Orange Sea Pen

Weekly Sea Discovery: One organism or many? The budding and beautiful orange sea pen

Suitably named after the old-fashioned quilled pen, sea pens have a very distinct shape. Sea pens are cnidarians and related to soft corals, sea fans, anemones and jellies. These bottom-dwelling invertebrates look quite different from their other stinging cousins. But up close, you can see a tell-tale cnidarian trait, which is the thousands of small polyps that make up its body. A polyp is an individual organism that is usually a member of a colony or can be a life stage in various cnidarians. The polyps bud off feather-like branches along the stalk and have tiny tentacles that they use to feed on microscopic prey.  

The orange sea pen is a common species found in the Pacific Northwest and ranges from shallow to deep water. It prefers muddy and sandy areas that they can bury in by collapsing their branches to avoid predators. Various sea stars and nudibranchs eat orange sea pens and can trigger different levels of attack-responses in them. 

For example, the leather star, one of its specialized predators, instantly causes the orange sea pen to burrow when it touches it. Yet other sea stars that are more generalized feeders might not create such a response. Sea pens sense their predators by chemical receptors on their polyps’ tentacles and can taste if it’s a more dangerous predator like the leather star. Other cnidarians like sea anemones also use their tasting sense to respond to their environment.

When disturbed, the orange sea pen may become bioluminescent and give off a blue-green glow. In other marine species this show of light and colour is an attempt to startle or confuse predators, but many of the species that eat orange sea pens do not use sight for hunting. The use of bioluminescence by orange sea pens continues to be a beautiful mystery.