Sea pen

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Sea pen Temporal range: Cambrian–Recent PreꞒ Ꞓ O S D C P T J K Pg N
"Pennatulida" from Ernst Haeckel's Kunstformen der Natur, 1904
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Cnidaria
Class:	Octocorallia
Order:	Pennatulacea Verrill, 1865
Family
see text

Sea pens are colonial marine cnidarians belonging to the order Pennatulacea. The order Pennatulacea, commonly known as sea pens, are colony-forming benthos belonging within subclass Octocorallia (Anthozoa, Cnidaria). Sea pens are found worldwide from shallow to deep waters, and they are important components in sandy and muddy environments. Thus far, there has been only one molecular study focusing on the phylogenetic relationships within the order Pennatulacea, which mainly treated deep-sea species, and thus information on shallow water species is still lacking.^[1] There are 14 families within the order and 35 extant genera; it is estimated that of 450 described species, around 200 are valid.^[2] Sea pens have a cosmopolitan distribution, being found in tropical and temperate waters worldwide, as well as from the intertidal to depths of more than 6100 m.^[2] Sea pens are grouped with the octocorals, together with sea whips (gorgonians).

Although the group is named for its supposed resemblance to antique quill pens, only sea pen species belonging to the suborder Subselliflorae live up to the comparison. Those belonging to the much larger suborder Sessiliflorae lack feathery structures and grow in club-like or radiating forms. The latter suborder includes what are commonly known as sea pansies.

The earliest accepted fossils are known from the Cambrian-aged Burgess Shale (Thaumaptilon). Similar fossils from the Ediacaran may show the dawn of sea pens.^[3] Precisely what these early fossils are, however, is not decided.^[4]