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Jellyfish are not fish. The are actually invertebrates which means they do not have a backbone. The jellyfish is made up of ninety-five per cent water.

Known as Scyphozoa, jellyfish are individual animals that function in similar ways to the hydrozoan 'jellyfish'. All jellyfish have stinging tentacles but the stings vary from severe to a mild and temporary prickling or burning sensation.

Comb Jelly / Phylum Ctenophora
 Box Jellyfish / Chironex fleckeri / Family Chirodropidae

Recognising jellies

Jellyfish are most recognised because of their jelly like appearance and this is where they get their name. They are also recognised for their bell-like shape and tentacles. Jellyfish species vary in size, some reaching two metres across the bell and trailing tentacles 30metres behind them. Some have bodies that are so clear you can see thought them.

Some jellyfish have a thick fluffy-looking frill between the upper bell and the lower tentacles, which actually contain countless mouths to trap small organisms from surrounding waters.

The upside-down jellyfish betrays its close relationship with the anemones by attaching itself, tentacles up, to sandy and muddy bottoms, often in the vicinity of mangroves. Like so many other cnidarians, this jellyfish farms zooxanthellae in its tissues.


Jellyfish live in the ocean. Jellyfish drift at the mercy of the currents, often accumulating in sheltered bays and estuaries. They swim in an unusual way, a pumping action. Muscles in their bodies contract and propel them through the water. This does little more than allow them to move up or down in the water. However, the four sided box jellyfish are powerful swimmers with good control and speed.


Jellyfish reproduction is often complex involving two different body forms, the attached polyp and the free-swimming jellyfish-like medusa stage.

Most jellyfish have a two-part life cycle. The first stage is the familiar free-swimming bell-like stage. The polyp is the lessor known stage, which forms just after egg and sperm combine. The polyp stage is normally very small and not free-swimming.

Prior to summer the polyp starts to shed parts of its body, each of which grow to form the swimming jellyfish stage.


Depending on the number and length of tentacles, jellyfish feed in various ways. Most catch their food sith their tentacles as it drifts by and is then carried to the mouth, which is located in the centre of the bell shaped body.

Some jellyfish eat small crustaceans and fish, some like minute planktonic organisms and some are like coral that have algae in their tissues which produces food by photosynthesis.

Box Jellyfish

The box jellyfish is the most dangerous marine animal to humans.

Often called the sea wasp or marine stinger, the box jellyfish (Chironex fleckeri) is the most dangerous of marine animals. It inhabits the shallow coastal waters around northern Australia during the summer months between October and April. The further north you travel, the longer the jellyfish season can be and in Darwin, the box jellyfish is present almost all year round.

The body or bell, which is box-like in shape, can vary in size from 5-25 cm across and has groups of tentacles (up to 15) at each of the four corners. In an adult the average length of tentacles is about three metres.

Jellyfish move in the water by filling their bell with water and then forcing it out again and they can travel at about 2 knots (as fast as your normal walk). Their tentacles, which are covered in millions of stinging cells or nematocysts, drag along behind them as they move. The box jellyfish does not actively hunt but relies on food to bump into it. This is how humans are usually stung when swimming or wading in shallow water. The jellyfish are difficult to see as they are almost invisible in the water. When an animal (or human) comes into contact with the tentacles the stinging cells are triggered and fine, hollow threads pierce the victim allowing the venom (poison) to enter. The tentacles attach to the victim with the help of a sticky material produced at the same time.

What to do if stung by a jellyfish

If possible flood the sting area with vinegar, which will stop the firing of the stinging cells. Do not rub the sting area with hands, a towel or sand. If breathing stops begin resuscitation. Keep the affected area still and seek medical assistance.

What you can do to prevent being stung

  • Swim at protected beaches with safety nets.
  • Swim in areas patrolled by lifeguards.
  • Avoid swimming in the mouths of creeks, fresh or salt water, where jellyfish are likely to be in summer.
  • Wear protective clothing such as a stinger suit or pantyhose (the tentacles which normally pierce the skin are too short to pass through the pantyhose or stinger suit).
  • Don't move too quickly in the water. Splashing and running will cause the water to swirl and increase the chance of the tentacles touching the skin.
  • Take first aid materials with you to the beach, particularly vinegar.