PAUL O’DOWD
There are many types of marine animal that are referred to as “stingers”. These include of course the Box Jellyfish and many true jellyfish, as well as Hydroids like the “Man o’ War”, and an assortment of creatures that actually nip and bite but whose effect is generally sting-like as far as your hide is concerned.
Most stingers however, including our very own, famously deadly ones, belong to the group Cnidaria; the extremely ancient group of animals for which either coral or jellyfish would be the perfect poster-child.
This group is unique in its possession of highly specialised cells, called cnidocytes, which are studded with high pressure, touch activated, hydraulic harpoons, called nematocysts. Each nematocyst is capable of firing a microscopic, thread-like, eversible tube into the flesh of target animals. Most nematocysts deliver, through this tube, a range of chemicals to subdue and digest prey items.
Included in this cocktail, for some at least, are the components for ingenious, self assembling molecular devices which form reinforced, open window frames in cell walls. This attack spills the contents of cells into their surroundings which equilibrates the inside of the cell with the fluid it resides in, shutting down all electrical activity and thus function.
In us that means, amongst many other unpleasant things, no more heartbeat and massive impacts on the circuitry of the nervous system.
Many nematocysts also act as anchoring devices, with spines, adhesives and other projections from the outer tube wall meshing tightly with the tissue into which they are discharged.
The surface of a cnidarian’s tentacle, and other surfaces as well, is studded densely with cnidocytes, each loaded with nematocysts, ready to instantly activate and discharge their “explosive poison hose” into any flesh they brush against.
In this ready state, the hose-like thread is “Outside-In”. Imagine a very long necked bottle made of rubber. Now fold the lip of the bottle inward and start pushing the neck down inside itself and into the bottle. Continue until the bottle looks like a ball with a long, tightly coiled tube projecting inward from a big fold on its surface.
This model is very much like the structure of an undischarged nematocyst. A nematocyst normally sits at equilibrium with the fluids of its cnidocyte. When certain chemical conditions are met, and then the trigger hair is touched, the nematocyst dumps calcium ions into the cnidocyte, causing the cnidocyte to rapidly flood that area with water. The sudden increase in pressure within the cnidocyte violently squeezes the nematocyst which undergoes an explosive eversion, powerful enough to punch through flesh.
The force with which the nematocyst discharges into the prey is astronomical. Based on the speed of discharge, the process involves G-forces measured in the millions! Just over five million G in fact…
Probably lucky that the threads are seldom longer than 600 nanometers, which is a fraction of the width of a human hair and only just long enough to get under your outer skin layer in the more sensitive bits.
There are different kinds of cnidocyte which are specialised into particular roles.
Some fire glue, some fire entangling threads or barbed harpoons, some inject specific venom compounds which hit the heart or neurotoxins to incapacitate the prey, others introduce powerful enzymes and digestive compounds into the prey,
These different “classes” of cnidocyte are arranged into “batteries”, or bands of particular functionality. These bands encircle the tentacle and can be seen with the naked eye in some species. There might be an anchoring band, followed by a venom band after which you might see more anchors or a digestive band… and so on…
This explains the distinctive “bar-code” banding seen in the scars that result from severe stings. Certain bands of cnidocyte are armed with chemistry that literally digests the flesh in their strike zone and the damage creeps outward from there.
This means that while the bands of cnidocytes loaded with powerful cardiac neurotoxins may have been what killed a Box Jelly victim, the dramatic bar-code like damage to the skin is actually caused by the parts of the tentacle from where the digestive functions were delivered.
The tentacle epidermal layer has a huge surface area as a result of billions of minute folds in its surface, a bit like the walls of your own guts.
This entire surface is densely studded with cnidocytes, including within the deep recesses of the epidermal folds.
The tentacle surface also secretes a mucous which performs a great multitude of functions, including preventing the nematocysts from firing into the animal’s own tissues.
When the tentacle touches an animal, oils from the prey’s skin stain the protective mucous coating.
Sensory cells in the tentacle tissue detect this chemical change and prime the weaponry in the area for firing, much like flicking off the safety on a gun. Or more like millions of guns simultaneously…
Upon this chemical verification of edibility, mechanical interference with the trigger hair is all that is required to cause the nematocyst to dump it’s calcium and fire.
Once anchored, the more the prey struggles, the more the tissues of the tentacle are deformed and the more the folds in its surface are stretched, revealing ever more undischarged weapons.
Strong muscles in the tentacle then begin to winch the prey into the cnidarian’s mouth where it enters a digestive chamber for further unpleasantness. It turns out that the digestive chamber is lined with powerful digestive cnidocytes which literally “sting the prey into soup”.
Of course, what this design means for us, is that the difference between a survivable Box Jelly sting and a deadly one can be down to the way the adherent tentacle is handled.
Many unfired nematocysts still await mechanical activation and will fire en mass as the tentacle is peeled off the victim’s skin.
Vinegar deactivates the undischarged nematocysts, making the tentacle safer to remove for both the rescuer and the victim.
For this reason, a person with adherent tentacle material still on their skin should be left that way until the tentacle gets soaked with vinegar.
Understandably, many people do not want to get up in a jellyfish’s face to get a good look at these incredible features. Your loss…
My advice for when you’re out on the reef and you see a Jellyfish, is to go and have a good look at it! Swim alongside it for a while… Just use your brain and don’t stick your fingers in its tentacles!
More sedately, you can experience the wonders of cnidarian biology in the safety of a rock pool or aquarium by sitting with an anemone or a coral colony for a while. With a bit of patience you will see them performing a variety of their daily chores, like cleaning themselves and catching tiny prey. You may even witness some of their famous symbiotic associations, of which the Clown Fish is just one of many…
…Just don’t be tempted to try the Tentacle Touch Test unless you know exactly what you are looking at, or are shooting for a Darwin Award…
