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Guynia annulata is known from the Pliocene to the Recent, and is widespread in today's oceans in tropical and temperate regions. It occurs in relatively deep water (28-1300 m) and is exclusively azooxanthellate. Although this lack of reliance on symbiosis allows Guynia to live beyond the euphotic zone (layer of light penetration) and at cold temperatures, it limits the animals to relatively small size. Adult Guynia annulata are only 1.0-1.3 mm in calicular diameter, but may be up to 10 mm long.
Guyniids are exclusively solitary in growth form, never forming colonies. They are inconspicuous, cryptic animals, often having elongate, worm-like growth forms. Vermiform coralla are often mistaken for serpulid polychaete worm tubes, but are easily distinguished with a microscope by having septa inside their tubular corallum. Little is known about their ecology and relative abundance in the oceans.