The nematocyst is a specialized, single-use organelle found within the cells (cnidocytes) of animals belonging to the phylum Cnidaria, such as jellyfish and sea anemones. These cells are distributed across the organism’s tentacles and body surface. The primary function of the nematocyst is to serve as an explosive tool for capturing prey and defending against predators. Its rapid, forceful discharge allows soft-bodied cnidarians to subdue much larger or faster-moving organisms.
The Static Structure of the Nematocyst
Before firing, the nematocyst is a highly pressurized, self-contained structure that stores immense potential energy. The main body of the organelle is a bulb-shaped capsule that holds the coiled components and the venom cocktail. This capsule is built to withstand high internal pressures, sometimes reaching 150 bar.
Within the capsule, a hollow, inverted tubule is tightly coiled, resembling a miniature harpoon or thread. The opening of the capsule is covered by a hinged lid known as the operculum, which keeps the system sealed until activation. The entire apparatus is governed by the cnidocil, a hair-like projection on the exterior of the cnidocyte that acts as a combined mechano- and chemo-receptor.
The coiled tubule itself is stored in an “inside-out” state. This configuration is maintained under tension, creating a powerful molecular spring mechanism ready to be released. The structural components of the capsule wall are composed of complex proteins, which contribute to the organelle’s biophysical properties.
The Phenomenon of Rapid Discharge
The firing sequence begins when the cnidocil detects both mechanical touch and specific chemical cues from potential prey. Activation initiates a rapid change in the ion concentration within the cnidocyte, causing the release of calcium ions. This sudden release creates a strong concentration gradient across the cell’s plasma membrane.
The change in ion balance leads to a massive influx of water into the capsule due to extreme osmotic pressure. This rapid increase in water volume and internal hydrostatic pressure provides the explosive force required for discharge. The operculum bursts open, and the coiled tubule is forced to evert as it is rapidly ejected.
This process occurs in as little as 700 nanoseconds. The acceleration of the tubule is staggering, estimated to reach up to 5.4 million times the force of gravity. This immense acceleration allows the tubule to achieve high pressure at the point of impact, enabling it to penetrate even the tough shells of crustaceans. The initial pressure generated inside the capsule, estimated to be around 150 bar, drives the forceful first stage of penetration. Once discharged, the nematocyst is a single-use structure, requiring the cnidarian to develop a new cnidocyte for subsequent stinging actions.
Delivery and Effect of Cnidarian Venom
The fired tubule injects a complex mixture of bioactive compounds, collectively known as venom, into the target organism. This venom is a sophisticated cocktail that includes proteins, peptides, enzymes, and non-proteinaceous compounds like biogenic amines. The specific composition varies widely across cnidarian species, but common components include neurotoxins, pore-forming toxins, and enzymes.
Neurotoxins work by exhibiting rapid, targeted activity on ion channels, which interferes with the nervous system to quickly immobilize prey. Pore-forming toxins self-assemble within the cell membranes of the target, creating holes that lead to osmotic lysis and cell death. Enzymes such as phospholipase A2 (PLA2) and proteolytic enzymes are also present, which can break down tissues and accelerate the spread of other toxic components.
The biological outcome depends on the target and the species of cnidarian. For small prey, the venom quickly causes paralysis and death, allowing the cnidarian to consume it. In larger animals or humans, the venom can cause effects ranging from localized pain and skin irritation to systemic symptoms like cardiotoxicity or paralysis.