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Sea pens are marine animal belonging to the superfamily Pennatuloidea, which are colony-forming benthic within the order Scleralcyonacea. The order comprises 16 families and 44 extant genus, with around 235 accepted species.
Sea pens have a cosmopolitan distribution, being found in tropical and temperate waters worldwide, from intertidal shallow waters to of more than .
The earliest accepted sea pen fossils are known from the Cambrian-aged Burgess Shale ( Thaumaptilon). Similar fossils from the Ediacaran may show the dawn of sea pens. Precisely what these early fossils are, however, is not decided.
As octocorals, sea pens are colonial animals with multiple polyps (which look somewhat like miniature ), each with eight . Unlike other octocorals, however, a sea pen's polyps are specialized to specific functions: a single polyp develops into a rigid, erect stalk (the rachis) and loses its tentacles, forming a bulbous "root" or peduncle at its base. The other polyps branch out from this central stalk, forming water intake structures ( siphonozooids), feeding structures ( autozooids) with , and reproductive structures. The entire colony is fortified by calcium carbonate in the form of spicules and a central axial rod.
Using their root-like peduncles to anchor themselves in sandy or muddy substrate, the exposed portion of sea pens may rise up to in some species, such as the tall sea pen ( Funiculina quadrangularis). Sea pens are sometimes brightly coloured; the orange sea pen ( Ptilosarcus gurneyi) is a notable example. Rarely found above depths of , sea pens prefer deeper waters where turbulence is less likely to uproot them. Some species may inhabit depths of or more.
While generally sessile animals, sea pens are able to relocate and re-anchor themselves if need be. They position themselves favourably in the path of currents, ensuring a steady flow of plankton, the sea pens' chief source of food. Their primary predators are and , some of which feed exclusively on sea pens. The sea pens' ability to be clumped together and spatially unpredictable hinders sea stars' predation abilities. When touched, some sea pens emit a bright greenish light; this is known as bioluminescence. They may also force water out of their bodies for defence, rapidly deflating and retreating into their peduncle.
Like other , sea pens reproduce by coordinating a release of sperm and eggs into the water column; this may occur seasonally or throughout the year. Fertilized eggs develop into larvae called planulae which drift freely for about a week before settling on the substrate. Mature sea pens provide shelter for other animals, such as juvenile fish. Analysis of rachis growth rings indicates sea pens may live for 100 years or more, if the rings are indeed annual in nature.
Some sea pens exhibit glide reflection symmetry, rare among extant animals.
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