Cnidocyte

A cnidocyte (also known as a cnidoblast or nematocyte) is an explosive cell containing one giant secretory organelle or cnida (plural cnidae) that defines the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). Cnidae are used for prey capture and defense from predators. Despite being morphologically simple, lacking a skeleton and many species being sessile, cnidarians prey on fish and crustaceans. A cnidocyte fires a structure that contains the toxin, from a characteristic subcellular organelle called a cnidocyst (also known as a cnida or nematocyst). This is responsible for the stings delivered by a cnidarian.

Structure and function

Each cnidocyte contains an organelle called a cnida or cnidocyst (e.g. nematocyst, ptychocyst or spirocyst), which comprises a bulb-shape capsule containing a coiled hollow tubule structure attached to it. The immature cnidocyte is referred to as a cnidoblast. The externally oriented side of the cell also has a hair-like trigger called a cnidocil. When the trigger is activated, the tubule shaft of the cnidocyst is ejected and in the case of the penetrant nematocyst, the forcefully ejected tubule penetrates the target organism. This discharge takes no more than a few microseconds, and is able to reach accelerations of about 40,000 g. Recent research suggests the process to occur as fast as 700 nanoseconds, thus reaching an acceleration of up to 5,410,000 g. After penetration, the toxic content of the nematocyst is injected into the target organism, allowing the sessile cnidarian to devour it.

Nematocyst (dinoflagellate)

A nematocyst is a subcellular structure or organelle containing extrusive filaments found in two families of athecate dinoflagellates (a group of unicellular eukaryotes), the Warnowiaceae and Polykrikaceae. It is distinct from the similar subcellular structures found in the cnidocyte cells of cnidarians, a group of multicellular organisms including jellyfish and corals; such structures are also often called nematocysts (alternatively, cnidocysts or cnidae), and cnidocytes are sometimes referred to as nematocytes. It is unclear whether the relationship between dinoflagellate and cnidarian nematocysts is a case of convergent evolution or common descent, although molecular evidence has been interpreted as supporting an endosymbiotic origin for cnidarian nematocysts.

In polykrikoids the nematocyst is found associated with another extrusive organelle called the taeniocyst, a complex that has been described as synapomorphic for the genus Polykrikos.

The full range of functional roles of the nematocyst is not well understood, but it has been observed to be involved in prey capture and feeding.