The Role of Strain Energy Stored in the bent Harpoon of Sea Anemone

Cnidarians (Jellyfish, Coral, and Sea Anemone) are known for their explosive cnidocyte cells, used mainly for predation; Indeed, they use these explosive cells in aqueous environments to capture prey and defend themselves in seemingly unfavorable conditions given the dense medium that imposes a high drag force. Previous studies have identified that the pressurized cell chamber as the main driver of the harpoon. However, cnidocytes vary in size, shape and therefore diversity in firing mechanisms is entirely possible. Recently, we have successfully measured the firing mechanism of a cnidocyte of Nematostella Vectensis (starlet sea anemone) using micro-high-speed videography. The video suggested that the firing mechanism utilizes the release of bent harpoon, which is a strain energy release, much like spring, different from the pressure driven mechanism. To further study the role of the bent harpoon, we created a mechanical counterpart using a thin NiTiNol wire. To preserve the associated physics, dimensionless ratio between strain energy inside the harpoon and the hydrodynamic energy loss due to the drag was kept at a comparable ratio. The resulting kinematics of the mechanical model showed that the potential energy stored in the bending of the harpoon drives the forward acceleration of the harpoon. This mechanism contrasts with the previous notion that the pressurized chamber is solely responsible for their extreme harpoon speed and thus demonstrate the diverse evolution cnidocyte firing mechanism.