A microfluidic device for quantitative investigation of zebrafish larvae's rheotaxis

Zebrafish is a model organism for various sensory-motor biological studies. Rheotaxis, or the ability of zebrafish to orient and swim against the water stream, is a common behavior that involves multiple sensory-motor processes such as their lateral line and visual systems. Due to the lack of a controllable and easy-to-use assay, zebrafish rheotaxis at larval stages is not well-understood. In this paper, we report a microfluidic device that can be used to apply the flow stimulus precisely and repeatedly along the longitudinal axis of individual zebrafish larvae to study their coaxial rheotaxis. We quantified rheotaxis in terms of the response rate and location along the channel at various flow velocities (9.538 mm. sec(-1)). The larvae effectively exhibited a similarly high rheotactic response at low and medium velocities (9.5 and 19 mm. sec(-1)); however, at high velocity of 38 mm. sec(-1), despite sensing the flow, their rheotactic response decreased significantly. The flow velocity also affected the response location along the channel. At 9.5 mm. sec(-1), responses were distributed evenly along the channel length while, at 19 and 38 mm. sec(-1), the larvae demonstrated higher rheotaxis responses at the anterior and posterior ends of the channel, respectively. This result shows that although the response is similarly high at low and medium flow velocities, zebrafish larvae become more sensitive to the flow at medium velocity, demonstrating a modulated rheotactic behavior. Employing our device, further investigations can be conducted to study the sensory-motor systems involved in rheotaxis of zebrafish larvae and other fish species.