Hydrozoans are a curious set of marine animals which go largely unnoticed until you brush yourself against a colony. And then you know all about them! Fire coral is just one example of a hydrozoan. The most famous (or should we say 'infamous'!) hydrozoan for Australian beach goers is the Portuguese Man'O'War or Blue Bottle. Hydrozoans usually consist of small colonies of polyps that are packed with stinging cells. They live part of their lives as polyps but also have a tiny jellyfish stage called a medusa. This jellyfish stage is so small that it usually goes completely unnoticed.

• To be a cnidarian that spends most of your life as a polyp, but reproduces during a short time as a tiny medusa.
• To eat by collecting particles out of the water (suspension feeding) or by actively catching prey using tentacles
• To live as a colony of polyps attached to the bottom of the sea. Some of you re made up of colonies, floating on the surface of the ocean.

Stingers, Blue bottles, Portuguese Man'O'War, Fire coral, Physalia, Millepora, Hydra.

Hydrozoans have two different stages during their life cycle. One is a polyp stage that usually forms large and noticeable colonies. These are generally shaped like small fern or plant. The other is the medusa stage which looks like a microscopic jellyfish. The polyp stage can take a number of characteristic forms. Fire corals (genus Millepora) look almost identical to reef-building corals, and have zooxanthellae and skeletons. Others like the Blue Bottle or Portuguese Man'O'War are bright blue and float on the top of the ocean with lots of trailing tentacles. Still other types of hydrozoans look like tiny ferns or encrust the bottom of the ocean like brightly coloured paint. A microscopic inspection of the surface of hydrozoan colonies reveals polyps that have tiny clear tentacles.

Hydrozoans live in all oceans. In tropical oceans, fire corals are often found out where they can provide sunlight for their symbiotic zooxanthellae and can be abundant members of coral reef ecosystems. In other parts of the world, hydrozoans usually make their colonies near the entrances to caves or near ledges and overhangs. This is where water currents bring plenty of food for the many hungry mouths that make up a hydrozoan colony.

Hydrozoans are usually carnivores or detritivores. They live mainly by trapping prey using their sticky tentacles and powerful nematocysts. This method of feeding is similar to other cnidarians (see soft corals for description). Hydrozoans that float in the ocean (like Blue Bottles or Portuguese Man'O'War) have long trailing tentacles that are lined with powerful stinging cells. This are dragged through the water, capturing unwary fish. The tentacles are then hauled up to other special "eating" polyps that consume the fish.

The predators of hydrozoans have not been well studied but probably include a wide range of fish, crustaceans and molluscs like sea slugs. Many sea slugs (eg. the Feather Boa sea slug, Pteraeolidia ianthina) eat hydrozoans and steal their weapons! They manage to concentrate the hydrozoan's stinging cells into the tips of their own tentacles (cerata), ready to use in their own defence. Sort of like being a bomb disposal expert - sneeze and you get stung!

Like many other cnidarians, hydrozoans clone themselves over their lifetimes to produce large colonies of genetically identical polyps. Unlike sea anemones and corals, however, the eggs and sperm cells are not produced by these polyp colonies. Instead, hydrozoans produce a tiny jellyfish stage called a medusa that literally swims away and produces the egg and sperm. These are released from the medusa and are fertilised in the water column. A swimming planula larva develops, settles on the bottom of the ocean after a few days and begins to form the next generation of polyps. And so the great hydrozoan cycle rolls on!

Hydrozoan colonies are often host to a range of animals. Some of the more famous examples are the special fish that live within the tentacles of Blue Bottles or Portuguese Man'O'Wars. These fish (aptly named Portuguese Man'O'War fish) have the same beautiful blue colour as their host, and are immune to its powerful sting. Here they are able to feed on tid bits that the are left over from the Portuguese Man'O'War's meals. What's in it for the Portuguese Man'O'War is not clear, but perhaps the fish cleans away bits of tissue that might otherwise hinder the feeding activities of the hydrozoan.

Next to the box jellyfish, hydrozoans probably have one of the most powerful stings of any cnidarian. Beach goers stung by Blue Bottles or Portuguese Men'O'War are often in so much pain that they have to get out of the water. Fire coral can ruin a dive and is one of the reasons that divers still tend to wear wet suits even warm tropical waters. Apart from being dangerous because of their sting, hydrozoans do not have much to do with humans. They are of little commercial value and are probably not under any direct threat except for the more insidious one of habitat destruction.