Hydrozoa (hydrozoans) are a class of very small, predatory animals in the phyllum cnidaria. They can be solitary or colonial and mostly live in saltwater. A few genera within this class live in freshwater.
Some examples of hydrozoans are the Freshwater Jelly, the freshwater polyps (Hydra), Obelia, the Portuguese Man o' War (Physalia physalis), the chondrophores (Porpitidae), "air fern" (Sertularia argenta) and the pink-hearted hydroids (Tubularia).
Most hydrozoan species include both a polyp and a medusa stage in their life cycle, although there are a number that have only one or the other.
The polyp form is usually colonial, with multiple polyps connected together by tubelike hydrocauli. The hollow cavity in the middle of the polyp extends into the associated hydrocaulus, so that all the individuals of the colony are intimately connected. Where the hydrocaulus runs along the substrate it forms a horizontal root-like stolon that anchors the colony to the ground.
The colonies are generally small, no more than a few centimetres across, and may have a tree-like or fan-like appearance, depending on species. The polyps themselves are usually tiny, although some non-colonial species are much larger, reaching 2.4 to 3.5 in, or, in the case of the deep sea Branchiocerianthus, a remarkable 6.6 ft.
In some, one polyp may develop as a large float, from which the other polyps hang down, allowing the colony to drift in open water instead of being anchored to a solid surface.
The medusae of hydrozoans are smaller than those of typical jellyfish, ranging from 0.20 to 2.4 in in diameter. Although most hydrozoans have a medusoid stage, this is not always free-living, and in many species, exists solely as a sexually reproducing bud on the surface of the hydroid colony.
Here are some examples of various species of Hydrozoa:
Hydroid photo ©Richard Ling
Gonionemus photo ©Allen Collins
(clicking any photo will take you to the image in the photographer's Flickr collection)