Critter News: Sea Pens!

What can glow, burrow, and drift away? Sea pens!

When touched by a predator such as a leather star, sunflower star, or any of several nudibranchs (including opalescent, striped, orange peel, and diamondback), sea pens have several defenses. It may flash-glow a bio-luminescent greenish light (produced by one of the same chemicals that fireflies use to create their lights) which might startle the predator and/or warn other nearby sea pens. It might deflate its body and burrow rapidly into the substrate, leaving little if any of its body exposed. Or, it might just inflate itself with water, pull up its base, and drift away on the current.

Critter News: Sea Pens!Sea Pen
Sea Pen
Opalescent nudibranch, such as the one at the bottom left of the sea pen (see blue arrow), usually feed on very small sea pens that are less than a year old.


This soft coral begins life as a single polyp which has developed from a fertilized egg. This first polyp becomes the bulb and the central stalk (#1 white arrows). As it grows, more polyps are created from this bulb and stalk, including tiny water-intake polyps embedded in the central body region (#2 bluearrows) that pump water into the sea pen. Feeding polyps (#3 red arrows) emerge from blades along the stalk to gather plankton with their 8 tentacles. All the polyps in a single sea pen are genetically identical and cannot live in-dependently.

At about five years of age (and about 9.5” long), females and males begin to spawn for the first time, and may continue for many years until predators, disease or pollution shorten their potentially unlimited lifespan.

To see the sea pens at the Aquarium, visit the Puget Sound Fish exhibit.

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