Jellyfish represent some of the most well-known sea creatures. Seen from boats or the shore, they are easily recognised by their blubbery appearance. But how much do we known about jellyfish? Did you know that jellyfish are an important food item in some parts of the world? Did you know that jellyfish also have a stage in their life cycle when they are attached to the bottom of the sea and look more like a sea anemone? The truth is that while jellyfish are the most recognised sea animal, people know very little about them!

• To be a cnidarian that spends most of your as a medusa (the stage that looks like a typical jellyfish), floating in the ocean.
• To eat by collecting particles out of the water (suspension feeding) or by actively catching prey using tentacles.
• To have a tiny polyp stage that lives on the bottom of the ocean.

Jelly blubber, jellyfish, scyphozoan.

Jellyfish, as the name suggests look a lot like jelly. They have two major parts to their body design. The first is a muscular bell or disk that is rhythmically contracted to push the jellyfish through the water. The second is a set of tentacles attached to the bell which are covered in stinging cells. These stinging cells are quite powerful and can stun fish in those species that hunt fish. Jellyfish are usually clear or coloured blue to avoid detection by predators. Jellyfish come in a range of sizes. Usually, they are about the size of a shoe box or smaller. Some species, like those that live in cold Antarctic waters, can get to be the size of a small car!

Jellyfish are found in every ocean of the world. They are usually pelagic and swim throughout the water column in search of food. They are not usually associated with reef like structures, and prefer the open ocean. Often they show daily patterns of migration between the surface of the ocean and the deep, remaining deeper by day and migrating to the shallows at night. While most jellyfish swim all the time, there is at least one species that can be found resting on the bottom. This is the "Upside Down Jellyfish" (Mastigius) that is found on many coral reefs. It has the same dinoflagellate protists (zooxanthellae) living in its tissues that reef-building corals have. Because it lives off the photosynthetic products from the zooxanthellae, it sits on the bottom with its tentacles in the air like a sea anemone, making sure it's hard-working symbionts have enough light to do their work. Is this the jellyfish version of a couch potato?

What jellyfish eat is very much dependent on the particular species in question. Some jellyfish are carnivores that hunt either fish, zooplankton or larger invertebrates. These species tend to have powerful stinging cells in their tentacles. Other species are suspension feeders that collect the tiny particles from the water around them. These species have hardly any sting in their stinging cells, but have very sticky tentacles. The Upside Down Jellyfish depends on food from its zooxanthellae and must always have sunlight during the day, otherwise it starves.

Jellyfish may eat other animals but they are not immune from being eaten themselves. Fish probably are important predators on jellyfish, but guess what? One of the major foods of sea turtles in some parts of the world are jellyfish! In the open ocean, jellyfish are probably one of the most important parts of the food chain - just because they are so abundant in these environments. Jelly fish can number in the hundreds in a single square metre. Probably the greatest surprise of all to European Australians is the fact that jellyfish are a delicacy in many parts of the world. That is, humans are an important predator of the "ole jelly blubber"!

Jellyfish grow extremely rapidly, up to 30 cm across the bell in about 12 months. They start life as a tiny polyp, looking like a tiny sea anemone. This tiny polyp then produces a series of tiny jellyfish (<2 mm across) called medusae that swim toward to surface of the ocean. Here they grow into the larger jellyfish we are all familiar with. When mature, jellyfish produce eggs and sperm that they release to the water column where fertilisation occurs. The result is a larva called a planula that eventually settles on the bottom of the ocean and forms the tiny polyp stage and the cycle restarts. Many jellyfish live only one year or less (annual). Others, like the huge jellyfish in the Antarctic, probably live to greater ages and might be as old as 5-10 years old.

Jellyfish often have symbiotic fish and prawns that live among their tentacles. These organisms are immune to the sting of the tentacles of the jellyfish and are probably tolerated because they do a bit of house cleaning in return for the protection of the jellyfishes tentacles.

Some jellyfish (eg. those that hunt fish) have stings that can affect humans. Others (suspension feeders) are relatively harmless. Although they are fished in many parts of the world, a fishery based on jellyfish here is only just beginning to operate. Because they have largely gone unnoticed in Australian waters, jellyfish are probably not under great threat at the moment.