Nematocyst initiation is a chemical and mechanical process. When a prey comes close to the hydra’s attractor, the trigger is thrown. Chemicals in the prey’s body increase the water permeability of the hydra’s nematocyst wall. The osmotic pressure or hydrostatic pressure of the capsule increases. Water quickly enters the bag. A chemical called poly-ℽ-glutamate is secreted inside the sac. The operculum opens as soon as the prey touches the nidocile. Sutraka is thrown out with lightning speed. This entire event takes place in just 3 milliseconds.
Once the nematocyst is released, it cannot be returned to the nematocyst. That is, once thrown, it cannot be used again. No more nematocysts are formed in the same nidocyte. Such nidocytes slowly enter the gastrointestinal tract and are mixed with food and digested. Within 48 hours new nidocytes are generated and used.