Isolated nematocysts (capsules of stinging cells in Cnidaria) from the freshwater polyp Hydra vulgaris [Weber, J., Klug, M. & Tardent, P. (1987) Comp. Biochem. Physiol. 88B, 855–862] are under a constant internal osmotic pressure of 12.5 MPa. The capsular wall which withstands this considerable pressure has an average elasticity modulus of approximately 1 GPa, enabling the cyst to swell from its relaxed state to more than double its volume. If the large concentrations of Mg2+ and Ca2+ within isolated nematocysts are substituted by alkali ions, the capsular volume increases by 15% and the final osmotic pressure rises to 15.3 MPa whereas after substitution by Ca2+ a decrease to 90% of the volume and 6.5 MPa is observed.
Evidence obtained from the osmotic behavior of nematocysts, as well as data from in vitro exchange of their cations, are consistent with a physicochemical model in which the internal osmotic pressure of nematocysts and thus their ability to discharge is governed by the cationic composition of the content and the salt concentrations in the environment.