Jellyfish

A jellyfish is a life stage of cnidarians (Cnidaria). Jellyfish themselves are not a systematic taxonomic group. They are among the oldest animals in the history of the earth. More than 2,600 species are known.

For many jellyfish, a generation change is typical, in which a fixed stage (polyp) and a free-moving medusa or jellyfish stage regularly alternate. The polyp produces free-swimming medusae by asexual means, e.g. by sprouting, which form a larva by sexual reproduction (via germ cells), which settles and differentiates again into a polyp. The free-swimming developmental stage, the actual jellyfish, is therefore only part of the life cycle of a species.

This life cycle is typical for cnidarians of the classes Scyphozoa (umbrella jellyfish), Cubozoa (cube jellyfish) and Hydrozoa.

Most jellyfish have long tentacles that are equipped with cnidocytes. They use these to catch prey and for defense. The cnidocytes (nematocytes) are scattered over the tentacles and contain highly effective toxins. They have a sensory hair on their surface that is directly connected to the toxin-containing cnidocyte capsule. When touched, the nettle capsule explodes. An internal stylet-like apparatus is shot out at a pressure of 150 bar and anchors itself in the prey's tissue. Within a few nanoseconds, the toxin contained in the nettle capsule is injected. In most cases, the nettle thread is also studded with spines. After use, the nettle capsules are shed and reformed.

The nettle toxins are proteins (cytolysins, neurotaxins) that have a cell or nerve-damaging effect. The clinical picture of nettle infestation (see Dermatitis medusica below) depends on the duration, extent (area of infestation) and intensity of contact with the nettle organs as well as the physical constitution of the person affected (age, weight, state of health).

Accidents with jellyfish are usually treated on the spot. In medical facilities, patients are seen with stinging from vacation countries that generally occurred some time ago.

Flower animals (Anthozoa)

With over 7000 species, this is the largest class of cnidarians. The best-known representatives are the sea anemones, which grow as "flower-like" polyps on solid ground. They are found in all seas from shallow water to depths of 10,000 meters. The neurotoxins of this jellyfish species are harmless to humans.

Umbrella jellyfish (scyphozoans)

Umbrella jellyfish also occur in large numbers in the North Sea and Baltic Sea. The most common representatives here are the box jellyfish and the fire jellyfish. One of the many aquatic (marine) skin diseases is a"seabather`s eruption" caused by planular larvae, an early, free-swimming (planktonic) developmental stage of marine umbrella jellyfish (Baaske KM et al. 2025). The strong cnidarian venom in the tentacles of these jellyfish causes welt-like skin reactions, not often accompanied by blisters and blisters, including skin necrosis.

Box jellyfish

Visitors to the North Sea and Baltic Sea often see these harmless jellyfish with their large umbrellas. The venom of the box jellyfish does not cause a noticeable skin reaction in humans.

Yellow hair jellyfish (fire jellyfish)

This jellyfish species occurs in the North Atlantic, the North Sea and the western Baltic Sea. It is characterized by its yellow-orange umbrella and long tentacles. The stinging leads to painful redness and itching; blistering reactions or cardiovascular problems are less common.

Cube jellyfish (Cubozoa)

Their cuboid or cube-shaped form gives them their name. Their tentacles can be up to 3 meters long. They inhabit all tropical and subtropical seas worldwide and are fast swimmers. Occasionally they have also been found off the Portuguese coast and in the Mediterranean.

Carukia barnesi

Carukia barnesi, a very small cube jellyfish whose umbrella is barely more than 1.0 cm in size, triggers Irukandji syndrome on contact with the skin, with severe back pain radiating abdominally for days to weeks. No deaths are known

Sea wasps (Chironex fleckeri)

The best-known representative of the cube jellyfish is the sea wasp , which occurs frequently on the north and east coast of Australia. The sea wasp is considered to be the most poisonous marine animal of all. Every year, more people die from sea wasp envenomation than from shark attacks! The neurotoxic poison leads to paralysis of the skeletal and heart muscles and, depending on the extent of the poisoning, to cardiac arrest.

The sea wasp season lasts from November to May. During this time, bathing in seas rich in sea wasps should be avoided. Sea wasps washed up on the beach should not be touched. The cnidocytes remain intact for weeks, even when the jellyfish has dried out.

Hydrozoans

The most important representative is the Portuguese man o' war (Physalia physalis), which lives off Australia and on the coasts of the Pacific and Atlantic. Its stinging venom can be fatal in extreme cases in weakened people or children. Even if the shape of this jellyfish species is reminiscent of a classic jellyfish, it actually consists of a whole colony of different polyp colonies. Physalia physalis is mainly found in tropical and subtropical regions (also off the coasts of the Canary Islands, Portugal, northern Spain and in the Mediterranean off Mallorca and Ibiza). Its tentacles are up to 30 m long.

Contact with sources can lead to entanglement of different types and strengths; see below Dermatitis medusica (Jellyfish dermatitis).

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