Hydrozoans

Commonly known as 'water animals', this group has the most varied body forms of all cnidarians. A particular characteristic of hydrozoans is the division of labour between the various polyps of the colony. Some specialise in defence, others feed and share nutrients with the colony through an interconnecting digestive tube, and others are reproductive organs.

Recognising hydrozoans

Lace Coral / Hydroid / Distichopora sp. / Family Stylasteridae

Hydrozoans can simply be broken into three categories:

Fern-like colonies of animals - Aglaophena and Lytocarpus

Hydroids can easily be mistaken for plants. One form commonly found on the reef, aglaophenia and lytocarpus resemble a clump of ferns that feel like nettles to the touch. They are colonies with a central stalk and numerous side branches full of tiny specialised polyps. This species is found in sunny situations where it grows to a particularly large size because of the presence of zooxanthellae. Other hydroids can be quite tiny.

Others closely resemble corals like fire coral

Its shape is variable, ranging from large sheets to branching staghorn-like forms. They have a skeleton of calcium carbonate that superficially resembles hard corals. The skeleton is relatively smooth and contains minute holes in which the hydroid polyps live. Fire corals give a strong burning sensation when touched.

Others are like jellyfish

The most highly developed hydrozoans is the blue bottle. Most people think the blue bottle is a jellyfish however they are not, they are actually a colony of individuals.

Hydrocorals closely resemble hard corals and are important reef builders. They secrete a hard skeleton and can form large colonies varying in form from branches to upright sheets and blades, mounds and encrusting growths.

Habitat

Fern like hydrozoans can be found living on rocks and jetty piles. Other hydrocorals are frequently found on the reef front areas as well as delicate lacy plates found under ledges.

Floating Hydrozoans

Physalia looks very different from its attached relatives. Known variously as the Portuguese/Pacific man-of-war and bluebottle, it is a floating hydrozoan. Unlike 'true' jellyfish, it is not an individual but a colony of specialised polyp-like animals. One medusa-like individual creates the blue float and below this hang long tentacles, well supplied with nematocysts which are responsible for capturing, killing and holding prey, or repelling predators. These tentacles can contract, pulling prey up to the special feeding polyps situated under the air bladder. Also found under the bladder are the male and female reproductive polyps.

Other floating hydrozoan colonies include the blue Porpita and Velella, or 'by-the-wind' sailor. Living at the surface, these are in an ideal position to host zooxanthellae which provide them with extra nutrition.