Cnidocyte

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A cnidocyte, cnidoblast, or nematocyte is a type of venomous cell unique to the phylum Cnidaria (corals, sea anemones, hydrae, jellyfish, etc.). The cnidocyte cell provides a means for them to catch prey and defend themselves from predators. Despite being morphologically simple, lacking a skeleton and usually being sessile, cnidarians prey on fish and crustaceans. A cnidocyte fires a structure that contains the toxin, from a characteristic sub-cellular organelle called a cnidocyst (also known as a cnida or nematocyst). This is responsible for the stings delivered by jellyfish.

Contents

Structure and action

Each cnidocyte cell contains an organelle called a cnidocyst (or nematocyst), which comprises a bulb-shape capsule containing a coiled hollow thread-like structure attached to it. The externally-oriented side of the cell also has a hair-like trigger called a cnidocil. When the trigger is activated, the shaft of the cnidocyst penetrates the target organism, and the hollow thread is everted into it. This discharge takes no more than a few microseconds, and is able to reach accelerations of about 40,000 g[1]. Recent research suggests the process to occur as fast as 700 nanoseconds, thus reaching an acceleration of up to 5,410,000 g[2]. After penetration, the toxic content of the nematocyst is injected into the target organism. The rapid activity of the injected neurotoxins immediately paralyzes the mobile prey, thus allowing the sessile cnidarian to devour it.

Discharge mechanism

The nematocyst capsule stores a large concentration of calcium ions, which are released from the capsule into the cytoplasm of the cnidocyte when the trigger is activated. This causes a large concentration gradient of calcium across the cnidocyte plasma membrane. The resulting osmotic pressure causes a rapid influx of water into the cell. This increase in water volume in the cytoplasm forces the coiled nematocyst to eject rapidly. The coiled nematocyst is a hollow tube that exists inside the cell in an "inside out" condition. Imagine a rubber glove with only the fingers inside out and tucked into the palm of the glove. If you blow into the cuff of the glove, the fingers will pop out quickly creating a glove into which you can put your hand. The pressure of water flowing into the cnidocyte forces the water into the tubular nematocyst causing it to right itself as it comes rushing out of the cell with enough force to impale a prey organism.

Prey detection

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