Saviour Grech, writing in The Malta Independent on Sunday of 18 June, was irritated by the diametrically opposed advice on the treatment of jellyfish stings being offered in local newspapers, including the information I gave David Lindsay when he interviewed me on the subject of the jellyfish infestations we are experiencing at present (TMIS, 11 June).

Anybody with an Internet connection will find a plethora of remedies on the treatment of jellyfish stings that include those that have appeared in local papers as well as many others. Recommendations include treating the affected area with vinegar, isopropyl alcohol, ammonia, baking powder, meat tenderiser, oils of various sorts, hot water, and even urine. Mr Grech might be irritated further, or perhaps amused, to learn that many of these treatments may work in one set of circumstances but not in others, and that the same treatment applied twice to the same person might work one time but not the other. How is this possible? To understand, one must look at the way jellyfish sting and the purpose of this article is to explain this, and perhaps throw some light on this confusing subject.

Jellyfish and their relatives (the most familiar of which are the sea anemones and corals) are equipped with stinging cells whose primary function is feeding and defence against predation. Most species of jellyfish capture and feed upon any animal that bumps into their tentacles as they drift about in the water. Obviously, the size of the prey they take depends a great deal on the size of the jellyfish; most species of small jellyfish take small sized prey, mainly the diminutive animals of the plankton. For such fishing to be effective, many species have tentacles that are much longer than the diameter of the bell (the umbrella-shaped part of the jellyfish’s body) such that the animal is effectively fishing a volume of water that may extend to some 25 times or more the diameter of the bell. This is why one can get stung even if one is not very near to a jellyfish. This is also why one can get stung although there are no jellyfish visible in the immediate area. The tentacles, which may be so transparent as to be practically invisible, can become detached from the animal and float about in the water but are as deadly as when still attached. Incidentally, this is the reason why mechanical means of removing jellyfish en masse from the water, such as by the use of nets, do not make the water safe to swim in – all but very gentle handling will result in tentacles dropping off or fragmenting, so while the animals may be removed, their tentacles linger on as invisible stinging threads.

The tentacles, and other parts of the jellyfish’s body, are equipped with stinging cells that are technically called “cnidocytes”, of which a single tentacle may carry from thousands to hundreds of thousands, and which may be arranged in groups to give batteries of stinging cells. Each cnidocyte has a very complex structure, known as a nematocyst, that takes the form of a capsule containing what appears as a thread packed up inside (see left diagram). Under appropriate stimulation, the capsule will burst open, and the thread inside will be released as a long filament that shoots out with great force, reminiscent of a jack-in-the-box that jumps out when the lid is opened (see right diagram).

True jellyfish have four types of nematocysts, of which one type, the so-called penetrant type, is the one that “stings”. In this type, the thread that shoots out of the capsule is long and hollow. It shoots out with great force (in relatives of the jellyfish, a speed of two metres per second has been measured), penetrates the body wall of the prey, lodges firmly by means of the backwards pointing barbs that cover the base of the thread, and injects toxin (poison) through the hollow thread. In effect, the nematocyst acts like a combined harpoon and hypodermic syringe, attaching the prey to the tentacle and at the same time subduing or killing it by injecting a powerful cocktail of toxins. In the case of a human brushing against a jellyfish tentacle, the “harpoons” embed themselves in the skin, injecting their contents and causing the stinging sensation. How badly stung one gets depends on how many nematocyst threads become embedded, what kind of toxin the jellyfish has, and on how sensitive the person is to the toxin. It also depends on which part of the body is stung, since penetration is more difficult where the skin is thick.

Different species of jellyfish have different mixtures of toxins, some much more potent than others; they have stinging nematocysts with different penetrating powers, so how badly one gets stung is related in part to the species doing the stinging. It also depends on the reactions of the victim. The instinctive reaction to a jellyfish sting is to pull back and to rub the affected area. Pulling back usually has the effect of ripping off the tentacle so that this remains stuck to the body by means of the “harpoons” that have already fired. Rubbing the area has the effect of causing nematocysts that have not yet discharged to fire, increasing the dose of toxin and hence the pain.

Here is where the various remedies against jellyfish stings come in. These are designed to remove the fragments of tentacle that remain stuck to the skin, to prevent undischarged nematocysts from firing and to alleviate the pain. To understand why some remedies work and some do not, and why some work one time and not another, one needs to look at what causes the nematocysts to fire in the first place. This is a very complicated mechanism that is certainly not equivalent to opening the lid of a jack-in-the-box. Most cnidocytes are equipped with a short “trigger hair” on the outside of the cell that, when mechanically stimulated, initiates a complicated process that results in the release of calcium into the nematocyst capsule, followed by a large influx of water so that the pressure forces out the thread. Once the nematocyst has discharged, it cannot be used again and the cnidocyte dies and is replaced.

If firing the nematocyst were simply a matter of touching the trigger hair, the jellyfish would have a problem, since it would mean that the slightest brush would fire hundreds of nematocysts, which then need to be replaced, and if the animal brushes against itself, it would sting itself. Individual cnidocytes therefore have a threshold for firing. This means that they can tolerate a certain degree of mechanical stimulation without the nematocysts firing. However, if stimulation exceeds a certain level (the threshold), then the nematocyst fires. This threshold is not constant but is affected by many factors. These include the acidity or alkalinity of the surrounding water, the chemical composition of the water, chemicals released into the water by prey or present on the surface of any object that touches the jellyfish, and on the level of nervous stimulation of the batteries of cnidocytes provided by the jellyfish itself. The threshold for firing will therefore vary greatly depending on circumstances. For example, it has been shown that a hungry jellyfish will have low thresholds and will fire nematocysts at the slightest stimulation by prey; as the prey releases body fluids from its wounded body, more and more nematocysts are induced to fire. However, a satiated jellyfish has high thresholds and will only fire its nematocysts after much greater stimulation than when hungry.

So what is the bottom line to all this? There are several. The first is that different species of jellyfish in different parts of the world have different toxins, different physiologies and different responses to chemical agents. This means that a treatment that works with one species may not work with another or may make matters worse. Thus vinegar (an acid) prevents undischarged nematocysts from firing in some species but actually causes them to fire in others. This is the danger of “importing” treatments designed for use in one part of the world to another.

The second is that the same species will respond differently to the same chemical, depending on its mode of application. Thus, washing the wounded area with a fast stream of fresh water (such as by taking a shower) may mechanically remove adherent tentacles before the nematocysts have a chance to fire, but gentle bathing with fresh water usually makes matters worse by causing undischarged nematocysts to fire. Sea water should only be used as a last resort, since it may not only include fragmented tentacles, but may also introduce pathogens into the wounds. For gentle bathing, a sterile saline is recommended.

The third is that the effectiveness of remedies also depends on the person affected – for example, which part of the body was stung, what chemicals (sun tan oil, cosmetics etc.) were applied to the area before the stinging occurred and how vigorously the affected area was rubbed before the remedy was applied, among others.

There is no sure remedy for jellyfish stings that will work on all people at all times, and what works on one occasion may make matters worse on another. This is why the strategy that I recommended to David Lindsay was to pick or wash off as much of the adherent tentacles as possible and apply an ice pack (not ice directly to the wound, however). This will deaden the pain as well as slow down the action of the toxin. Of course, if one is sensitive to jellyfish toxins, then one should seek medical advice.

Patrick J. Schembri is professor of biology at the Department of Biology of the University of Malta