eLife assessment:
Sampaio and colleagues utilize an elegant approach to manipulate fluid dynamics in zebrafish Kupffer's vesicle to ask if fluid movement or something in the fluid governs the break in symmetry. These valuable results support a role for fluid movement and detection as important in breaking symmetry in a ciliated left-right organizer and help set a time window when fluid flow is critical for this process. However, as the fluid extraction experiments affect both chemical and physical features, the authors need to provide further convincing evidence to support their mechanosensory hypothesis or temper the claims.