Extra-visual use of spatial-geometry in zebrafish remains unaffected after pharmacological ablation of the lateral line

The fish are able to reorient themselves, that is to find a salient place in their own space of life and action, after passive disorientation, within environments characterized by a precise geometric shape, usually rectangular, making use of metric spatial cues (length, distance, corners) and sense (left / right), i.e., geometric attributes of the navigation’s space. This ability is widespread among vertebrates and it has been extensively explored within rectangular enclosures, characterized by opaque visible surfaces, with or without conspicuous landmarks (an entirely blue wall) or local ones (different panels at the four corners of the arena). Evidence, in this regard, suggested a view-based processing of spatial information during reorientation behavior. However, the use of fish as an animal model in this research area allows us to test the role of other sensory modalities besides sight: one of these is the lateral line (LL), which is made up of mechanical sensors (neuromasts), capable of detecting hydrodynamic stimuli, that is, the pressure variations of the water induced by the presence of salient stimuli, and potentially also capable of building a map of the surrounding space. Therefore, in order to investigate whether fish could use the LL to reorient themselves using only the spatial geometry of a non-visible environment, the reorientation behavior of zebrafish (Danio rerio) was observed: fish were trained inside a transparent rectangular arena, after inducing a pharmacological ablation of the LL, by administering gentamicin. A staining procedure of mitochondria with the DASPEI fluorescent vital dye was performed to visualize the LL neuromasts of controls vs. gentamicin-treated fish. Despite the treatment, the results showed that the zebrafish reoriented correctly, leading to the exclusion of LL as a pivotal way to solve the geometric reorientation task. On the other hand, an analysis of movement strategies has highlighted mainly a perimeter exploration, often closely adhered to the surfaces of the rectangular arena, thus emphasizing the role of touch and its integration with other senses in constructing an internal representation of the environmental geometry. However, it is still little investigated in visual and non-visual spatial tasks both in animals and in human beings.