Critter Corner

By: Bill Knauer

This month I’d like to talk about a relative of the soft corals found here in Alaska, the Orange (or Gurney’s) Sea Pen (Ptilosarcus gurneyi). One Sea Pen is composed of hundreds of individual polyps. In this species, a bulbous foot buried in the substrate holds the orange plume-like polyp covered stalk (central axis) in place. It ranges in color from dark orange to light orange to white. The plume is decorated with a series of leaf-like branches and each branch holds numerous little feeding polyps that come off of a central axis. Other less conspicuous polyps are found opening onto the surface of the main stem and their function is to force water into and out of a series of canals that ventilate the colony. Water provides a hydrostatic skeleton. When a Sea Pen is irritated or stimulated in some way, it will contract into its bulbous foot by forcing water out of the colony. It may take a while before it becomes inflated again. A large specimen is 20 inches (50cm) long when extended. Although generally sessile, the Sea Pen can move about 15-20 inches by a slow creeping movement of its foot. The Sea Pen filter feeds on plankton and other small crustaceans. The Sea Pen is found in sandy mud flats from 25 feet down to depths of 150 feet. Its range is from Alaska to southern California. Predators of the Sea Pen are the Rose Star, the Leather Star, and two nudibranchs,. Sea Pens are also beautifully luminescent producing a bright greenish light when stimulated mechanically by stroking. Luminosity is inhibited by exposure to direct light. Another very close relative of the Sea Pen, found also in Alaska, is the Sea Whip (Osteocella septentrionalis). The Sea Whip is usually found in deeper waters than the Sea Pen and can reach 4 feet or more in height. Keep your eyes open for interesting creatures when diving in Alaska and elsewhere.