Cnidocytes (Stinging Cells)

Cnidocytes, also called ‘stinging cells’, are specialized cells exclusively found in the epidermis of cnidarians (phylum Cnidaria), such as jellyfish, corals, and sea anemones.¹ They are most abundant on their tentacles and around the mouth. Since cnidarians cannot actively chase prey, these cells help them quickly pierce their skin, thereby immobilizing them.

Each cnidocyte contains a large secretory granule called the cnidocyst, which comprises a bulb-shaped capsule and a hollow tubule that remains coiled within. Near the apical end of the cnidocyte, just beside the opening of the capsule, lies a hair-like sensory trigger called the cnidocil. When this trigger is activated, the cell fires, and the coiled tubule is ejected with great force. In an inactivated state, an operculum or lid covers the opening of the capsule.

There are 3 types of cnidocysts, each having a distinct function.

1. Nematocysts: The most common type of cnidocyst, these are adapted for skin penetration and are often associated with venom delivery in both predator and prey. Their discharge tubule is typically armed, with a shaft bearing spines or barbs.
   Found in: All cnidarians
   
2. Spirocysts: They are primarily adhesive, having a spineless, spirally coiled tubule that sticks to or entangles prey.²
   Found in: Sea anemones and some corals
   
3. Ptychocysts: These are characterized by long, spineless, sticky tubules that help build protective tubes around their bodies.³
   Found in: Tube anemones (exclusively)

Function

A cnidocyte is triggered by contact with prey or a predator (stimulus), initiating a rapid series of events that culminate in the discharge of the cell.

1. Contact with prey or a predator stimulates the cnidocil.
2. The operculum of the capsule opens immediately.
3. Calcium ions are released from the capsule into the cytoplasm. This movement creates a concentration gradient across the membrane.⁴
4. The gradient generates osmotic pressure, causing water to rush into the cell.
5. The internal pressure and volume of the cnidocyte increase rapidly.
6. The coiled tubule is forcefully discharged outward.

In nematocysts, the barb on the tubule penetrates the skin, often releasing a toxin that incapacitates the prey or predator. In other cnidocysts, the tubule may simply adhere to the target and weaken it.

To learn more about how a cnidocyte stings, refer to the jellyfish stinging mechanism.