The structure of the nematocyst thread and the geometry of discharge in Corynactis viridis Allman

Abstract

Electron microscope studies reveal that the undischarged nematocyst thread is not (as the light microscope image suggests) a cylinder containing a compact mass of barbs, but a screw, as first shown in the electron micrographs of Bretschneider (1949). In the process of discharge, the screw surface is converted to a cylinder without significant change in surface area. This transformation is markedly anisometric, the length of the thread increasing almost threefold, while the overall increase in diameter is less than 50 %. The screw shape of the undischarged thread is due to the presence of three helical pleats in the membrane; and discharge results in the smoothing out of these. The cavity of the thread is smaller in the undischarged condition—because of the presence of pleats—and is largely filled by the whorls of asymmetrical barbs (three to a whorl); the tips of the barbs are pressed closely together, while their spatulate bases are distributed in open hexagonal array over the pleated surface of the thread. Barbs readily become detached from the surface of the discharged thread, leaving a complex, striated scar. The discharged thread is a slightly tapering tube in holotrichous isorhizas, and the barbs show systematic changes in size and proportions with taper. Electron micrographs show that the cavity of the undischarged thread is filled with a flocculent material, as is the space between the capsule wall and the thread. This material is presumably the highly hygroscopic proteinaceous working substance responsible for the increase in volume of the capsular fluid—at least up to 200 %—on hydration. The undischarged thread and its contents, isolated under anhydrous conditions, are conspicuously hygroscopic and perform movements of elongation and rotation as water vapour is admitted to or removed from the system. The transformation of a membrane in the form of a screw surface to a cylinder, such as occurs in the discharge of the nematocyst thread, is only possible if portions of the membrane in the trough of the screw increase in area, or portions of the pleats decrease in area. The apparent constancy of the area of the thread throughout discharge suggests that both processes may occur.