Cnidaria include organisms such as jellyfish, sea anemones and corals. They are primitive coelenterates i.e. bag-shaped organisms with only one body opening. They have a mouth and an abdominal cavity, but no anus. Usually tentacles are present around the mouth. This is a very old phylum. Fossils have been found in rocks from the late Precambrian period. The four present-day classes have existed since the Ordovician period. All Cnidaria are carnivorous. Certain jellyfish and sea anemones are eaten in some countries such as Japan and Korea. Some corals are used in jewellery. Every year numerous swimmers have skin contact with these creatures, leading to local skin irritation with or without serious systemic effects. Every year a small number of people die because of these creatures. In view of the increase in the number of travellers to exotic regions, a doctor working in Europe can expect problems which must be viewed in this context.

Cnidaria are formed from an external ectoderm or epiderm and an internal endoderm or gastroderm. They are diploblastic or formed from two germ layers. Between these two layers is a gelatine-like mass, the mesoglea. This gives them a blubber-like consistency. Muscle cells are found in the innermost and outermost layers. They provide propulsion. There is also a primitive network of nerves. There are two basic forms in the phylum: (1) a polyp, which is fixed to the seabed with one foot, like a sea anemone, and (2) a free-swimming jellyfish (medusa). Sometimes these life stages follow one another (this is called metagenesis). Many polyps together may form a colony, as is the case with many corals and siphonophores. In sexually reproducing species, after fusion of male and female gametes a small planula larva is formed. At this stage it is a small ciliated, pear-shaped creature which over the course of time will come to rest on a solid substrate. After metamorphosis a polyp is produced. This may reproduce asexually via budding or branching outgrowth (stolonic growth). At a certain moment, depending on the size of the colony and/or external environment influences, asexual small jellyfish will form. They are free-living and in the course of time will produce gametes in their turn. There are many variations of this general life cycle.

The Cnidaria encompasses four taxonomic classes:

• Hydrozoa
• Anthozoa
• Scyphozoa
• Cubozoa

This class includes animals such as the small Hydra, which spends its whole life as a polyp on the bottom. The creature can move itself by performing a somersault. Other Hydrozoa may go through a jellyfish stage, characterised by the presence of a membrane (velum) at the mouth, by which they can be differentiated from true jellyfish. Some species form colonies. These may be either pelagic (floating) or benthic (fixed to the bottom). Such a colony consists of countless individuals where each can have its own speciality, unlike true jellyfish. One example of a floating colony is the Portuguese man-of-war, Physalia physalis. This animal is a siphonophore. The creature has no propulsion of its own and is moved passively by the wind and ocean currents. Some Hydrozoa colonies are sessile (they are fixed to a solid substrate). One example of this is fire coral, which belongs to the Milleporidae, e.g. Millepora tenera, M. platyphylla and M. alcicornis. They have a solid calcareous skeleton. These polymorphous colonies have a similar appearance as "normal" coral. Skin contact produces severe local irritation quite quickly. Organisms such as Aglaophenia cupressina and Lytocarpus species (Plumularidae) can also cause lesions after contact with bare skin.

This class includes true corals and anemones. In the class of the Anthozoa there are two subclasses: Octocorallia and Hexacorallia. This refers to the basic symmetry of their body structure. The Octocorallia which includes the soft corals (order Alcyonacea), sea pens and sea pansies (order Pennatulacea), sea fans, whip corals, organ-pipe corals (Stolonifera), blue coral (order Helioporacea) and horny corals (order Gorgonacea) are not dangerous to humans. If there is a current, a sea fan is typically at right angles to the direction of the current in order to have the largest filter surface. The Hexacorallia include the Scleractinia (madrepores or hard corals) which help to provide the splendour of tropical coral reefs, the Zoantharia (cf. Palythoa), the Corallimorpharia, the Antipatharia or black corals, the sea anemones or Actinaria and the medically insignificant Ceriantharia. Skin contact with certain Actinodendron or Dofleinia species of sea anemone may cause death. Actinodendron plumosum is also known as fire anemone. Madrepore corals or stone corals are the best known corals from reefs. They never have a jellyfish stage. Their stinging cells cannot penetrate human skin. Corals form three types of reefs: (1) close to the coast (fringing reefs), (2) barrier reefs which are separated from the coast by a channel and (3) atolls, ring-shaped islands around central lagoons. Corals without symbiotic algae do not perform photosynthesis and may occur down to 3,000 metres. Those which do perform photosynthesis depend on very pure, shallow, clear and sufficiently warm sea water, which is why they only occur in some tropical regions. In countless reefs nowadays, the corals are affected by a number of diseases, coral bleaching being the best known. The problem is so severe and wide-spread that we are witnessing a mass extinction event of gigantic proportions.

The size varies from 1 cm to 2 metres in diameter. If there is a polyp phase, the polyp forms a jellyfish via a very typical method of reproduction (strobilation). The digestive cavity has four partitions, which gives them a four-fold symmetry.

Cubozoa are similar to classic jellyfish, but differ in their more square shape and the four groups of tentacles. There is no strobilation in the life cycle. The jellyfish develops directly from the polyp stage. The Australian Chironex fleckeri or sea wasp is the best known representative of this group. It is a potentially deadly creature.

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Many Anthozoa, Hydrozoa and Scyphozoa have symbiotic unicellular algae. Symbionts belonging to the Dinoflagellata are also known as zooxanthellae. They occur exclusively in marine animals. Zoochlorellae are symbiotic green algae which occur in both sea water and freshwater species. In some species the symbionts can produce toxic substances.

Aequoria aquatica (= A. victoria) is a jellyfish or hydromedusa which lives in the cold waters of the north Pacific. The animal measures 5-10 cm, rarely up to 20 cm. It contains a bioluminescent protein -aequorin- that emits blue light. It also contains a second protein: green fluorescent protein (GFP). It is GFP which converts the blue light given off by aequorin to green light. The fluorescence in a living animal occurs at the rim of the animal's bell. It does not produce a soft overall glow as some photographs might show (do not mistake the reflection of a flash for fluorescence). The purpose and advantage of this bioluminescence are not clear. GFP is a cylindrical protein which contains 238 aminoacids. It carries a central fluorophore which does not contain a chemical prosthetic group but consists of a few specific aminoacids. Solutions of GFP look yellow under typical room light, but glow bright green in sunlight. The protein absorbs ultraviolet light from the sunlight, and then emits it as lower-energy green light. The gene has been cloned. This opened new avenues of investigation in cell, developmental and molecular biology. Fluorescent GFP has been expressed as a functional transgene in bacteria, yeast, slime mold, plants, worms, insects, zebrafish and in mammalian cells, even in living rabbits (cfr the "GFP bunny"). The flexibility as a noninvasive marker in living cells allows for numerous applications.

Cnidaria contain stinging cells, called cnidocytes. The stinging apparatus or cnida (plural cnidae) is a complex structure formed by the Golgi organelle in the cnidoblast. These stem cells begin by producing the stinging apparatus and then migrate to their final destination in the body of the animal. There are three basic forms of cnidae: nematocysts (venom), spirocysts (adhesive, only in Anthozoa) and ptychocysts (only in Anthozoa, in the order of Ceriantharia). After mechanical or chemical stimulation nematocysts eject a long pointed thread which contains venom (Gr. cnidos = thread). This penetrates the prey at high speed and administers the toxins as an injection. There are 16 kinds of stinging cells, depending on whether the stinging thread is open or closed, whether there are lateral spikes and/or a shaft and also depending on diameter. The ejection of the stinging cells is triggered by locally increased concentrations of certain ions, but is probably also under partial neurological control of the animal (animals which have eaten recently sting much less easily). Ejection of the stinging cells occurs at high speed and is one of the swiftest biological processes known (several times the speed of sound). Some animals such as sea snails (Nudibranchia) can eat stinging cells without their being fired. Even more unusual is that the cells then migrate through the body, appear on the surface of the animal and are used for its own defence. These animals often have a beautiful appearance with fierce warning colours.

• Sea bather’s itch or planula dermatitis (Gr. planos = roaming; cfr planets = "wandering stars"). At certain times of the year certain Cnidaria simultaneously produce massive amounts of planula larvae. The sea anemone Edwardsiella lineata and the thimble jellyfish Linuche unguiculata are common causes. The planula larvae carry stinging cells. When divers swim during such a period, the larvae come into contact with the skin, and may cause local irritation. When the person is back on board or ashore and dries him/herself, most of the larvae are removed without having fired their nematocysts. But often people do not dry themselves under their swimwear, which means there is longer contact and an itching rash occurs on the covered parts of the body. Differentiation from cercariae dermatitis is easy because the latter only occurs in freshwater (cf. schistosomiasis).

• Palythoa. Certain corals of the genus Palythoa contain an extremely powerful neural and cardiac toxin in their tissues: palytoxin. The chemical structure is that of a polyketide with a molecular weight of 2700 Dalton. It is one of the most powerful non-protein poisons known (more powerful than tetrodotoxin and saxitoxin). Sometimes divers experience tingling and paresthesia when they swim in closed pools which contain large numbers of Palythoa corals. These animals are not remarkable at first sight. Palythoa toxicus is a small creature measuring 9 mm, with no striking colour or shape. Palythoa caribaeorum and P. vestitus were formerly used as a source of poison for hunting.

• Sea wasp, also known as box jellyfish or Chironex fleckeri. This box jellyfish is found in the coastal waters of northern Australia (from Gladstone in Queensland to Broome in Western Australia), but not in the Great Barrier Reef. It is the most dangerous jellyfish known to man. Usually this jellyfish measures approximately 10-15 cm, but larger animals, up to 30 cm also occur. They may weigh up to 6 kg. The animal is transparent in the water, which makes it difficult to see. The tentacles are 1-2 metres long. It has 4 main bundles of tentacles (pedalia), which split to form about 60 finer tentacles. Each tentacle bears many millions of nematocysts, which are fired after contact with the skin. Massive injection of venom may follow. The result is acute intense local pain and 5-10 mm wide red weals on the skin at the site where the tentacles came into contact. A cross-hatching pattern of the skin lesions is typical. Confusion, agitation, syncope and collapse with respiratory and cardiac arrest may follow (sometimes within 5 minutes). Every year people die because of this jellyfish. Most cases occur in children, in shallow water. When humans survive there is often skin necrosis with permanent scars. In view of the speed of the symptoms, first aid is literally of vital importance. Once on dry land, any nematocysts still present on the skin which have not fired, need to be neutralised with large amounts of diluted acetic acid (e.g. kitchen vinegar). Antiserum may be administered IM by paramedics on the spot, but it is better to give this IV. It is based on purified sheep immunoglobulins. Its effectiveness has been demonstrated by in-vitro neutralisation tests, by tests on animals and in clinical practice. Antiserum reduces the systemic effects, the pain and the dermatonecrosis. Antiserum is given for all lesions, except for small lesions on parts of the body which are of no cosmetic importance. If no antiserum is available, the pressure immobilisation technique should be applied after inactivation (see chapter on neurotoxic snake bites). This is easier said than done, however, due to the fact that sometimes large body surfaces are affected. When the victim reaches hospital mechanical respiration and narcotic analgesics may be necessary. As prevention, it is advisable not to swim in endemic regions from September to March. It is also better never to swim alone and to wear special "stinger suits". Even when swimming on beaches which are protected by anti-box jellyfish nets, there is no protection from irukandji stings (see below).

• Chiropsalmus quadrigatus. This jellyfish is similar to Chironex fleckeri, but smaller (7 cm) and has short tentacles, rarely more than nine in number. The venom has dermatonecrotic and haemolytic properties, but the amount injected is much less than that of the box jellyfish. It is true that stinging results in severe pain and hypotension, but the subsequent course is less severe than in C. fleckeri. A few exceptional deaths have been reported from the Philippines, but none from Australia. Residual scar formation is minimal. In experiments antiserum to Chironex also neutralises the venom from Chiropsalmus quadrigatus, but there are no further clinical data.

• Carukia barnesi. This small jellyfish measures 2 cm and is responsible for an unusual and dramatic illness, known as irukandji syndrome. It is possible that other cubozoa are also able to provoke the syndrome, but little is known about these animals. It occurs in northern Australia, chiefly northern Queensland. Unlike Chironex fleckeri, Carukia barnesi occurs in deeper waters of the reef. The stinging itself produces moderate pain, with little associated tissue damage. Approximately 30 minutes later the patient develops a complex of symptoms, including severe back and abdominal pain, pain in the limbs and joints, nausea, profuse sweating and agitation. Paresthesia, local goose bumps (piloerection), hypertension and tachycardia are common, probably as the result of endogenous catecholamine release. It is best to hospitalise the patient, administer painkillers (often opidoids) and begin antihypertensive treatment, e.g. with phentolamine (alpha blocker). It is possible that IV magnesium sulfate is useful, but further study is needed. Transient dilated cardiomyopathy has been described and it is advisable to carry out serial echocardiography in patients with severe symptoms. In rare cases patients die due to these small creatures. Lethal cerebrovascular accidents can occur. It is still unclear whether vinegar neutralises the jellyfish’s nematocysts. Skin scrapings permit identification of the nematocysts.

• Physalia species (Portuguese man-of-war). This is not a true jellyfish, but a floating colony of polyps. Physalia physalis is found in the Atlantic ocean and the smaller Physalia utriculus, known as bluebottle, in the Indopacific. The genus is recognised by a float or pneumatophore, filled with gas, including carbon monoxide. This float is approximately 15 cm long. The animals sometimes rock from side to side, alternately immersing one or the other side to prevent drying out. The tentacles of P. physalis can be up to 10 metres long. P. utriculus only has one long tentacle. Physalia physalis occupies a special place in the history of medicine. It was through experiments with this species (and also with Anemonia sulcata), that two French oceanographers, Richet and Portier, discovered the phenomenon of anaphylaxis in 1902. While searching for the basis of immunity, they discovered hypersensitivity. This is a basic concept in allergic phenomena. It won them the Nobel prize in 1913. Stinging results in local pain and skin lesions, headache, nausea, vomiting, abdominal pain and rarely collapse. During First Aid the tentacles should be removed mechanically (with tweezers). Do not rub the skin with sand as more nematocysts will be stimulated. The as yet unstimulated nematocysts on the skin may be stimulated with vinegar, making the lesions worse, but there is still doubt about this. If using vinegar for unknown jellyfish stings it is always advisable to first try out the fluid on a small area of skin (30’’) to evaluate the reaction.

• Stomolophus nomurai is a large jellyfish which is found in the Yellow Sea between China and Korea, and other places. Stinging by this creature is characterised by pulmonary oedema which occurs within 2-24 hours. It has been responsible for several deaths.

• Minor problems caused by jellyfish. Carybdea rastoni (jimble) is a small jellyfish, measuring 2 cm in diameter and has 4 tentacles which may be 30 cm long. The species is widely distributed in warm seas. Stinging produces only moderate pain with local swelling and erythema. This may persist for several weeks. No deaths are known. Pelagia noctiluca (mauve stinger) is a many-coloured toadstool-shaped jellyfish, which measures 12 cm and is responsible for brief local skin irritation. After contact, often a luminous slime remains on the skin. There are no systemic symptoms. Aurelia aurita (moon jelly) is saucer-shaped with a typical figure 8 pattern on the upper side (reproductive structures). It has short tentacles. The creature measures up to 50 cm in diameter. Chrysaora hyoscella is transparent whitish yellow. It is easily recognised due to the brown lines which stream out from the centre, which explains its common name, the compass jellyfish. It has a diameter of 25 cm. Cyanea sp. (sea blubber or hairy stingers) are widely distributed, even as far as the Arctic waters. They measure 30 cm (Cyanea lamarckii, Australasia) or larger, up to 2 metres (Atlantic specimens of Cyanea capillata). Due to its mass of tentacles the creature is also called lion’s mane jellyfish. The Atlantic species is a transparent bell with a reddish brown centre. In the Indopacific the creatures have a purplish pink body. The consequences of stinging include local pain, nausea and abdominal pain, sweating, muscle cramps and dyspnoea. No deaths due this species have been reported. Vinegar here causes stimulation of the nematocysts and must not be used. The same applies to Chrysaora species. It is better to use sodium bicarbonate solution or ammonia. In the absence of anything better, urine may be used, but how effective this is, remains an open question.