Hydrozoa (hydrozoans) are a taxonomic class of individually very small, predatory animals, some solitary and some


colonial, most living in salt water. The colonies of the colonial species can be large, and in some cases the specialized individual animals cannot survive outside the colony. A few genera within this class live in fresh water. Hydrozoans are related to jellyfish and corals and belong to the phylum Cnidaria.

Some examples of hydrozoans are the freshwater jelly (Craspedacusta sowerbyi), freshwater polyps (Hydra), Obelia, Portuguese man o' war (Physalia physalis), chondrophores (Porpitidae), "air fern" (Sertularia argentea), and pink-hearted hydroids (Tubularia).