Structural organization of pseudobranch of the hill-stream fish, Schizothorax richardsonii: a pilot study

Abstract Background The pseudobranch is a gill-like structure found uniformly in all the cyprinids and several other groups of teleosts. It is located in the opercular cavity anterior to the first gill, attached to the dorso-lateral wall of the operculum on both sides. Although various types of pseudobranch have been explored in several fish species belonging to different ecological habitats, their presence in hill-stream fishes is limited so far. Therefore, the present investigation has been undertaken to study the structural organization of the pseudobranch in Schizothorax richardsonii (snow trout). Results The morphological and histological investigations undertaken on the S. richardsonii revealed the presence of a well-developed pseudobranch, close to the first gill. It is attached to the anterior dorso-lateral wall of the operculum and covered by the opercular membrane. The pseudobranch was composed of parallel rows of pseudobranchial lamellae supported by blood capillaries and branches of facial and glossopharyngeal nerves. The blood capillaries of lamellae were lined on either side with large epithelial cells, i.e., “pseudobranchial cells,” which is a characteristic feature of the covered type pseudobranch. Various cell types, including pseudobranchial cells, pilaster cells, and alarm cells, were observed in the pseudobranch. The pseudobranchial artery, its capillaries, and alarm cells were observed in the deeper layers of the pseudobranch. Conclusion The pseudobranch of S. richardsonii can be categorized as a covered type II B pseudobranch. Its structure was very similar to the pseudobranch of other cold-water fishes like Oncorhynchus mykiss and Barilius bendelisis , but it is quite different from other cyprinids namely Ctenopharyngodon idella and Labeo rohita . On the basis of structure, topography, and types of cells, the pseudobranch of S. richardsonii could be expected to serve multiple roles in the physiology of fish. However, further detailed research is required to investigate the specific functions of the pseudobranch in this fish.