When disturbed, a sea pen forces water out of the colony, making it possible for the sea pen to retreat into its bulbous foot. Sea pens are octocorals — each polyp has eight tentacles. Sea pens glow with a bright-greenish light when stimulated.
As their name suggests, sea pens can look like writing pens, or they can resemble feathers, whips or worms. But these diverse and delicate underwater animals, which are a type of coral, are also hard to pin down: A single sea pen is both an individual and a colony.
Sea pens are a type of octocoral, named for the eight stinging tentacles they use to capture plankton (tiny floating plants and animals) to feed themselves. The basic unit of a sea pen, like other corals, is a polyp, which consists of a sac-like body cavity enclosed by a mouth and surrounded by a ring of tentacles. Once it has found the perfect substrate — sand, mud, rubble or, in a few cases, solid rock — a larva settles down and becomes the primary polyp. It buds off into daughter polyps, and the sea pen grows. All are supported on a stemlike structure.