Most hydrozoans live in the ocean, although a few species—such as the freshwater Hydra—live in freshwater environments.

Hydrozoans have soft, simple bodies. They use special stinging cells called cnidocytes to catch food like tiny plankton. Many species go through two different life stages: a stationary polyp stage, when they are attached to a surface, and a free-swimming medusa stage. Some hydrozoans live alone, while others form colonies made up of many connected individuals, such as the Portuguese man o’ war.