Fig. 1.

Behavioral complexity increases as the nervous system develops. Throughout development, animals perform spontaneous movements (green), which occur without the delivery of an extrinsic stimulus. The first behaviors to emerge are spontaneous coiling movements, which become frequent before the end of the first day post-fertilization. Further development leads to the ability of larvae to perform simple responses to environmental stimuli (blue), including whole-body responses (such as struggles and escapes) to noxious stimuli, which can be evoked by 3 dpf. As larvae develop, behavior becomes more complex and animals gain the ability to modify their responses to stimuli by learning (pink). They additionally gain the ability to respond to a broader range of stimuli (ranging from whole-field illumination changes to the presence of members of their own species) and acquire the ability to more finely tune their spontaneous movements. Recent studies have traced these functional developments back to neurodevelopmental processes. The age of onset (colored text) or age at which a given behavior is assayed (indicated by ‘a.’) is indicated next to each schematized behavior. hpf, hours post-fertilization.