Born in complexity: How the early life environment shapes zebrafish larvae phenotype

The biotic and abiotic environments are potent drivers of individual plasticity. Zebrafish, a popular research model around the world, has been extensively used for understanding the proximate cause and mechanisms of phenotypic plasticity to environmental enrichment. Despite awareness of providing high-standard rearing condition for increasing fish welfare, rearing of zebrafish larvae involves little more than stable temperature and daily water changes. We asked whether including Lego bricks as additional visual and mechanosensory stimuli during early rearing would induce developmental plasticity across morphological (growth, forebrain cell proliferation), physiological (stress response), and behavioural (anxiety, habituation) endpoints. We raised zebrafish in Petri dishes with complexity (Lego bricks) from fertilization to 5 days post-fertilization (dpf) and then compared phenotypes to those of traditionally reared larvae (barren). Relative to barren-reared larvae, those raised with complexity were shorter and did not increase whole body cortisol in response to an acute agitation stressor. Forebrain cell proliferation was not different between treatments, however, larvae raised with complexity habituated sooner to an adverse stimulus and showed a tendency towards reduced anxiety-like behaviour, suggesting the anxiolytic effects of environmental enrichment may be initiated at early life stages. Taken together, our results invite consideration into how current practices in raising zebrafish may influence neurodevelopment and behavioural research findings, and may be applied to improve outcomes in fish breeding programs for conservation initiatives.