The Amazing Cnidoblast
The coral polyp is a member of the Phylum Cnidaria. This Phylum includes the sea anemone and jellyfish. The stem of the Greek word for nettle is Cnid-. The most unusual characteristics of this Phylum are the nettlelike stinging nematocysts -- poison dartlike microscopic hairs used to capture prey.
The nematocyst is located inside the one-celled Cnidoblast (pronounced Ny-doh-blast) which in turn is located on the tentacles of the coral polyp, which lives inside its calcareous cup. At night the polyps partially emerge from their cups to feed. Each Cnidoblast has a trigger, the cnidocil, which is usually activated by physical contact but sometimes by chemical stimuli (such as minute quantities of animal juices in the water), causing the cell to burst open and extrude its contents. Inside each cnidoblast is a coiled thread, a nematocyst, which explodes out of the cell. Only those nematocysts located in the stimulated area are discharged. The mechanism for release is probably intensified water pressure in the cell, brought about by a sudden increase in the permeability of the cell membrane. Water rushes in and the pressure increases until cell pops open and the nematocyst is forced out, turning inside out as a sock does when pulled off one's foot. Thus the barb or coiled thread that is inside the nematocyst as it lies in the cnidoblast becomes the tip and comes into contact with its prey.
There are at least 17 kinds of nematocysts. They are generally grouped in into three functional categories:
1. Volvents have a whiplike end that wraps around spines of prey.
2. Glutinants have a sticky secretion that sticks to the prey.
3. Penetrants have barbs at their tips and penetrate the prey, often injecting a toxin causing paralysis.
A cnidarian may not have all of these types, but penetrants are especially common. It is remarkable that all this complex apparatus is contained inside one cell. To get the proper perspective, note the nucleus of the cell.
So pity the poor plankton! These minute animals and plants are as a rule relatively helpless, being able to swim only feebly against the currents. As they brush by the tentacles of the polyps, they trigger the explosion of the cnidnocysts that extrude the long, hollow, microscopic threads (nematocysts), which unravel in a burst, penetrating or sticking to the planktonic organism and capturing or paralysing it. The tentacle then retracts rapidly, wiping the organism across the mouth, which takes in the prey, passing it down the throat into the gastrovascular cavity where it is digested.
The toxins produced by corals are usually either harmless to man or cannot normally be introduced into the body because human skin is too thick. An exception to this is the toxin produced by the fire corals of the genus Millepora, which can cause a painful localised welt.
Based on Eugene H. Kaplan's A Guide to Coral Reefs, 1982