Irukandji species resemble the larger Box Jellyfish, Chironex fleckeri, but have a single tentacle, instead of a cluster of tentacles, at each corner of their box-shaped bell. They range from 0.5 to 18 centimetres in bell height, with some tentacles up to a metre long. A sting from one of these jellyfishes can cause Irukandji syndrome, a collection of symptoms that typically requires hospitalisation.
The severity of Irukandji syndrome varies according to the species, with outcomes ranging from mild neuralgia to death. Symptoms may include severe pains in various parts of the body, headaches, nausea, restlessness, sweating, vomiting, an increase in heart rate and blood pressure, potentially fatal pulmonary oedema (fluid on the lungs), and psychological phenomena such as a feeling of impending doom.
First documented by the toxicologist Hugo Flecker in 1952, the syndrome was named after the Irukandji people, whose country stretches along the coastal strip north of Cairns, Queensland. The first species linked to the symptoms was Carukia barnesi. In 1964 Jack Barnes, another toxicologist, decided to prove that this tiny jelly was the cause of Irukandji syndrome. He captured a live specimen and allowed it to sting himself, his son and a lifeguard. All three victims ended up in hospital!
It was not until the 1980s that researchers realised Carukia barnesi was merely one of several species involved in the potentially fatal syndrome. Although at least nine or ten species have been recognised, only a few have been scientifically described. There are many common names for Irukandji jellyfishes, including Morbakka, Moreton Bay Stinger, Fire Jelly, Tamoya, Broome Irukandji, Darwin Carybdeid, Dampier Irukandji, Outer-reef Irukandji and Halo Irukandji, but it is unclear whether these are all separate species.
Irukandji jellyfishes were once thought to be restricted to the northern waters of Australia. Since then, however, stings have been recorded worldwide, in tropical and much cooler waters as far north as the British Isles, Japan and the Florida coast of the United States. The jellyfishes are now also known from southern Australian waters, and similar syndromes have been described elsewhere in the Pacific. Every summer, more than 60 people are hospitalised with the syndrome.
The medical treatment and public health management of Irukandji syndrome (and of Box Jellyfish stings) is a priority for areas that rely on tourism and for diving-based industries, such as Broome’s pearling industry. The challenge is that various species are involved and some of these are unknown, as are their ecology and breeding times. This makes it difficult to develop antivenom and also to determine guidelines for when it is safest for swimmers and divers to go in the water.
One of the species represented in the Australian Museum collections is Morbakka fenneri, which was described in 2008 by Dr Lisa Gershwin, a visiting researcher. Commonly called Morbakka or Fire Jelly, the species had been known among marine scientists and surf lifesavers for years, but it had never been scientifically described. Gershwin thought the species occurred only in Queensland and northern New South Wales until she recognised and identified specimens collected previously from Sydney Harbour and Botany Bay. Such a wide geographical distribution is certainly unusual, and experts suspect that the smaller form that occurs from Coffs Harbour to Sydney may indeed be a different species altogether.
Further molecular research into these enigmatic jellyfishes may separate species more accurately and completely. This information would help to determine better ways of managing the public health and economic issues associated with Irukandji syndrome.