The neural correlates of migration in fish

Negota, Rabelani
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The hippocampal pallium and its homologues show remarkable plasticity, including volumetric changes and changes to cell density and even cell morphology in response to changes in the external environment. Across vertebrates, this brain region has been involved in navigation and spatial memory. Many studies have revealed specialisations associated with large home ranges, environmental complexity, and migratory behaviour in birds, rodents, and mammals. Much research has been done on fish movement patterns as it relates to migratory activity, as this is of significant ecological and economic importance. However, studies of the neural correlates of fish migration are few. The study aimed to identify potential specialisation of the fish pallium for migratory behaviour. This project compared the dorsolateral pallium of resident fish species (Pometopon grande and Diplodus capensis) and migrant fish species (Pomotomus saltatrix and Lichia amia). Fish were procured through donations from local anglers on the coast of KwaZulu-Natal and EasternCape, South Africa, and approximately three brains of each species for analysis. For each species, studies of neuronal morphology, using Golgi impregnation technique, and the volume of pallial nuclei was made (from Nissl-stained sections). Specifically, neurons of the dorsomedial (a homologue of the amygdala) and dorsocentral (a homologue of the isocortex) were made to identify potential specialisations of dorsolateral pallium neurons. Generally, dorsolateral and dorsomedial neurons are similar, with dorsocentral neurons being remarkably different in the volume of dendrites. However, neuron morphology shows no correlates of migratory behaviour. However, differences in the relative proportion of the different pallial nuclei imply some adaptation in migrants, including overall reduced brain size and selective enlargement of the dorsolateral pallium in migrants. Overall, our findings suggest that migratory fish may be relying more on responding to specific cues for navigation during migration instead of spatial memory centred in dorsolateral pallium. Anatomical areas associated with sensory cue perception could potentially show more adaptation for migratory behaviour
A research report submitted in partial fulfilment of the requirements for the degree of Master of Science in Medicine to the Faculty of Health Sciences, School of Anatomical Sciences, University of Witwatersrand, Johannesburg, 2022