hydroid

Hydroid is a term used to describe the polyp form of many hydrozoan cnidarians, and can also refer to colonial freshwater hydroids such as Hydra. In many hydrozoans, the life cycle alternates between a sessile, colonial polyp stage (the hydroid) and a free-swimming medusa stage; in several lineages the medusa is reduced or absent.

Hydroids are usually colonial and display a branching structure anchored to a substrate by a basal stem called a hydrorhiza. The colony is composed of specialized polyps: feeding polyps (hydranths) and reproductive polyps that bear gonophores, which in turn release medusae. Some species have a protective cup, the hydrotheca, around each hydranth.

Reproduction occurs asexually by budding, expanding the colony. Sexual reproduction occurs when gonophores produce free-swimming medusae, which fertilize and form new hydroid colonies from planula larvae. In some hydrozoans, no medusa stage is produced and reproduction is entirely by budding.

Habitat and ecology: Hydroids are found in marine and freshwater environments, from rocky shores to rivers. They can form fouling colonies on ships, docks, kelp, or coral, and are predators of small planktonic organisms. Hydra, the best-known freshwater hydroid, is a solitary organism that reproduces by budding and is widely used in developmental biology.

Notable examples include Obelia and Sertularia for colonial marine hydroids, Campanularia, and the freshwater Hydra. The term helps distinguish the polyp form from the medusa form in hydrozoan life cycles.