HydrozoansHydrozoa

Hydrozoans are small marine invertebrates, typically forming colonies of interconnected polyps, though some exist as solitary individuals. Colonies vary widely in appearance, often growing as delicate, feathery plumes, fern-like structures, or extensive encrusting mats. Individual polyps are minute, but colonies can reach several tens of centimetres across. Many species are translucent or white. A notable group, the fire corals (Millepora spp.), develops hard, calcareous skeletons that are typically yellow, brown, or pink. These fire corals structurally resemble stony corals but are distinguished as hydrozoans by their characteristic pore structure and the potent stinging cells they possess.

Hydrozoan colonies attach to reef structures, rocks, and rubble, from shallow reef crests to depths of at least 40 m (130 ft). They are predators, extending tentacles armed with stinging nematocysts to capture small planktonic organisms drifting in the current. Contact with these nematocysts, especially from fire corals, can cause a painful skin irritation. Many hydrozoan species also have a free-swimming medusa stage, which are typically small, transparent, and drift in the water column as part of the zooplankton.