Nematotsiidid

Nematotsiidid (nematocysts) are specialized stinging organelles embedded in cnidocytes, the cells of cnidarians such as jellyfish, sea anemones, corals, and hydrozoans. Each nematotsiid consists of a capsule filled with a highly coiled tubule, often reinforced with barbs or spines, and capped by an operculum. A trigger hair called the cnidocil on the cnidocyte detects mechanical or chemical stimuli and initiates discharge. When activated, water rapidly enters the capsule, generating high internal pressure that forces the operculum to open and the tubule to everte at great speed, sometimes injecting toxins into prey or threats.

There are several forms of nematocysts, specialized for different tasks, including penetrants that pierce tissues, volvents that coil and shoot, and glutinants that adhere to surfaces. The toxins delivered by the tubule can immobilize prey and deter predators; toxin composition varies among species and cnidarian groups. The discharge mechanism is highly rapid and is a key factor in the effectiveness of cnidarian feeding and defense strategies.

Ecology and human interactions: Nematocysts play a central role in the feeding and defense strategies of most cnidarians. Stings from nematocysts can cause pain, redness, and swelling in humans, and in rare cases more serious reactions. Handling cnidarians or swimming in areas with active stinging organisms should be done with care.