Hydra in Flow – Investigating the Flow Response of Hydra via Microfluidic Systems ^*

Hydra , a simple freshwater cnidarian, occurs in both moving and still bodies of water. Flow and corresponding forces are ubiquitous factors in Hydra ’s environment. Even in still water, brief exposure to a burst of flow (e.g., due to wind) can influence the surface attachment and dispersal of polyps. Additionally, alignment with flow may play a crucial role in minimizing forces and regulating feeding behaviors in Hydra . However, the response to flow (particularly in the presence of gravity) has remained underexplored. Using vertically oriented microfluidic chambers, we investigated the biomechanical response of Hydra vulgaris and two additional Hydra species to fluid flow. For Hydra vulgaris , strong surface attachment was observed for all flow rates (0 - 100 mL/hr, corresponding to average velocities of 0 - 2.2 mm/s for the chambers). The experiments indicated alignment of the body column with the flow direction at high flow rates (≥ 50 mL/hr). Alignment with flow was examined by quantifying the angle between the organisms (using a vector connecting head-to-foot) and the flow direction for multiple individuals (N = 9). Most individuals of H. vulgaris exhibited alignment under high flow. While preliminary, comparisons with H. hymanae and H. oxycnida suggested species-specific differences H. hymanae showed tentacle deformation but lacked clear alignment. H. oxycnida did not remain attached under flow. Additional experiments explored the combined effects of flow and osmolarity. These results support the use of microfluidic tools to examine flow-related behaviors and highlight the potential for comparative biomechanics across Hydra species.