To navigate around their local environment, animals must recognise their own position with respect to their goal. This task can be achieved through a representation of space in their brain, built upon learning and remembering environmental features. Previous research has focused on horizontal navigation but most animals must also move vertically. This is exemplified by flying or swimming animals, which move with six degrees of freedom (unlike surface-constrained individuals that move with three). By using experimental and theoretical approaches, we consider how pelagic and benthic fish deal with the problem of 3D navigation.

We explore mechanisms and processes of spatial cognition in vertebrates by considering both the sensory input and the behavioural output. In doing so, we can also make inferences about information storage and processing. We show that vertical and horizontal components of space are stored separately in the fishes’ representation of space and that the vertical axis relies on a particularly salient spatial cue - hydrostatic pressure. We also demonstrate that freely swimming fish are able to accurately encode metric information from the volume through which they move. By comparing our work to the classical model system that is used in neuroethology, the surface-bound rat, we argue that fish have a supramodal representation of space that is seated in the lateral pallium and that is homologous to place cells found in the hippocampus of mammals. Further, we suggest that space is represented isotrophically in fish – in other words, the postulated neurones that encode space fire with a spherical distribution. More generally, we hypothesise that the representation of space in vertebrates’ brains might be shaped by the degrees of freedom of movement that binds the animal.