Removal of bound calcium from nematocyst contents causes discharge

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Abstract
Nematocysts are the stinging organelles of jellyfish, sea anemones and other cnidarians. Each consists of a closed capsule filled with fluid. In the resting state, part of the surface of the capsule is inverted, forming a tubular thread, which is everted explosively on excitation. The mechanism of explosion is not understood; it may be relevant to exocytosis in general, as the nematocyst is a specialized type of exocytotic secretion originating in the Golgi apparatus. Picken and Skaer observed that the capsular fluid showed a large depression of freezing point, suggesting that the osmotic pressure might be as high as 140 bar. They tentatively ascribed the explosion to a sudden increase in permeability of the capsule wall, allowing a rapid osmotic influx of water. There is evidence that the material of the thread may be capable of some degree of extension, and it has been suggested that osmosis plays no part in discharge. A new theory is presented: the capsule wall is permeable to water even in the undischarged state; discharge is initiated by an increase in the osmotic pressure of the capsular fluid which is brought about by removal of bound Ca ions. [The work was carried out using Rhodactis rhodostoma.].

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