Stung: just what is a jellyfish?

“No worries” said the skipper, a noted marine biologist, already in the water, “You don’t find stingers out in the open ocean. Just jump in, you’ll be fine.”

Famous last words in my experience.

On one memorable occasion a few years ago now, we were becalmed aboard our 39 foot Cavalier Noctiluca, 180 nautical miles from New Caledonia having endured eight horrendous days of stormy passage from New Zealand. This was my first bluewater passage and it was proving to be a true baptism of fire.

Both the motor and our new self-steering gear had given up the ghost on the second day out of Opua when the storm hit. This meant resorting to a tiny storm jib, the only sail possible in these conditions.

Nonetheless, despite its small size, we were still powering along at speeds in excess of seven knots. The waves were huge and ominous and our poor old lady was pitching and yawing all over the place making moving around extremely challenging.

Meals were hurried ‘food in the machine’ affairs eaten straight from the can, our clothes were salt-stiffened and grotty, we were both sleep-deprived and, to add insult to injury, no cups of tea were possible to soothe and lift the spirits.

This much anticipated trip was definitely not meeting my expectations of an idyllic sailing adventure highlighted by gourmet dinners, sparkling champagne and opportunities to mull on the wonders of nature as the sun set in the evenings. Hence, when on the eighth day the wind dropped right out and the sun shone, the enticement of the sparkling azure water and the opportunity to have a jolly good scrub, proved too much.

Reassured by the afore-mentioned marine biologist’s assertion that there would be no stingers, I stripped off and jumped overboard, bravely overcoming my irrational fear that the bottom was some three kilometres below me and that the water teemed with countless unknown marine beasties, full of evil intent.

As I hit the water, I felt several incredibly painful lashes across my boobs and shoulders and, sure enough when I clambered back on deck whimpering, the tell-tale welts from a number of jellyfish stings were immediately observable. They continued to remind me of their presence for almost a week after the event.

Although I did not think so at the time, I was very lucky because if the culprit had been one of the jellyfish that cause any of the symptoms of Irukandji syndrome, the outcome would almost certainly have been catastrophic, especially as we were so far offshore.

A bit on their biology

Jellyfish, or jellies as they are better known as they are definitely not fish, belong to the phylum Cnidaria and the subphylum Medusozoa. However, their phylogenetics are complex and evolving as more research is conducted.

Currently, it is accepted that there are four major classes within the phylum: the Scyphozoa, sometimes called true jellyfish; the Cubozoa, commonly known as the box jellyfish; the Hydrozoa and the Staurozoa or stalked jellyfish. They are found in every ocean from the surface to the deep sea, even down to depths of 3700 metres in the Mariana Trench.

Exclusively marine, except for some freshwater Hydrozoans, jellies have had an extraordinarily long evolutionary history: over 500 million years, making them one of the oldest multi-organ animals to have evolved. Composed of over 95 per cent water, these soft-bodied gelatinous animals usually take the form of umbrella-shaped bells, although some have a flattened disc shape. All have limited control over their movement, but by pulsating their hydrostatic skeleton, some can navigate and move.

Although jellies do not have a true central nervous system or brain, they have a loose network of nerves located in the epidermis, interspersed with ganglia.

This enables them to detect various stimuli, including odours and pressure such as the touch of other animals. In addition, the network allows transmission and spread of nervous information to all areas in their bodies.

Did you know that jellyfish can see? They can detect light because the lower surface of the bell or disc bears light sensitive sensory organs called ocelli. Some species are able to detect colour.

The edge of the bell also has the trailing tentacles which are studded with stinging cells called nematocysts. Nematocysts are discharged for the capture of prey or in defence against predators.

The injectable portion of a nematocyst is coated with toxin and it is this that causes pain and the other symptoms associated with being stung. The shape of nematocysts differs in different species and this can be useful in deciding on treatment protocols after a victim has been stung.

Often, particularly in the case of some box jellyfish, the sting is only moderately irritating initially. Severe symptoms can be delayed for 5 minutes to 120 minutes depending on the species involved. Thus a timely microscope examination of skin scrapings can allow earlier medical intervention.

Most jellies do not have specialised digestive, respiratory or circulatory systems. Often they have a stalk-like manubrium that hangs down from the middle of the ventral surface and this is surrounded by oral arms that connect to a central aperture which serves as both a mouth and anus. This opens into a gastrovascular cavity, which facilitates both digestion and absorption of food.

Jellies are carnivorous, feeding on plankton, crustaceans, fish eggs, small fish and other jellyfish. They usually hunt passively using their tentacles as drift nets.

Jellies have separate sexes although they can reproduce both sexually and asexually. At maturity and when there is plenty of food, whole populations can spawn daily, the event being controlled by light. Thus, at either dawn or dusk, adults synchronously release eggs and sperm into the surrounding water.

Multiple phases follow fertilisation; a larval planulae stage which is free swimming, until a suitable substrate is found; a polyp phase which, through budding, produces free-swimming ephyrae and then the adult medusa phase, where the organism is recognisable as a jellyfish. In some species, some phases are skipped.

Lifespans vary. Some species live only a few hours, others several months and, in the case of some deep-sea species, lifespans can be several years. Most large coastal jellyfish live two to six months.

They have few predators. Most predation is from their own kind: species of jellyfish that specialise in preying on other jellies. Other known predators include tuna, shark, swordfish, sea turtles and Pacific salmon.

Australian jellies

In the tropical waters off the Queensland coast, marine stingers can be present year round, although there is a higher risk during the period October to May. At these times, jellies are prevalent in the waters around the mainland, the islands as well as out on the reef.

The primary indicator of stinger season is linked to the increase of light northerly winds immediately after the south-easterly trades subside. Onshore breezes have the ability to concentrate animals and to move them into areas where people swim or undertake other water activities, thus increasing their risk of being stung. Jellyfish bloom formation is, however, a complex process dependent on a number of other factors apart from wind: ocean currents, nutrient availability, temperature, season, reduced predation and water oxygen levels.

Currents can concentrate populations of jellies in particular areas. Hence the common stinger ‘hotspots’ in the Great Barrier Reef tend to be on the south western sections of islands or the south western areas of rocky headlands with far fewer stings recorded on the more open north western sections of island and headland.

However, it needs to be remembered that while all jellyfish have the capacity to discharge nematocysts, not many have the ability to penetrate the skin, not all toxins are the same and people differ in their response to being stung. Nonetheless, Australia is host to some notorious box jellyfish, which are able to produce extremely potent toxins with the capacity to inflict severe reactions and in some cases death in their hapless victims.

Most deaths have been attributed to Chironex fleckeri and two of the Irukandji species: Carukia barnesi and Malo kingi.

Chironex fleckeri

Chironex venom is particularly nasty because within 2m to 5m the victim’s cells become highly porous leading to massive losses of potassium, cardiovascular collapse and death. This jelly is widespread throughout the Indo-Pacific region, including Vietnam, Papua New Guinea, the Phillipines, Hawaii and, possibly, New Caledonia.

Carukia barnesi

This jelly was named after Dr. Jack Barnes who was searching for the jellyfish causing Irukandji syndrome. Carukia barnei is very small and transparent with a diameter of only 25 millimetre to 35mm. Not much is currently known about its biology. It has four retractable tentacles of variable length 50mm to 500mm and both the body and the tentacles have nematocysts. It is also known to have image-forming eyes although, anomalously, no brain capable of processing visual information.

Malo kingi

This species differs from all other cubozoans in having halo-like rings of tissue encircling the tentacles, with club-shaped appendages inserted end-on around the periphery of the rings. The species was only named in 2007 after the sting fatality of a 44 year old American tourist.

There are at least 16 Irukandji species. These, almost transparent, box-shaped jellies are very hard to see in the water and its small size enables it to move freely though stinger nets; although stinger suits prevent penetration of the nematocysts.

All Irukandji species have four tentacles bearing numerous nematocysts which fire on contact with prey, delivering a highly-potent toxin into the epidermis of victims. The toxicity of the venom is far in excess of that from a cobra or a tarantula and although the initial sting may only be mildly irritating with little visible evidence to the skin, symptoms increase in severity after some five to 120 minutes, depending on the species.

The victim experiences excruciating muscle cramps in the arms and legs, severe pain in the back and kidneys, a burning sensation of the skin and face, headaches, nausea, restlessness, sweating, vomiting, an increase in heart rate and blood pressure and psychological phenomena such as the feeling of impending doom. Known as the Irukandji syndrome, these symptoms are in part caused by the release of catecholamines: hormones produced by the adrenal glands including dopamine and adrenalin and which are normally released into the blood stream at times of physical or emotional stress i.e. the flight/fight response. In addition, the venom also contains a sodium channel modulator which alters the normal function of nerves and muscle cells.

Currently no antivenom has yet been developed. Treatment is symptomatic with antihistamines given to control inflammation, antihypertensive drugs to reduce high blood pressure and intravenous opiates used to control pain.

Are numbers on the rise?

In some places, blooms of jellyfish have shown a substantial, visible impact on coastal populations and have resulted in clogged nets for fishermen, an apparent increased risk of being stung for tourists and even choked intake lines for power plants.

This has led to the general perception that the world’s oceans are experiencing increases in jellyfish due to human activities such as increased agricultural and industrial runoff and the overharvesting of fish.

It is true that agricultural and industrial runoff has increased the nutrient loads in tropical coastal waters off the Queensland coast, resulting in eutrophication and lowered oxygen levels in seawater in some areas. Furthermore, laboratory studies have shown that jellies appear to thrive better in nutrient-rich, oxygen-poor water with higher salt concentrations than other marine organisms. This potentially may give it an advantage when eutrophication events occur.

We also know that jellies have few predators and feed by touch, not by vision; potentially allowing them to feed effectively at night in turbid water. We also know that if the balance of a particular ecological niche is disturbed, because of the removal of one of the niche species, then another species quickly moves to fill that niche.

Thus, in theory because jellyfish feed on the same prey as adult and juvenile fish, the removal of fish by overfishing could markedly reduce the competition for available food allowing jellyfish populations to increase significantly. We have also seen in some areas of the world, populations of invasive jelly species from other habitats tend to expand rapidly because of lower predation levels in a new habitat.

However, we do not have any definitive evidence. We know very little about the biology of jellyfish although there is some evidence to show that jelly populations fluctuate in approximate 10 and 20 year cycles, in concert with solar and climate cycles.

Globally, they are little surveyed and why their numbers fluctuate like this is not entirely understood. The paucity of historical records also makes it very difficult to determine if there is a global trend of jellyfish proliferation.

Global warming, however, will provide a rising baseline against which climate cycles will cause fluctuations in jelly populations. Most scientists agree that there is an urgent need to put a stop to the continued abuse of our oceans. Unfortunately, the probable acceleration of anthropogenic effects may well lead to further problems with jellies.

Will they move down the Queensland coasts in large numbers as was suggested after the recent Irukandji incidents off the Fraser Coast? Maybe, but in the meantime take care in the water, especially during the stinger season. Avoid known box jellyfish habitats if you are not sure the dive site or swimming area is safe.

On a lighter note

Jellies do have some beneficial attributes.

In some countries, such as China, Japan and Korea, they are considered to be a delicacy.

The jellyfish is dried to prevent spoiling and, once dried, can be stored for weeks at a time. When required for consumption, the product is soaked overnight in water after which it is shredded, or eaten raw or cooked with a dressing of oil, soy sauce, vinegar and sugar. I have eaten it in soup many times and found it to be delicious.

On a more pragmatic note, did you know that jellies are used to make diapers, tampons and paper towels because as a ‘hydromash’, they have been found to be super-absorbent!

Jenny Kailie
M.O.S.S Australia
Jeanneau SF30 OD
JPK August 2023
NAV at Home