Emergence of consistent intra-individual locomotor patterns during zebrafish development

The analysis of larval zebrafish locomotor behavior has emerged as a powerful indicator of perturbations in the nervous system and is used in many fields of research, such as neuroscience, toxicology or drug discovery. The behavior of larval zebrafish, however, is highly variable, resulting in the use of high numbers of animals and the inability to detect small effects. In this study, we analyzed whether individual locomotor behavior is stable over development and whether behavioral parameters correlate with physiological and morphological features of the larvae, with the aim to better understand variability and predictability of larval locomotor behavior. We found that locomotor activity of individuals is consistent within the same day and becomes predictable during development especially during dark phases, when larvae are performing exploratory light-searching behavior and display increased activity. Stimulus induced startle responses were less predictable for an individual, and response strength did not correlate with inherent locomotor activity. Moreover, locomotor activity was not associated with physiological and morphological features of the larva (resting heart rate, body length, size of the swim bladder). These findings highlight the areas of intra-individual consistency, which could be used to improve the sensitivity of assays using zebrafish locomotor activity as an endpoint.