Hydroida

The order Hydroida belongs to the class Hydrozoa within the phylum Cnidaria. These invertebrates are primarily marine, though some inhabit freshwater. Hydroids can exist as solitary individuals or as colonies.1

The typical life cycle involves a free-swimming larva that settles to become a sessile polyp. This polyp stage can then produce medusae (jellyfish). However, in many hydroid species, the medusa stage is reduced or absent, with reproduction occurring directly from the polyp colony. Colonial hydroids are often branched structures composed of many individual polyps, or zooids, joined by a common living tube. These colonies are protected by a tough outer sheath called the perisarc. Some hydroids, like the genus Hydra, are solitary and lack a medusa stage. Other hydroids, such as Hydractinia, form colonies that encrust surfaces and have specialized polyps for feeding, reproduction, and protection.123