Morphomolecular identification of <i>Polylabris lingaoensis</i> Tingbao, Kritsky, and Jun, 2007 infecting the twobar Seabream ( <i>Acanthopagrus bifasciatus</i> )

A total of 40 fish samples of the twobar seabream, <i>Acanthopagrus bifasciatus</i> (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 <i>A. bifasciatus</i> gills. Parasitic prevalence and the mean value of intensity were 32.5% and 10, respectively. This parasite species has all the morphological characteristics of <i>Polylabris</i>, a genus erected by Euzet and Cauwet, 1967 (Microcotylidae), and mostly resembles <i>Polylabris lingaoensis</i> 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 <i>Polylabris</i> species was conducted through sequencing of the partial nuclear large subunit (<i>28S</i>) of the ribosomal RNA (<i>rRNA</i>) and the mitochondrial gene of cytochrome c oxidase subunit I (<i>COI</i>). <i>28S rRNA</i> genetic sequences were found to be conserved across several <i>Polylabris</i> spp., whereas, <i>COI</i> sequences were found to be more informative even with intraspecific variation. Therefore, the isolation of <i>P. lingaoensis</i>, provided herein, represents its first report on a marine sparid fish from Saudi Arabia. Furthermore, the reported sequences are also new genetic data for <i>P. lingaoensis</i>. Our findings will serve as the basis for any future study on monogeneans in Saudi waters.