Title: Thermal navigation in larval zebrafish
Description: Navigation in complex environments requires selection of appropriate actions as a function of local cues. To gain a quantitative and mechanistic understanding of zebrafish thermal navigation, we have developed a novel assay that requires animals to rely exclusively on thermosensory information in the absence of other cues such as vision or mechanosensation. We show that zebrafish use both absolute and relative temperature information to restrict their locomotor trajectories to a preferred temperature range. We identify components of movement that are modulated solely by absolute temperature, as well as components that are modulated by both absolute and relative temperature. Specifically, we find that dwell time between movements and displacement per movement depend solely on absolute temperature, whereas turn magnitude and turn direction bias are modulated by absolute and relative temperature. To evaluate whether these sensorimotor relationships could explain thermal restriction in our navigation assay, we performed Monte Carlo simulations of locomotor trajectories based on all or subsets of these relationships. We find that thermosensory modulation of turn magnitude and turn direction bias constitute the core navigation strategy in larval zebrafish, while modulation of dwell time accelerates the execution of this strategy at noxious temperatures. Modulation of turn direction bias represents a novel strategy not found in invertebrate models, whereby animals correct unfavorable headings by preferentially turning in a preferred turn direction until they obtain a favorable heading. Modulating turn direction bias in response to recent sensory experience is an effective strategy for selecting favorable headings in organisms that do not have a dedicated sampling phase before each reorientation event.