When people think of jellyfish, the first thing that comes to mind is usually something about their sting. This capability is characteristic of most jellies and their relatives in the phylum Cnidaria. The name of the group is actually derived from their possession of structures known as cnidae. Each is located within a specialized cell that houses the cnida, which is a capsule with an attached hollow thread. Nematocyst is the more familiar term applied to specialized cnidae that are characteristic of scyphozoan and other types of jellies, and other cnidarians.
Imagine trying to capture live prey without the aid of teeth, a jaw and hard protective body parts. What if your body also consisted of delicate gelatinous tissue that would easily be destroyed by a struggling prey. That’s the challenge that jellies face every time they attempt to feed. Nematocysts come handily to the rescue. Rather than being designed for attacking people, stinging nematocysts function primarily for the capture of prey, and secondarily as a defense mechanism. A wide variety of nematocysts have been classified. Many function to inject toxins to immobilize prey, while others serve to entangle and hold onto the intended meal by wrapping around it.
An undischarged nematocyst is housed within a cell known as a nematocyte. Most nematocytes are located on the tentacles of the jelly, which are the primary food capturing part of the body. Scyphozoan jellies also concentrate them around the mouth and on the gastric filaments of the stomach. The nematocyst capsule within the nematocyte is covered by a trapdoor-like operculum. Inside the capsule is the long, cylindrical tubule of the nematocyst. At the base of the tubule is an enlarged area known as the shaft. Both the shaft and the tubule may be endowed with an impressive set of spines (at least when viewed with scanning electron microscopy!). Characteristics of the tubule, spines and shaft are used in classifying the bewildering array of nematocyst types. You may see terms such as heterotrichous (tubule spines of unequal size), homotrichous (spines of equal size), atrichous (tubule without spines), eurytele (shaft dilated at its far end), haploneme (tubule without a well-defined shaft), heteroneme (tubule with a well-defined shaft), and isorhiza (tubule diameter the same throughout). Obviously there were some dedicated early researchers who didn’t have much of a social life! Species of cnidarian jellies vary in the types of nematocysts they possess, and this can be used to some extent in classifying and identification. A common nematocyst style among scyphozoan jellies is the heterotrichous microbasic eurytele (say that fast 3 times), one in which the shaft is relatively short and has a widened portion at the far (distal) end, with the spines largest near the shaft.
Nematocysts are continuously produced within cells known as nematoblasts. Since they are not reused following discharge and it is energetically costly to produce them, it’s to the advantage of the jelly to fire only when necessary. Both mechanical (touch) and chemical stimuli may act to trigger nematocyst firing. Contact with members of their own species generally doesn’t result in firing, which makes sense when you see a dense swarm of sea nettles that are frequently touching. When a potential prey item, such as a larval fish or another type of jelly makes contact, the result is quite different. Discharge is initiated by the opening of the capsule operculum. Immediately the tubule begins to evert out with a twisting motion. Although not completely understood, discharge appears to involve an increase of osmotic pressure within the capsule and perhaps a release of tension within the capsule wall. Combined with the spines, the twisting acts to drill the tubule into the unfortunate victim. The tubule then separates from the capsule and remains imbedded in the flesh.
Within a fraction of a second, hundreds or even thousands of nematocysts discharge with sufficient force to penetrate the skin or exoskeleton of the prey. Nematocysts can discharge independently of each other in certain cases, or be influenced by interactions with surrounding cells or even the simple nerve net system. They can discharge even after the jellyfish has been dead for hours or days, much to the chagrin of beachgoers with a penchant for fondling beached gelatinous blobs.
See nematocyst discharge in action, from the Pacific Cnidaria Research Institute: box jelly nematocysts
Nematocysts inject a complex slew of chemical agents into their prey or human victim. The numerous spines help to anchor the tubule into the prey and also serve as sites for the discharge of the toxic brew. Toxins and other substances may have a direct deleterious effect, or cause an immune reaction. It’s not clear whether different nematocyst types have characteristic toxins. Different species of jellies do, however, vary greatly in the suite of toxins they inject. Those that specialize in preying on fish, such as the sea wasp (Chironex) or the Portuguese man-of-war (Physalia), will tend to have very potent toxins that quickly immobilize the prey (and hence are quite painful to humans). Jellies that favor more gelatinous fare, like the egg-yolk jelly (Phacellophora), don’t need to concentrate on subduing the prey. Instead they often have nematocysts that are quite sticky and good at holding slimy blobs. Others like the moon jelly (Aurelia) rely more on bell mucus to capture zooplankton and thus have a comparatively reduced reliance on nematocysts.
Jellyfish toxins include a poorly understood array of complex chemicals, many of which are proteinaceous. Many have deleterious effects on cell membranes and cause them to rupture. This may, for example, lead to the breaking up of red blood cells, certainly not a desirable response to a sting. Other toxins have disruptive effects on the action of nerve and muscle cell membranes and impair their normal function. Throw in toxins that degrade collagen, break down proteins and lipids, and disrupt cellular influx of ions like calcium, and you can see why jellyfish mean business.
So behold the amazing nematocyst. Although small in stature, the combined efforts of multitudes of these microscopic workhorses is sufficient to subdue creatures that seemingly should have no problem against a delicate gelatinous blob. Nematocysts are one more reason to admire our gelatinous friends, and they are key to the success that jellyfish and their cnidarian relatives have had in conquering all marine habitats.