New data on the presence of <i>Sparicotyle chrysophrii</i> (Polyopisthocotyla: Microcotylidae) on the gills of <i>Sparus aurata</i> (Teleostei: Sparidae) in Tunisian coastal waters, with insights into the genetic structure and phylogenetic analysis

This study represents the first integrated epidemiological and molecular characterisation of the monogenean <i>Sparicotyle chrysophrii</i> in wild and farmed gilthead seabream <i>Sparus aurata</i> populations across seven Tunisian localities. Prevalence in the different localities ranged 37.5 % - 63.6 % in wild fish and 29.4 % - 90.0 % in farmed fish. Population genetic variation and phylogenetic relationships were inferred by incorporating novel and existing haplotype data from various Mediterranean regions, using the structural ribosomal RNA (rRNA) for the large subunit (28S) and the cytochrome oxidase subunit I (COI) gene. Phylogenetic analysis of the 28S rDNA sequences revealed a monophyletic group with a distinct clade closely related to <i>Microcotyle</i>. The haplotype network exhibited a star-like pattern, supporting recent demographic expansion across the Mediterranean Sea. This expansion was reflected in genetic diversity indices, which showed high haplotype diversity and low nucleotide diversity. Negative neutrality test values further suggested a recent population expansion. Phylogenetic analysis of COI sequences revealed no significant phylogenetic differentiation, with samples from various Mediterranean regions clustering into a monophyletic clade. The mismatch distribution for Tunisian samples displayed a unimodal pattern, confirming this demographic expansion. Genetic distances based on COI sequences revealed a 1.15 % divergence between Tunisian and Algerian samples, with the lowest pairwise FST value, suggesting a shared evolutionary history, as supported by the haplotype network. AMOVA analysis of Tunisian samples revealed that 92.77 % of the variance was attributed to individual differences, with a moderate FST value, indicating the absence of significant genetic structuring within populations. The presence of shared haplotypes confirmed the potential for pathogen transfer between wild and farmed fish populations. Overall, the COI data suggested that host biological characteristics play a significant role in shaping the genetic structure of <i>S. chrysophrii</i> populations, as evidenced by the lack of significant phylogenetic divergence across the Mediterranean Sea.