Fall is right around the corner, and this month’s critter is a preview of the brilliant colors to come. The orange sea pen, also called the fleshy sea pen or Gurney’s sea pen, resembles a colorful autumn tree waving in the “breeze” of moving water currents.
Like its slender sea pen cousin Stylatula elongata, the orange sea pen is actually a colony of tiny animals called polyps working together to form a single organism. It starts out as one primary polyp that grows and expands to form the base (the bulbous part that anchors the animal in the sediment) and the rachis (the central stalk). As it grows, it adds more polyps, assigned to different jobs depending on their location in the colony.
On the sea pen’s long feathery leaves are rows of autozooids: feeding polyps that wave their eight tentacles in the water to catch drifting plankton. These polyps pull double duty, also responsible for producing eggs and sperm that get released into the water column. The siphonozooids, or pumping polyps, are found in the orange regions on the sides of the rachis. Their function is to take in or expel water, allowing the colony to inflate or deflate.
Make like a tree and leave
The orange sea pen is surprisingly mobile, inflating its siphonozooids with water and drifting like a leaf on the wind when it wants to relocate. It can also deflate, partially retracting into its fleshy base when predators come calling. The amount of retraction has been shown to be specific to the approaching predator, which suggests that the pen can actually sense who is creeping up on it! Unfortunately, even a retracted pen can have some soft tissue exposed, and the sea pen’s predators are determined — and hungry.
Orange sea pens make up one of the largest sources of food on the open sea floor and are important to a wide variety of benthic predators, including sea stars like the leather star, the common sun star, and at least three different nudibranch (sea slug) species.
In the limelight
Luckily for the orange sea pen, making a run for it and hunkering down aren’t its only defenses. When the pen is physically disturbed, it can bioluminesce, producing a greenish-blue light that runs up and down its stalk and startles predators. It can also produce a mild toxin called ptilosarcone, although the function of this substance has not been well studied.
Young sea pens are especially vulnerable to predation. They are incredibly slow-growing, taking over a year to reach about an inch tall. Orange sea pens increase their chances of survival with sheer numbers — a single pen can produce about a million eggs during its 10-year lifetime.
The orange sea pen may not be the only species of sea pen in Puget Sound, but it is certainly the most iconic. Its cheery form brightens up dreary sandy or muddy sea floors in shallow subtidal waters from the Gulf of Alaska to California, often occurring in dense beds with many pens per square meter.
Because of the orange sea pen’s tendency to occur in patches and in shallow water, we don’t encounter them very often in our benthic grab samples, but when we do, they are a fun and welcome sight. The adults are so easily identified that we can count, weigh, and measure them in the field and release them alive back into their environment.