Comparative Neurobiology of Olfactory Processing in African Catfish (Clarias gariepinus): An Original Analysis of Peripheral and Central Neural Architecture

This dissertation provides an original comparative neurobiological analysis of olfactory processing in Clarias gariepinus, examining peripheral receptor neuron diversity, olfactory bulb organisation, and central olfactory projections using histological, immunohistochemical, and electrophysiological methods. Clarias gariepinus has exceptional chemosensory capacities that underpin its predatory and social behaviour, but the neuroanatomical architecture supporting olfactory processing in this species has not been systematically characterised at the cellular and circuit level, representing a fundamental gap in the neuroethology of Africa's most important aquaculture fish. The dissertation employs a systematic neuroanatomical design using adult fish from controlled breeding programmes in Lagos State, South West Nigeria. Olfactory rosette morphology and receptor neuron types are characterised by scanning electron microscopy and immunohistochemistry for olfactory marker protein, calbindin, and tyrosine hydroxylase. Olfactory bulb cytoarchitecture is described from Nissl and Golgi-stained sections and compared with available teleost models. Electrophysiological recording from the olfactory bulb and secondary olfactory centres quantifies neural responses to amino acids, bile salts, and sex pheromone fractions. The dissertation identifies four morphologically distinct olfactory receptor neuron types including a previously undescribed microvillar subtype expressing a novel odorant-binding protein, presenting an original structural contribution. Electrophysiological data demonstrate high-sensitivity discrimination of bile salts at femtomolar concentrations, relevant to prey detection. The findings provide an original neurobiological foundation for understanding chemosensory-guided behaviour in Clarias gariepinus with implications for aquaculture behavioural management. Keywords: olfactory neurobiology, Clarias gariepinus, olfactory bulb, receptor neurons, neuroethology.