Abstract
Simple SummaryAir-breathing fish constitute a broad evolutionary group of fish, which are generally characterized by distinctive phenotypical plasticity. These fishes usually inhabit waters where oxygen deficiency occurs periodically, which is why they have developed a variety of accessory respiratory organs (AROs) that may be used in an obligatory or a facultative manner. Knowledge of the structure of these organs is important for both the breeding and the conservation of these fish species. The aim of this study was to conduct a comparative histological analysis of two types of AROs found in the Anabantoidei suborder and the Clariidae family, both of which are freshwater fish taxa of high ecological and commercial importance.Accessory respiratory organs (AROs) are a group of anatomical structures found in fish, which support the gills and skin in the process of oxygen uptake. AROs are found in many fish taxa and differ significantly, but in the suborder Anabantoidei, which has a labyrinth organ (LO), and the family Clariidae, which has a dendritic organ (DO), these structures are found in the suprabranchial cavity (SBC). In this study, the SBC walls, AROs, and gills were studied in anabantoid (Betta splendens, Ctenopoma acutirostre, Helostoma temminckii) and clariid (Clarias angolensis, Clarias batrachus) fishes. The histological structure of the investigated organs was partially similar, especially in relation to their connective tissue core; however, there were noticeable differences in the epithelial layer. There were no significant species-specific differences in the structure of the AROs within the two taxa, but the SBC walls had diversified structures, depending on the observed location. The observed differences between species suggest that the remarkable physiological and morphological plasticity of the five investigated species can be associated with structural variety within their AROs. Furthermore, based on the observed histology of the SBC walls, it is reasonable to conclude that this structure participates in the process of gas exchange, not only in clariid fish but also in anabantoids.
Highlights
Fish, like other animals, breathe oxygen, and its availability is crucial for their survival.An aquatic environment offers a much lower oxygen content compared to atmospheric air, which is why fish evolved special anatomical structures that enable efficient oxygen uptake from the water
The gills of all the investigated individuals were located in gill chambers (Figure 1A,B), with the cores of the gill arches covered by a thin layer of loose connective tissue, along with primary and secondary lamellae (Figure 1C,D)
The lanceolate-shaped, branched gill rakes were observed as having a bony core and acidic mucosal cells
Summary
Like other animals, breathe oxygen, and its availability is crucial for their survival.An aquatic environment offers a much lower oxygen content compared to atmospheric air, which is why fish evolved special anatomical structures that enable efficient oxygen uptake from the water. In some species, this ability has regressed with time, in others, it either remains active to the present day or was reacquired after the preceding regression [2,3]. In the latter group of fish, oxygen uptake takes place through the gills, a special form of respiratory organ equivalent to the lungs of terrestrial vertebrates, and through accessory respiratory organs (AROs), which are frequently found in fishes living in poorly oxygenated waters throughout the world [4]. Some of the most well-known representatives of air-breathing fish are species belonging to the suborder Anabantoidei (order: Anabantiformes) and the family Clariidae (order: Siluriformes)
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