Unraveling the biology of Tetrancistrum polymorphum (Paperna, 1972) Kritsky, Galli & Yang, 2007 (Monogenoidea: Dactylogyridae) parasitizing the dusky spinefoot, Siganus luridus (Rüppell): morphological, ultrastructural, phylogenetic, and histopathological profiling.

An integrative reassessment of Tetrancistrum polymorphum (Paperna, 1972) Kritsky, Galli & Yang, 2007 (Monogenoidea: Dactylogyridae) was conducted based on specimens collected from the gills of Siganus luridus (Rüppell) (Acanthuriformes: Siganidae) from the Egyptian Red Sea. Although this species has been previously reported and morphologically described using light microscopy, its ultrastructural features, molecular identity, and pathological impact on the host have remained unexplored. In the present study, detailed morphological and morphometric analyses were performed using light microscopy to confirm species identification. Scanning electron microscopy (SEM) provided complementary information on the external morphology, allowing improved visualization of surface topography of the tegument, copulatory complex, and haptor. SEM observations revealed longitudinal tegumental folds, small dome-shaped papillary structures, subtle cephalic elevations, and the external configuration of haptoral structures, with anchors visible at the surface while bars remained embedded within deeper tissues. Molecular characterization based on partial 28S rDNA sequences provided crucial genetic data for this species and supported its phylogenetic placement within Dactylogyridae. Histopathological examination of infected gills demonstrated that T. polymorphum induces epithelial necrosis, lamellar fusion, vascular congestion, and inflammatory cell infiltration, leading to marked disruption of normal gill architecture. By integrating morphological, ultrastructural, molecular, and histopathological data, this study moves beyond traditional redescription and provides an integrative reassessment of T. polymorphum based on these complementary approaches, contributing to a better understanding of its taxonomy, phylogenetic relationships, and host-parasite interactions.