Mechanics of Hydra Detachment from Subrates: The Role of Substrate Rigidity and Starvation

Abstract

Hydra is a fresh water hydrozoan living as a solitary polyp with a sedentary feeder lifestyle attached to a substrate. In times of food shortage they are reported to detach from their substrate and move either by drifting or ‘somersaulting’.However, the quantification of the mechanical strength of the adhesion of Hydra has not been reported before. Here, we measure the force required to detach Hydra vulgaris and Hydra magnipapillata from a surface and the role of nutritional state and substrate rigidity. For detachment force measurement, we have developed a calibrated flow system, based on a syringe pump. We find the detachment shear stresses are similar whether the animal is well-fed or starved in the two species tested - H. vulgaris and H. magnipapillata. On the other hand, adhering to a hard substrate like a glass cover slip requires more force to detach H. vulgaris as compared to a soft substrate like polyacrylamide gel. Detachment stresses also differ across the two species in the same state. Taken together, it suggests that mechanics of the substrate and ambient flows in the water body could affect passive locomotion of Hydra, while suggesting the magnitude of muscle-based forces required to actively detach it from the substrate.