Hydroids are plant-like organisms and belong to the Order „HYDROIDA“. The hydroids do not form a separate taxonomic group of animals, but include several subclasses of Hydrozoans. They are animals and closly related to jellyfish, sea anemones and corals. Many have an complicated lifecycle but generally they go through three stages. First as free swimming planktonic larvae that settles and forms sessile polyps. Second as a small jellyfish like stage where the polyps produce free swimming medusa; and third finnaly the medusa in turn produce planktonic larva.
Hydroids usually exist as colonies, the polyps are protected by a horny perisarc which. All hydroids are carnivorous animals, catching prey in the water column with the aid of stinging and grappling nematocysts. They will be found attached to seaweeds and rocks. They are not always easy to find as many are small or form seaweed like colonies.