Seasonal Histomorphological Study of the Gills of Common Carp (Cyprinus carpio) in the Tigris River, Iraq

Abstract

The present study aims to describe the morphological and histological characteristics of the gills in male Common Carp (Cyprinus carpio) across four seasons and to highlight the seasonal effects on gill structures under varying temperature conditions. For this purpose, 52 specimens were collected from a rearing aquarium in the Tigris River, within the Taji region of Baghdad, over the course of a year. The fish were sacrificed, and the specimens were fixed in a 10% neutral buffered formalin solution. Routine histological techniques were applied, and the samples were stained using Hematoxylin and Eosin (H&E), Periodic Acid-Schiff (PAS), and Van Gieson (VG) stains. Morphological analysis revealed that the gills of C. carpio consisted of four pairs of gill arches located inside the branchial chamber, equipped with gill rakers and secondary lamellae, all covered by the operculum. The epithelium of the primary lamellae comprised several cell types, including chloride cells (ionocytes), which were rarely observed in the secondary lamellae, as well as pavement cells, non-differentiated supporting cells, and primary epithelial cells. The secondary lamellar epithelium contained all these cell types except for non-differentiated cells and additionally featured pillar cells. Histologically, the gill raker epithelium was identified as a non-keratinized stratified squamous type containing goblet (mucous) cells and two types of taste buds: elongated superficial and spherical. The gill arches’ blood supply was provided by afferent branchial arteries (ABAs) and efferent branchial arteries (EBAs), while afferent filament arteries (AFAs) and efferent filament arteries (EFAs) supplied the primary lamellae. The secondary lamellae were vascularized by afferent and efferent lamellar arterioles. The results demonstrated significant differences (P < 0.05 and P < 0.01) in gill epithelium characteristics across the four seasons. Statistical analyses revealed that inter-lamellar distance varied significantly (P < 0.01) between seasons, which was attributed to temperature fluctuations. The study concluded that carp could adapt to a limited temperature range (23–30°C). However, when water temperatures dropped below the lower threshold (8°C) during winter, the carp’s ability to adapt diminished, leading to increased histological changes compared to other seasons.