Identification of paraneuronal pseudobranchial neurosecretory system in a hill-stream carp, schizothorax richardsonii

Pseudobranchial neurosecretory system (PSNS) belongs to the category of “diffused neuroendocrine system” (DNES) and is located in the gill region of certain fishes close to pseudobranch/ carotid gland/ carotid labyrinth. Earlier investigations revealed the presence of PSNS in fishes that are either air-breathing or have an ability to tolerate low oxygen concentrations of water. A recent report revealing the presence of PSNS in the cold water carp Barilius bendelisis opened up the possibility of existence of this system in hill-stream carps as well. Therefore, to confirm this it was aimed to investigate the presence of the PSNS in another hill stream carp Schizothorax richardsonii. Specimens of snow trout were collected from Alaknanda River, Srinagar, India and dissected on-site to procure the tissues of PSNS close to pseudobranch and processed for paraffin microtomy with Acid Violet staining. The histological observations confirmed the presence of PSNS close to pseudobranch. The PSNS was endowed with different neurosecretory stages of pseudobrancial neurosecretory cells (NSCs) forming neurosecretory cell mass (NSM) along with their numerous cell processes forming thick bundles, entangled with blood vascular structures as neurohaemal complex at several places. This is the first report of the presence of PSNS in S. richardsonii and the system has been discussed from a new functional and evolutionary point of view in hyperoxic cold waters to maintain the respiratory homeostasis of the fish.