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Nematocysts
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. 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. 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. 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. For
views of nematocysts in action, see Jellyfish
Art
All photographs © David
Wrobel and may not be used or copied without permission! |
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. 
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. 
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. 
