The slender sea pen

Sea pens are marine cnidarians that belong to the order Pennatulacea. They are colonial organisms, composed of specialized polyps.

Stylatula elongata
This critter of the month, Stylatula elongata, is often found in groups. Photo courtesy of Chris Grossman,

Where there’s a quill, there’s a way

The sea pen resembles an old-fashioned feather quill pen, and is fittingly named Stylatula elongata.

Sea pens are a type of octocoral, or soft coral, which are related to jellyfish and anemones. They may not get as much love as their glamorous cousins, the reef-building hard corals, but sea pens are fascinating creatures that can live to be 100 years old, glow in the dark and live in the soft sediments of Puget Sound.

Many heads are better than one

A sea pen is not one single animal, but a colony of many tiny animals called polyps. Each colony contains several types of polyps which contribute to its survival in different ways. The feeding polyps, called autozooids, wave their tentacles in the water column to catch drifting plankton. Siphonozooids, a different kind of polyp, are responsible for distributing food throughout the colony using a complicated water circulation system.

Classification of Stylatula elongata

Glow with the flow

Although sea pens may appear to be inanimate plant-like organisms yielding to the will of the water currents, they actually have a surprising number of defensive tricks up their sleeves. If disturbed, the polyps can take in water, inflating the colony so it can drift to a new location. They can also quickly expel water, deflating the colony and allowing it to retreat into the mud.
S. elongata

S. elongata autozooid polyps with tentacles extended.

Another way sea pens respond to stimulation is by glowing in the dark, through a process called bioluminescence. Bioluminescence occurs when a chemical reaction takes place within the sea pen and energy is released in the form of light. These showy displays resemble the lights on a movie marquee as the flashes circulate up and down the rod-like stem of the sea pen, and are used to startle potential predators.

In rod we trust

Like other sea pens, S. elongata is supported by a hard white skeletal rod made of calcium that looks like a lollipop stick. At the base of the rod is a muscular, bulbous holdfast called the basal peduncle that anchors it in the soft sediment.

S. elongata rod and sclerites.

LEFT: Part of S. elongata with the supporting skeletal rod exposed. RIGHT: Close-up of polyp clusters with needle-like spokes called sclerites.

Arranged in spirals around the rod are clusters of polyps which create the sea pens’ feather-like appearance. S. elongata can be distinguished from other species of sea pens by the exposed fans of needle-like spikes called sclerites which protect the soft, fleshy polyps from its two main predators, nudibranchs (sea slugs) and sea stars.
S. elongata peduncle

A muscular, bulbous holdfast called the basal peduncle anchors S. elongata in the soft sediment in which it lives.

Sea pen on the decline?

Despite having so many strategies for survival, sea pens are on the decline in some parts of Puget Sound. Scientists aren’t sure why this is happening, but their long lifespan (100+ years) may be considerably shortened by disease, pollution, disturbance and other unknown factors.
About the Author: 
Dany Burgess and Angela Eagleston are scientists who work for the Marine Sediment Monitoring Program at the Washington State Department of Ecology.