Features of the morphology of the respiratory organs of Clarias gariepi-nus (Burchell, 1822) of the catfish family

Fish farming plays a significant role in ensuring the global food supply. For achieving maximum yields in the global production of edible fish products, aquaculture has become the most dynamically developing sector. Within this context, the cultivation of Clarias catfish (Clarias gariepinus) holds particular importance due to its biological advantages in terms of growth and development when compared to other artificially cultured fish species under aquaculture conditions. This article presents findings on the morphological structure of the respiratory organs (gills and lungs) of the Clarias catfish, obtained using macroscopic, morphometric, and statistical research methods. Observations revealed distinctive features in the structure of respiratory organs in air-breathing (bimodal) fish, of which the Clarias catfish is a representative. It was shown that during the phylogenetic development of air-breathing fish, modifications occur in their gas exchange and circulatory systems–specifically, the development of a double circulatory loop–like that observed in amphibians. In these animals, paired lungs form in addition to gills, enabling them to breathe atmospheric air. Thus, in the evolutionary development of the respiratory organs of air-breathing fish such as the Clarias catfish, structural adaptations are observed compared to fish that rely solely on gill respiration. These changes ensure efficient gas exchange between the animal’s body and the surrounding environment. Such adaptations are characterized by the improvement of respiratory structures typical of amphibians, in which paired lungs form alongside gills, allowing the organism to utilize atmospheric oxygen. In unfavorable aquatic conditions, Clarias catfish can migrate to alternative water bodies. In Clarias catfish, the gills are in the pharyngeal region on the gill arches, with two rows of gill filaments attached to their outer edges. A dense network of fine blood capillaries is found within these filaments. The lungs (right and left) of the Clarias catfish are composed of branched anatomical structures situated on the second and fourth gill arches. These are paired hollow sacs with alveolate walls, connected to the esophagus and adapted for aerial respiration. The lungs originate from the gill cavities and branches in a tree-like manner to form the parabrachial organ, which includes both the right and left lungs. Each lung comprises medial and lateral lobes. Morphologically, the lungs exhibit three surfaces: dorsal (facing the dorsal part of the trunk), ventral (facing the ventral side), and mediastinal (facing the midline). The lungs also possess distinct cranial, caudal, and lateral borders. According to the results of morphometric analysis, the absolute lung mass in Clarias catfish is 5.24 ± 0.09 g, and the relative lung mass is 0.33 ± 0.02 %. The absolute mass of the left lung is 2.52 ± 0.17 g (48.13 ± 2.71 %), while the right lung weighs 2.72 ± 0.16 g (51.87 ± 2.71 %). Among the lung lobes, the medial lobes are more developed, while the lateral lobes are less developed. Specifically, the absolute mass of the medial lobe of the left lung is 2.16 ± 0.12 g (41.46 ± 2.94 %), and the lateral lobe is 0.36 ± 0.11 g (6.68 ± 1.84 %). The absolute mass of the medial lobe of the right lung is 2.24 ± 0.11 g (42.83 ± 1.5 %), while the lateral lobe weighs 0.48 ± 0.1 g (9.03 ± 1.7 %). Based on linear measurements the length of the medial lobe of the right lung is 21.0 ± 0.78 mm, the width is 32.2 ± 1.0 mm, and the thickness is 12.8 ± 0.33 mm, the lateral lobe of the right lung measures 11.2 ± 0.25 mm in length, 17.5 ± 0.4 mm in width, and 6.0 ± 0.48 mm in thickness, the medial lobe of the left lung measures 20.2 ± 0.45 mm in length, 31.4 ± 0.63 mm in width, and 12.0 ± 0.6 mm in thickness, the lateral lobe of the left lung measures 10.9 ± 0.73 mm in length, 17.3 ± 0.68 mm in width, and 5.8 ± 0.35 mm in thickness. According to the lung development index (length-to-width ratio), which is 64 %, the lungs of the Clarias catfish are classified as the expanded-shortened type.