<p>The study presented herein examines the vortex structures generated by a magnetic stirrer inside a container filled with seawater and the dinoflagellate <i>Pyrocystis lunula</i>. This bioluminescent organism can emit light when water is stirred, due to a natural chemical reaction. The relationship between dinoflagellate bioluminescence and hydrodynamic flow fields has been recently investigated, particularly focusing on water induced shear stress. In the present study, the funnel depth of vortices observed in water stirred experiments are compared with existing vortex models, and the shear stress distribution within the flow field is estimated. The results indicate that, for the range of angular velocities analysed and the geometry of the experimental setup, <i>Pyrocystis lunula</i> bioluminescence can serve as an indicator of the vortex structure. Furthermore, this study confirms the validity of <i>Pyrocystis lunula</i> as a biological sensor of tangential shear stress in fluid domains, specifically within vortical flows. The dinoflagellate species may serve as a low-cost tool to visualize fluid structures in hydrodynamic experiments in which the flow field must remain undisturbed by external instrumentation. Additionally, the study suggests a potential approach to designing applications with dinoflagellates: by enabling the formation of controlled light patterns, hydrodynamics may serve as a functional design parameter.</p>