Abstract
This narrative review synthesises current knowledge on the effects of habitat complexity, defined as the variation in physical structures within ecosystems, on fish cognition, brain morphology, and personality. Globally, habitat complexity is declining through a process known as habitat simplification, with largely unknown effects on animal cognition. Owing to their remarkable diversity and capacity for neural plasticity, fish provide an ideal model for investigating these cognitive consequences. We first examine how habitat complexity shapes brain structure both between and within species. The results highlight that species from more complex habitats often evolve larger brains and specific brain regions, like the telencephalon and cerebellum, which are crucial for advanced cognitive and motor functions. Conversely, a lack of structural complexity, such as experienced in hatchery environments, can lead to smaller brains in fishes, though this effect can be mitigated by physical environmental enrichment. The paper then explores the impact of habitat complexity on four key areas of fish cognition, i.e., general cognition including simple learning and executive functions, spatial cognition, social cognition, and numeracy. We then propose an intricate three-way interaction between habitat complexity, personality, and cognition, whereby changes in habitat complexity can indirectly alter cognition via changes in personality. The review concludes by identifying areas for future research and warns that the continued simplification of aquatic habitats may pose a significant threat to the cognitive abilities and adaptive capacity of fish in an increasingly altered world. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s10071-025-02042-0.