Exploring the Importance of Environmental Complexity for Newly Hatched Zebrafish

Simple Summary Growing up in an impoverished environment can profoundly affect brain development and cognitive abilities in animals. Most studies have looked at the effects on adults and we know little about how it impacts the early stages of cognitive development in fish. We studied zebrafish larvae, which are routinely reared in bare containers where they experience reduced sensory stimulation. We aimed to see if providing them a more enriched environment would have an effect on their cognitive abilities and behaviour. Larval zebrafish from the enriched treatment had better performance than controls in a number discrimination task but did not differ when required to discriminate two areas. In both experiments, larvae from the impoverished treatment showed a reduced locomotor activity. When essayed in a preference test, recently hatched larvae showed a marked preference for an enriched environment over an empty one. A better understanding of these effects is crucial for the welfare of captive zebrafish populations as well as for the quality and reliability of research on larval zebrafish.Abstract The effects of an early impoverished social or physical environment on vertebrate neural development and cognition has been known for decades. While existing studies have focused on the long-term effects, measuring adult cognitive phenotypes, studies on the effects of environmental complexity on the early stages of development are lacking. Zebrafish (Danio rerio) hatchlings are assumed to have minimal interaction with their environment and are routinely reared in small, bare containers. To investigate the effects of being raised under such conditions on development of behaviour and cognition, hatchlings housed for 10 days in either an enriched or a standard environment underwent two cognitive tasks. The results were mixed. Subjects of the two treatments did not differ in performance when required to discriminate two areas. Conversely, we found a significant effect in a number discrimination task, with subjects from impoverished condition performing significantly worse. In both experiments, larvae reared in impoverished environment showed a reduced locomotor activity. Given the effects that enrichment appears to exert on larvae, a third experiment explored whether hatchlings exhibit a spontaneous preference for more complex environments. When offered a choice between a bare setting and one with objects of different shapes and colors, larvae spent over 70% of time in the enriched sector. Deepening these effects of an early impoverished environment on cognitive development is crucial for the welfare of captive zebrafish populations and for enhancing the quality and reliability of studies on larval zebrafish.