Morphology and Histology of Major Organ Systems in the Killifish Esmaeilius sophiae (Teleostei: Aphaniidae).

Killifishes have emerged as valuable supplementary biological and biomedical research models due to their remarkable adaptations to extreme and variable environments. Despite their ecological and evolutionary significance, the histology and anatomy of freshwater members of the family Aphaniidae remain largely understudied. This study provides the first comprehensive microscopic analysis of Esmaeilius sophiae, a freshwater killifish endemic to Iran, using histological staining, light microscopy, scanning electron microscopy (SEM) and x-ray microscopy. Key organ systems-including gills, kidneys, liver, brain and reproductive organs-were examined to identify structural adaptations that support survival in dynamic freshwater habitats. The gills possess organised lamellae supported by pillar cells and a specialised mucosal epithelium, enhancing oxygen exchange in environments with fluctuating oxygen availability. SEM imaging revealed specialised glomerular structures in the kidney that likely facilitate efficient osmoregulation, as well as a distinctive scale morphology contributing to environmental protection. Additionally, the well-differentiated brain regions-particularly the optic tectum-and the unique otolith morphology of E. sophiae underscore its sensory and ecological specialisation. Our findings reveal key anatomical and histological adaptations in E. sophiae, establishing a foundational reference for future research in fish physiology, environmental adaptation and evolutionary biology.