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Poisonous animals
Cnidarians (Jellyfish, Corals and Anemones)
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General information on cnidarians


Cnidarians (Cnidaria, approx. 7,600 species) and comb jellies (Ctenophora, approx. 100 species) are combined in the phylum coelenterates (Coelenterata). They constitute a peculiar group of animals, as they lack a true mesoderm as possessed by most multicellular animals. Their bodies are radially symmetrical and consist of an outer and inner cell layer (ectoderm and entoderm). Between these layers there is often a thick, gelatinous matrix.

Comb jellies are medically unimportant, as they lack the venom-filled nematocysts (see below) characteristic of cnidarians. The diverse, species-rich phylum cnidarians comprises primarily marine representatives. They can be divided into the following classes:

  1. Hydrozoa (plume-like hydroids, fire corals, medusae, Siphonophora and many others, approx. 2,600 species),
  2. Scyphozoa (jellyfish, approx. 130 species),
  3. Cubozoa (box jellyfish, approx. 20 species),
  4. Anthozoa (sea anemones, soft and hard corals, sea whips, sea fans, approx. 4,850 species).


The anthozoans and most hydrozoans are firmly attached to the sea bottom, while the overwhelming majority of scyphozoans are free-swimming animals. Humans can be harmed by members of all 4 classes – contact with fire corals, plume-like hydroids, hard corals or sea anemones can lead to burning local reactions that heal poorly – but the most dangerous cnidarians, which can cause severe systemic envenoming, are various scyphozoans, cubozoans and also certain free-swimming hydroid medusae (SiphonophoraPhysalia sp.). The most important species in medical terms are found in the class of box jellyfish (Chironex fleckeri and Chiropsalmus sp., as well as Charukia barnesi).

Morphological characteristics

In cross-section through its longitudinal axis it is possible to see the typical, radially symmetrical form of the cnidarian. There is a central hollow space – in sessile forms the "foot", in jellyfish the bell – that forms the digestive region and opens externally through a mouth. Outgrowths in the form of tentacles emerge from this region and direct captured prey towards the mouth.

There is no heart, central nervous system or respiratory organs, but there are basic muscle and nerve fibres. These enable contraction and expansion of the body and tentacles and allow the free-swimming forms to propel themselves by means of rhythmic contractions.

There are many colony-forming species among the hydrozoans and anthozoans, which produce a hard or elastic fan-like skeleton through the secretion of calcium carbonate or scleroproteins (Fig. 4.16 and 4.17). The separate small individuals, which are only visible under close observation, are identical and joined to each other. The best-known examples are the hard corals (Madreporaria). Due to their expansive growth they play an essential role in the formation of coral reefs. Another form of colony building can be found among the medusae. Various groups of individuals take on different functions such as reproduction, digestion, capturing prey or locomotion (Physalia sp.).

Among the hydrozoans there are species that live in the sea as well as in fresh water. With the exception of some polyp stages found in river estuaries, the remaining classes are all sea dwellers. Among the free-swimming jellyfish and hydroid medusae there are small species whose bells have a diameter of only 1.5–2.5 cm (e.g. Gonionemus vertens) and others with a diameter of up to 2.3 m (Cyanea capillata).


Fig. 4.10 Schematic representation of a nematocyst in the various phases of discharge (see the text for a description).

a Resting phase.
b When the cnidocil is activated the lid opens.
c The barbed ring is ejected first.
d Final stage with everted venomous thread.


Cnidarians possess small microscopic nematocysts (Fig. 4.10). Along with nematocysts that have sticky threads or lasso-like strings, some cnidarians have venomous nematocysts. They are used to capture prey and offer these otherwise defenceless animals a valuable means of defence. There are countless nematocysts scattered over the external cell layer of the tentacles or sometimes also on the bells of jellyfish. They contain specific venoms; in the resting position they are closed by a lid (operculum) and contain a hollow thread (Fig. 4.10a). At one end it leads into a barbed ring, directly below the lid. Mechanical stimulation of the so-called cnidocil causes a rapid and massive flow of water into the nematocyst. The lid springs open and the barbed ring is discharged under high pressure and penetrates the protective shell/skin of the prey or enemy (Fig. 4.10b and 4.10c). The hollow thread then follows, so that the interior surface now faces outwards and the venom contained within is injected into the microscopically small wound (Fig. 4.10).

Contact with cnidarians causes the discharge of countless nematocysts that leave painful and usually clearly visible sting marks over the area of contact. With some of the most dangerous species, these marks can form characteristic patterns that may serve as features for identification. In Figure 4.11 (see below) the typical sting marks from Chironex fleckeri, Physalia sp. and Carukia barnesi are shown.

Fortunately the penetrating power of the nematocysts in many species is too weak or the venomous thread too short to penetrate human skin. There are estimated to be several billion nematocysts on a single jellyfish among the larger species! If there is contact with dangerous species over a large area of skin, a greater dose of venom penetrates through the skin, and the wide area of distribution in the well-supplied hypodermis means it can rapidly enter the blood circulation.

Nematocysts are generally not able to penetrate the thick skin on the palms of the hands and the soles of the feet. It has been observed that hair on the skin hinders penetration by nematocysts. In theory, this would mean that women should be more severely affected by stings than men. In any case, it is proven that the delicate skin of children offers poorer protection than that of adults.

The production of nematocysts is only known to occur in cnidarians; however, they are found in a number of other animals that acquire them from eating cnidarians as prey. These kleptocnidae ("stolen nematocysts") are known to occur in several species of sea slugs (nudibranchs), freshwater turbellarian flatworms, comb jellies and sponges and are used in self defense. After aquisition from the prey cnidarians aeolian nudibranchs store the nematocysts in special appendices, the so calld cerata. The mechanism by which the nematocysts can be ingested and stored undamaged in special organs of the host animals has not yet been conclusively established. Greenwood (2009) gives an overview on kleptocnidae.



Fig. 4.11

Ecological, morphological and medical characteristics of dangerous jellyfish in the Indo-Australian region.

a Chironex fleckeri lives primarily in flat coastal regions. Stings leave typical marks with a ladder-like "cross-hatching" pattern.

b Physalia sp. sail on the surface of the water and are drifted into waters near the coast. The tentacle marks are whip-like.

c Carukia barnesi is a thumb-sized jellyfish that lives in open water. They may enter reefs after strong storms. Stings are caused by the bell and are barely visible. Accidents with these jellyfish often occur in divers, and only very small areas of the skin that are not protected (throat, face) are affected.

Range of venom effects

See the Clinical flowchart for cnidarians.

Way of life

The most diverse forms of cnidarians have developed in tropical reefs, where many sessile forms characterise an entire ecosystem. Several of the free-moving species live close to the coast, others live at various depths throughout the open sea or "sail" long distances over the surface of the water, such as the Portuguese man-of-war (Physalia sp.).

The reproductive cycle of the coelenterates is remarkable. In general there is a generation change between sessile polyps, which reproduce vegetatively, and swimming medusae, which reproduce sexually. There are differences in the emphasis on the one or other generational form. Thus in the hydrozoa the emphasis is on the polyp stages, with some exceptions, and in the scyphozoa it is on the medusa stage. Anthozoans do not pass through a medusa stage. They release reproductive products into the water, which then develop directly into sessile adult animals again.


There is no doubt that the medical significance of cnidarians, in particular jellyfish, is underestimated throughout the world, in respect to both morbidity and mortality. With the exception of a few regions, such as the coastal waters of Australia, knowledge of the distribution of cnidarians and their medical significance is quite fragmentary. Moreover, in many accidents it is only possible to speculate about the cause, as the victim can often only give an insufficient description of the animal involved or may not even have seen it. The International Consortium for Jellyfish Stings, a group of experts who have occupied themselves with the problem of cnidarian stings for a number of years, are trying to document cases of envenoming due to cnidarians worldwide, among other activities. All case reports are recorded by P. Fenner, J. Williamson and J. Burnett according to a standardised method and published at regular intervals in their communications.

Victims of cnidarian stings are most frequently swimmers or divers in coastal waters, and cnidarians are among the most common causes of accidents with venomous marine animals.

Contact with sessile cnidarians and many types of jellyfish usually only results in unpleasant local symptoms. That said, there are several species that quite frequently cause more serious local effects, such as skin and soft tissue alterations that heal slowly and can result in scarring or even lasting damage (above all on the extremities). Apart from anaphylactic reactions, severe systemic envenoming is only caused by a few tropical and subtropical jellyfish. Besides Physalia physalis, which is known to have caused at least 3 fatalities on the East Coast of the USA, it is the box jellyfish that causes the most serious envenoming, the most dangerous of which is Chironex fleckeri. Recorded data on Chironex fleckeri show that to date, this most dangerous of all venomous marine animals has caused 90 fatal cases in the tropical Indo-Pacific of Asia and Australia, 70 of which occurred in the tropical waters of Australia. A closely related species, Chiropsalmus quadrigatus (now known as Chiropsella quadrigatus), has been held responsible for fatalities in the tropical waters of Asia and New Guinea. One verified fatality due to the Atlantic species Chiropsalmus quadrumanus occurred on the coast of Texas in the USA. Finally, a fatality due to Stomolphus nomurai was reported in southeast China.

Severe systemic envenoming can also be caused by the tiny Carukia barnesi. The Irukandji syndrome caused by this jellyfish has been observed primarily in Australia, but a similar syndrome can also be caused by Stomolphus nomurai, the genus Goninemus sp., which is found throughout the world, and many others (Warrell and Fenner 1993).


Precautionary measures are primarily aimed at swimmers and divers in warm to tropical coastal areas.

  • Follow the local warnings in areas where dangerous jellyfish occur.
  • In the tropical coastal waters of Australia, there are safe bathing areas surrounded by floating jellyfish nets. Do not swim beyond these nets.
  • Diving suits and the thinner "stinger suits", which were specially developed for bathers and swimmers and are available in sport shops in Australia, offer good protection against stings to the body. In addition, the susceptible areas on the throat and face should be protected with a hood. 
  • Recommendations for persons who have already been sensitised to jellyfish venom (Therapies).

Cleland and Southcott 1965, Endean 1982, Fenner et al. 1993, Heeger 1998, Halstead 1988, Mather and Bennet 1984, Storch and Welsch 1997, Williamson and Exton 1985, Williamson et al. 1987, 1996

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