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Why are Box Jellyfish toxic?

Historically, there have been two main theories as to why cubozoans (box jellyfish), are toxic.

  1. Box jellyfish are fragile animals that must subdue their prey quickly to prevent damage to themselves.
  2. Box jellyfish require potent toxin in order to protect themselves from predators, such as turtles.

Both of these theories appear to have little support.

It would appear that the more fragile species are less toxic than some of the more robust species. Turtles and various species of fish are known to consume species of cubozoan with little or no effects, therefore as a defensive role the venom would appear relatively ineffective.

Recent research has shown that venom may instead be related to the feeding ecology of box jellyfish. With the type of prey or predator that the venom is targeting, influencing the toxicity levels of marine invertebrates. Thus venom would directly reflect the ecology of the animal and its specific function.

For more information on this topic visit the research page


How does the toxin enter the body?

Venom is delivered by nematocysts (see image below), which occur primarily on the tentacles, but in some species may be present on the bell (body) as well.

Nematocysts consist of a bulb containing a coiled shaft.

Nematocyst: bulb and coil

Activation relies on one or more chemical cues or mechanical stimulus to fire. When activated, the spine-covered shaft everts and penetrates the cuticle of the organism being envenomed. Venom flows from the bulb, down the shaft and into the victim.

Nematocyst comparison

The image above shows two nematocysts. The top cell is intact, with the bulb containing the coiled shaft and toxin. The lower cell has been activated and has fired out the shaft like a hypodermic needle, penetrating the victims flesh and delivering the toxin from the bulb.

Nematocysts are highly specialised structures showing great variety both morphologically and functionally.

They come in all shapes and sizes

These stinging cells not only function as venom injectors but to grapple at the integument of the prey and serve as an adhesion devise. In this way multiple tiny injected doses of venom, applied over a wide area can be achieved, with toxin being injected directly into the blood vessels of the victim.

fired nematocysts with venom droplets

The image above shows a series of fired nematocysts still attached to the tentacle. The vivid blue circles that you can see on the end of the fine threads are drops of venom. The threads themselves are the everted shafts of the nematocysts.

To see a short video (16s) of real nematocysts discharging from a tentacle, click on the download video image below to go to the video clip page. Here you can choose your video size and explore some of the other interesting video clips.

Download video

While the variation between the morphology and function of these nematocysts is vast, the underlying mechanisms are the same.

Discharged nematocysts

A species of cubozoan may have several types of nematocysts. Together, the complete complement is termed the cnidome.

Preliminary research into the cnidomes of different species of cubozoans suggests that they may be species specific. It has also been suggested that these variations reflect differences in the feeding ecology of the animals, with different types of nematocysts in some cnidarians containing different types of venom and being used for vastly different processes during prey capture.

The toxins involved in causing Irukandji syndrome are unknown. At least 20 different types of proteins are present in Carukia barnesi venom.


More information available soon

How jellyfish stingMyths and facts




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Content by Jamie Seymour, Teresa Carrette and Michael Pieloor. Updated 02-Dec-2002 by Michael Pieloor
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