Morphomolecular identification of Polylabris lingaoensis infecting the twobar Seabream (Acanthopagrus bifasciatus).

A total of 40 fish samples of the twobar seabream, Acanthopagrus bifasciatus (Sparidae), were collected from January to June 2023 in the Arabian Gulf (Saudi Arabia). Fish samples were surveyed for ectoparasitic taxa. This study revealed the presence of one monogenetic species in A. bifasciatus gills. Parasitic prevalence and the mean value of intensity were 32.5% and 10, respectively. This parasite species has all the morphological characteristics of Polylabris, a genus erected by Euzet and Cauwet, 1967 (Microcotylidae), and mostly resembles Polylabris lingaoensis Tingbao, Kritsky, and Jun, 2007 on the presence of follicular testes ranging between 6-8, two rows of parallel microcotylid clamps in the haptoral structure with 35-45 per each, the conical form of the male copulatory organ, as well as the absence of genital armature. Molecular identification of this Polylabris species was conducted through sequencing of the partial nuclear large subunit (28S) of the ribosomal RNA (rRNA) and the mitochondrial gene of cytochrome c oxidase subunit I (COI). 28S rRNA genetic sequences were found to be conserved across several Polylabris spp., whereas, COI sequences were found to be more informative even with intraspecific variation. Therefore, the isolation of P. lingaoensis, provided herein, represents its first report on a marine sparid fish from Saudi Arabia. Furthermore, the reported sequences are also new genetic data for P. lingaoensis. Our findings will serve as the basis for any future study on monogeneans in Saudi waters.