Comparison of Discharge Mechanisms of Cnidarian Cnidae and Myxozoan Polar Capsules

Abstract

Discharge stimuli and cell structure of polar capsules and cnidae are reviewed for both Cnidaria and Myxozoa. The discharge process in Cnidaria (especially Anthozoa) can be summarized as follows: 1) cnidae can discharge either independently or under the influence of adjacent cells. There are at least three types of cell complexes: one responding to mechanical stimuli, one to mechanical stimuli only after chemosensitization, and another responding to vibrational frequencies. 2) Supporting cells for the complex requiring chemosensitization bear chemoreceptors to mucins, N-acetylated sugars, and certain proteins. Binding of these ligands modifies the length of sensory cilia, sensitizing the cnidocyte to mechanical stimuli. 3) Mechanical stimuli from prey triggers the sensitized cnidocyte and cnidae discharge; spiro-cyst tubes stick to prey and penetrating nematocysts puncture prey. 4) Body fluids from prey provide additional ligands, further influencing cnidocytes. 5) Venom from nematocyst increases struggling of prey, mechanically triggering more nematocysts. Chemical signaling between cells is reviewed. Discharge in Myxozoa is less studied, but can occur after treatment with strong bases, urea, concentrated hydrogen peroxide, potassium salts, and in some species, mucus. Similarities between Cnidaria and Myxozoa include radial symmetry, development of specialized structures containing inverted, helically coiled filaments that are sticky and capped with a “stopper,” and the filament discharge response of these structures to pressure, potassium ion, and extreme pH. Myxozoans differ from cnidarians in the lack of a nerve net, less cellular organization, different life cycle stages, and in the response to chemosensitizers, neurochemicals, and external Ca²⁺ removal.

Keywords:

• Myxozoa
• nematocyst
• polar filament
• cnidae
• eversion
• polar capsule
• Cnidaria.