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The hypothesis that exchange of intracapsular divalent cations with Na+ in seawater increases the internal osmotic pressure during discharge of nematocysts of marine cnidarians was tested by examining effects of externally applied cations on the volume and elemental composition of nematocysts isolated from acontia of the sea anemone Calliactis polypus. The volume of isolated nematocysts increased with increasing concentrations of cations if the cation was monovalent but appeared to decrease if the cation was divalent. Ca2+ reduced the internal osmotic pressure of the nematocysts more efficiently than Mg2+. X-ray microanalysis of nematocysts incubated in 1 M solutions of various salts showed that Ca2+ in isolated nematocysts was only partially replaced, if at all, by externally applied Na+ and Mg2+ while most Mg2+ was replaced by Na+ and Ca+. The present results suggest that exchange of intracapsular divalent cations with external monovalent cations increases the internal osmotic pressure, and that selective binding of Ca2+ to polyanions in the capsule decreases it. Whether the increase in the internal osmotic pressure caused by the cation exchange is large enough to trigger discharge remains to be investigated.

Effects of Cations on the Volume and Elemental Composition of Nematocysts Isolated from Acontia of the Sea Anemone Calliactis polypus