Cnidocyte

Image:Nematocyst-discharged.png

Cnidocytes are prey-capture and defensive cellsunique to animals of the phylum Cnidaria.

General introduction

Cnidarians (corals, sea anemones, hydrae, jellyfishetc.) are a morphologically simple, mostly sessile phylum of animals. Nevertheless, most of these animals are predators, feeding on mobile prey such as fish and crustaceans. In order to catch their prey and defend themselves from predators cnidarians have evolved a unique "weapon system" - the cnidocytes (nematocytes), single cells able to penetrate their target and inject toxic substances into it.

Structure and action

Image:Nematocyst-threads.jpg

Cnidocytes are single cells which contain a subcellular organelle named cnidocyst. The cnidocysts are composed of a hollow coiled thread-like structure attached to the bulb-shaped nematocyst capsule body. The externally-oriented side of a cnidocyte cell also has a hair-like trigger on it, called the cnidocil. When the trigger is activated the cell "fires" - the shaft of the cnidocyst penetrates the target and the hollow thread is everted into the target organism body. This discharge is one of the fastest biological processes, takes no more than few microseconds and reaches accelerations of about 40,000g. Following penetration, the toxic content of the nematocyst is injected into the target organism. The rapid activity of the injected neurotoxins serves to immediately paralyze the mobile prey, thus allowing the sessile cnidarian to devour it.

Mechanism of discharge

The nematocyst capsule stores a large concentration of calciumions, which are released from the capsule into the cytoplasmof the cnidocyte when the trigger is activated. This causes a large concentration gradientof calcium across the cnidocyte plasma membrane. The resulting osmotic pressurecauses a rapid influx of water into the cell. This increase in water volume in the cytoplasm forces the coiled nematocyst to evert rapidly.

How is the prey detected?

While the nematocyte itself is able, under some conditions, to fire independently, this presents several problems for the cnidarian. Firstly, how does it avoid stinging itself? Secondly, cnidocytes are "single use" cells, and need to be replaced (at a large energetic cost) following discharge. In order to regulate their use, cnidocytes are connected as "batteries", containing several types of nematocytes connected to supporting cells and neurons. The supporting cells contain chemoreceptors, which together with the mechanoreceptoron the cnidocyte (cnidocil) allow only the right combination of stimuli (prey swimming, chemicals found in prey cuticleor skin, etc.) to cause the cnidocyst to discharge.

Types of nematocysts

There are over 30 types of nematocytes found in different cnidarians. They can be devided into the following groups:

1. Penetrant: A harpoon-like structure used to penetrate.
2. Glutinant: Sticky surfaces used to stick to prey.
3. Volvent: A lasso-like string that is fired at prey and wraps itself around a cellular projection on the prey.
4. Ptychocyst: A special type of nematocyte found on burrowing (tube) anemones, which help create the tube in which the animal lives.

Depending on the species, one or several types can appear simultaneously on the organism.

Nematocyst toxicity

Nematocysts are extremely efficient weapon systems. A single nematocyst has been shown to suffice in order to paralyze a small arthropod (Drosophila larva). The most deadly cnidocytes (to humans, at least) 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." Other cnidarians, such as the jellyfish Cyanea capillata (The " Lion's Mane" made famous by Sherlock Holmes) or the hydrozoan Physalia physalis (Portoguese Man-O-War, "Bluebottle") can cause extremely painful and sometimes fatal stings. On the other side, aggregating sea anemonesmay have the lowest sting intensity, perhaps due to nematocystsnot 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.

In this context it is worth noting that venom from cnidarians, as well as other venomous animals such as scorpions, spidersand others, may be species selective: what we consider as "weak toxicity" due to the low activity on humans (or other mammals) may be very potent towards the natural prey or predators of these animals. Such specificity has been utilized to create novel medicines and bioinsecticides.

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 WaspAustralian Institute of Marine Science; dangers of box jellyfishde:Nesselzelle

lt:Knidocitas nl:Netelcel pt:Cnidócito

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Cnidocyte Wikipedia article Cnidocyte.