A sea pen on the ocean floor

Photo: Greg Amptman/Shutterstock

What do you get when you cross a feather, a starfish and a fern? You get a sea pen, one of the funkiest critters in the ocean.

Sea pens are so named for their quill-like shape, but they're actually a form of soft coral ("octocoral") composed of polyps that each have eight tentacles. Sea pens can set up camp pretty much anywhere — sand or rubble — and they add a colorful flair to any sea floor. 

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A yellow sea pen

Photo: Greg Amptman/Shutterstock

Sea pens come in a variety of shapes, sizes and colors — including glow-in-the-dark outfits. They can live in the most extreme environments, over 20,000 feet below the surface and as far south as Antarctica. 

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Sea pen polyps

Photo: Steven Maltby/Shutterstock

As a sea pen grows, it sprouts more polyps from its stemlike center. At the bottom of the sea pen, one of the polyps adapts into a bulge of water meant to weigh the organism down. Some sea pens only reach a couple of inches in height, while others tower more than 6 feet above the ocean floor. 

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Sea pens in Puget Sound

Photo: Greg Amptman/Shutterstock

Sea polyps reproduce and grow near one another — at least as long as that anchor keeps them from being carried away by the strong ocean currents. Sea pens have also been known to relocate themselves and anchor down in a more practical spot where there's more plankton to eat.

In the photo above, the sea pens have made their home near the site of a shipwreck in the Puget Sound off the coast of Washington state.  

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A porcelain crab climbs up a sea pen

Photo: Takashi Images/Shutterstock

Strange and beautiful sea creatures hide within the polyps of a sea pen. The porcelain crab uses the sea pen as an anchor as both animals filter-feed particles from the water. 

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Goby hiding on a sea pen

Photo: timsimages/Shutterstock

Gobies also have a commensal relationship with sea pens, meaning the fish benefits from the sea pen's protection without doing the sea pen any harm (or good). 

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Transparent blue sea pen

Photo: Jukka Siltanen/flickr

Sea pens appear to be a plantlike version of a sea star mixed with a sea slug, yet those creatures actually prey on sea pens. 

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Bent sea pen

Photo: Jung Hsuan/Shutterstock

They may seem like lifeless creatures bending to the current, but sea pens are quite responsive. If they are touched, they may retreat into their bulbous root. In this way, scientists estimate sea pens can survive for more than 100 years. 

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Anna Norris is an associate editor at Mother Nature Network. Follow her on Twitter and Google+.

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These soft corals resemble quills and hide all sorts of surprises in their polyps.