In many species the polyp, or hydroid, stage is colonial: as new polyps are created by budding, they remain attached to a branching common stalk, often hardened with nonliving material, forming a plantlike structure called a hydroid colony. The branching, hydroid colonies of Obelia are commonly found on North American seashores; the individual polyps are microscopic, but the feathery white or yellow colony grows up to 12 in. (30 cm) tall. Individual polyps project from the branches of such a colony; they produce the tiny, free-swimming Obelia medusae, barely visible to the naked eye. Other hydroid colonies are colonial coral and the sea pens; these have no medusa stage. All hydroid colonies have at least two types of individual polyps, specialized for feeding and reproduction respectively; some have additional specialized types. The purple sail, genus Vellela, and the Portuguese man-of-war, genus Physalia, are elaborate floating colonies composed of many types of specialized individuals, both polyplike and medusalike in structure; the entire colony is equipped with a float.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.