Cnidarians (jellyfish, coral, sea anemone, hydra) are characterized by their stinging cells that function mainly in pray capture and defense. The stinging cells contain the cnidocysts, capsules equipped with an injection system and act as biological syringe. These capsules contain highly folded thin tubule resembling a needle, and can hold pressure of 150 bar. Upon activation, the tubule unfolds and penetrates its target at a remarkable acceleration of 5ยท106g to deliver 10 times capsule volume. Recently a large group of parasites, known as Myxozoa was added to the Cnidaria phylum. These endoparasites have a complex life cycle involving two hosts mainly fish and annelid and the infection is initiated by 2 stinging capsules. Myxozoa fish infection can have dramatic negative effects on fish populations inhibiting growth and causing mortality through diseases such as ceratomyxosis and whirling disease. To understand the adaptations of myxozoan capsules, we studied the function and structure of the tubules of two types of myxozoans and compared our finding with the stinging capsules of the jellyfish Rhopilema nomadica. External tubules ultrastructure of myxozoans and jellyfish were observed by scanning electron microscopy (SIGMA-HD, Carl Zeiss, Germany) and provides evidence of different functional modifictions that were developed through evolution. Our findings demonstrate that myxozoan capsules can infect their host by a complex mechanism of injection and anchoring.
