Pennatulacea ( the "sea pens") is an order of around 200 described species of octocorals (or "soft corals") in the phylum Cnidaria. Various sea pens are found throughout the world's oceans from polar seas to tropical waters and from intertidal waters down to depths of more than 6100 meters. Like other octocorals, sea pens are actually colonies of individuals known as polyps. Most sea pens exhibit striking polyp polymorphism: One polyp grows very large and loses its tentacles, forming the central axis of the colony and anchoring itself in soft sediment; other polyps taking various forms and serving various functions (feeding, water circulation) branch off from this central polyp. The global diversity of Pennatulacea was recently reviewed by Williams (2011).
The common name “sea pen” and the order and family names “Pennatulacea” and "Pennatulidae" are derived from the perceived resemblance of the species in some genera (such as Pennatula, Pteroeides, and Virgularia) to quill pens.
(Williams 2011 and references therein)
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