Nematocyst of Aurelia aurita

The “nematocysts” that characterize the Cnidaria are the capsules containing poisonous stings with a highly delicate mechanical structure. Nematocysts are used not only to catch prey and protect the jellyfish from foreign enemies, but also to adhere to the base. Jellyfish belonging to the Ctenophora do not have nematocysts, but some instead have “colloblasts” that capture prey by adhesion. In addition, some Ctenophora and mollusks are known to place the nematocysts of captured Cnidaria on their body surface for their protection. The presence of nematocysts is the cause of “being stung by jellyfish” in humans and has attracted great attention. However, the molecular mechanism by which the poisonous stings are ejected from the nematocysts is still poorly understood.

The nematocyst itself is not a living cell but a structure that is created inside the cnidocyte and pushed out of the cell by exocytosis. In Aurelia aurita, nematocysts are present not only throughout the body surface but also inside the gastral cavity. In addition, nematocysts are most abundant on the tentacles, but they are also abundant on the upper surface of the oral arm and umbrella. Surprisingly, there are relatively few nematocysts on the back of the umbrella.