Hydrodynamic effects of sea star morphology and orientation to flow*

Live-animal experiments show that certain sea star species exhibit morphological plasticity in response to changes in hydrodynamic conditions. We investigated the hydrodynamic effect of changing shape by measuring drag and lift forces for 3D printed sea star models resembling Pisaster Ochraceus with varying arm length-to-width ratio (aspect ratio).  An ATI Industrial Automation six-axis load transducer was used to measure forces for the model assembly.  An automated positioning system oriented the models relative to mean flow direction. We found that drag forces increase with increased aspect ratio and lift forces decrease with increased aspect ratio. We also found that orienting the model with its arm rotated 36° relative to the flow direction reduces both the drag and lift forces compared to other orientations.  These preliminary results indicate that, in wave exposed areas, the hydrodynamic loads acting on sea stars are minimized by decreasing aspect ratio and optimally orienting the body.