hydroids

Hydroids are small cnidarians belonging to the class Hydrozoa. In common usage, the term refers to colonial hydrozoans that form branching, plant-like colonies on rocks, shells, seaweed, or submerged debris. A hydroid colony is composed of connected units called zooids, each specialized for a task. Feeding polyps, called hydranths, bear tentacles and capture prey with stinging cells (nematocysts). Other zooids include reproductive structures known as gonangia, which produce medusae or eggs, and, in some species, protective or sensory units called dactylozooids.

Most hydroids reproduce asexually by budding to enlarge the colony. In many species a second, sexual phase occurs when a free-swimming medusa is produced from gonangia; the medusa releases sperm and eggs, and the resulting planula larva can settle to form a new colony. Some hydroids have a greatly reduced or absent medusa stage and reproduce within the polyp colony.

Hydroids occupy a wide range of habitats, from marine to freshwater. Colonial forms such as Sertularia and Obelia are common on shells, algae, or wrecks, while freshwater species form solitary polyps such as Hydra. In ecosystems, hydroid colonies act as predators of small plankton and contribute to fouling communities on submerged surfaces.

Human interactions include nuisance fouling on boats and structures and, in some species, dermatitis from contact with their tentacles. Taxonomically, hydroids are hydrozoans; their life cycles and colony organization have made them a classic subject in studies of cnidarian development and life cycles.