A graceful creature of the seafloor, this sea pen resembles a plump, old-fashioned quill pen. Its colors range from dark orange to yellow to white. Each sea pen is a colony of polyps (small anemonelike individuals) working together for the survival of the whole. The primary polyp loses its tentacles and becomes the stalk of the sea pen, with a bulb at its base—the bulb anchors the sea pen in the muddy or sandy bottom. The various secondary polyps form the sea pen's "branches" and have specialized functions. Some polyps feed by using nematocysts to catch plankton; some polyps reproduce; and some force water in and out of canals that ventilate the colony.
Once plentiful in parts of Puget Sound, sea pen populations have declined in those areas. Large numbers of their predators—sea stars and nudibranchs—have also disappeared, leaving some sandy-bottom areas vacant. This affects populations of creatures at the top of the food chain, too. Scientists haven't determined the cause or causes of the disappearing sea pens, but their absence can indicate an ecosystem in trouble.
The red star, the leather star and three types of nudibranchs prey on sea pens.
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.