Cnidocyte
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Cnidocytes are prey-capture and defensive cells unique to animals of the phylum Cnidaria.
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Mechanism and structure
Cnidocytes contain specific organelles named nematocysts that are used for self-defense and prey capturing. Nematocysts are composed of a coiled thread-like structure attached to the bulb-shaped nematocyst capsule body. The externally-oriented side of a cnidocyst cell also has a hair-like trigger on it. When the trigger is activated the cell fires its nematocyst. Nematocyst discharge is one of the fastest biological processes and takes not more than few microsconds. The discharge is also accompanied by stimulation of specific chemoreactors on the animal's body.
Some nematocysts puncture the skin or exoskeleton of an organism and inject a paralyzing toxin. Others stick to the surface of the prey item and yet others coil themselves around cellular projections on the prey.
Types of nematocysts
There are three types of nematocysts. Depending on the species, one or several types can appear simultaneously on the organism:
- Penetrant: A harpoon-like structure used to penetrate.
- Glutinant: Sticky surfaces used to stick to prey.
- Volvent: A lasso-like string that is fired at prey and wraps itself around a cellular projection on the prey.
Miscellaneous
The most deadly cnidocytes are found on the box jellyfish's body. One member of this family, the "Sea wasp," Chironex Fleckeri, is "claimed to be the most venomous marine animal known," according to the Australian Institute of Marine Science. They state that it causes excruciating pain to humans, often followed by death, sometimes within two or three minutes, and that the chance of survival if stung while swimming alone is "virtually zero."
On the other side, aggregating sea anemones have the lowest sting intensity, perhaps due to nematocysts not penetrating the skin, providing only a feeling of touching sticky candies to human fingers. Besides feeding and defense, sea anemone colonies use cnidocytes to sting one another in order to win space.
The Ctenophora ("sea-gooseberries" or "comb jellies") are a group of transparent, jelly-like sea creatures belong to a different phylum; these animals have no nematocysts at all and are harmless to humans.
External links
- Dangerous marine animals of Northern Australia: the Sea Wasp (http://www.aims.gov.au/pages/research/project-net/dma/pages/seawasp-01.html) Australian Institute of Marine Science; dangers of box jellyfish