Preference for complex environments in larval zebrafish

Environmental complexity is increasingly recognized as a critical factor for animal welfare and cognitive development across vertebrates. While adult zebrafish (Danio rerio) show a strong preference for complex environments, little is known about the importance of environmental complexity in larval stages. This study investigated the preference for enriched environments in zebrafish larvae and the sensory modalities underlying this behavior. In Experiment 1, we assessed the preference for complex environments in larvae aged 4, 5, and 6 days post-fertilization (dpf). Results showed that larvae consistently preferred an environment containing objects of varying shape and color over a barren environment. The preference for the enriched environment was not immediately apparent upon introduction but became evident within the first 90 min, reaching its peak the following day. This pattern likely reflects an initial neophobic response to novel objects, which temporarily counteracted the attraction to environmental enrichment. In Experiment 2, larvae also preferred a sector enriched with photographic representations of objects. However, a direct comparison with real objects in Experiment 3 revealed that the photographs were less effective in attracting larvae. Larvae did not show attraction to transparent objects, which minimized visual input while stimulating the lateral line and olfactory systems. Monochromatic (white) objects were also ineffective in eliciting a response. These findings suggest that zebrafish larvae can perceive and respond to environmental complexity within 24 h after hatching, emphasizing the need to reassess husbandry practices and research protocols to align with their preferences and level of neural development at hatching.