae) in the European pilchard, Sardina pilchardus, from Egypt.

Sardine flesh is highly valued for its nutritional qualities; however, isopod parasitic infections can cause substantial economic losses in sardine production. Therefore, this study sought to investigate the infection rate and morphology of the isopod Ryukyua circularis in Elbeheira Governorate, Egypt. A total of 633 samples of European pilchard (Sardina pilchardus) specimens were purchased from local markets in Edko City, Elbeheira, Governorate, over 12 months. Isopod parasites were carefully removed from host fish specimens, with attachment sites and density recorded before immediate preservation in 70% ethanol for microscopical examination. Scanning electron microscopy (SEM) was used to identify the isopods. A PCR assay of the 28S ribosomal RNA gene was used for molecular characterization and phylogenetic analysis. The overall infection rate was 3% (19 infected out of 633). The infection rate was marginally elevated in female specimens (3.29%) compared to male specimens (2.91%). According to the season, the infection occurred only in colder seasons-winter (4.69%) and spring (5.26%). This reflects temperature-limited transmission or greater host aggregation in cooler months, supporting targeted seasonal surveillance for fishery management. Fish weighing <15 g had a higher infection rate (3.91%) than those weighing >15 g (1.34%), suggesting increased juvenile vulnerability, potentially guiding size-selective harvesting or monitoring to reduce parasite impacts on sardine stocks. The PCR amplified 443 bp of the target sequence of the 28S rDNA gene. BLAST searches revealed that the isopod sequence shared 77.88-79.91% identities with previously reported Ryukyua circularis isolates. Phylogenetic analysis positioned the present sequence within the Ryukyua circularis clade, along with sequences from India. This study presents the first report on the infection rate, SEM, and molecular characterization of Ryukyua circularis in sardines from Elbeheira Governorate, Egypt.