I’ve started writing about what is possibly one of my favorite marine creatures: the nudibranch, and more specifically, the Red Flabellina that I observed in the Hood Canal in Washington this past weekend.

When I began my research I was surprised by how little there was on this species of nudibranch, and indeed by some of my own observations of them, which in light of my research puzzled me until I dug even deeper. The best site I’ve found thus far related to nudibranchs and branches of inquiry regarding the Red Flabellina in particular, is the Sea Slug forum. I suggest checking that out if you’re interested in learning more about these slugs in action.

Red Flabellina (predaceous aeolis)

Other names include: Flabellina trophina, Flabellina fusca (an older variation of the name), Coryphella fusca

Size: 12cm long (4.9 inches)

Location: Japan and south Alaska to central Oregon

The Red Flabellina is a nudibranch (basically a part of the sea slug family). The name nudibranch translates to mean “naked gills.”

While I’ve spent a good deal of time scouring the internet for information on the Red Flabellina, my primary source for identifying it was Andy Lamb’s “Marine Life of the Pacific Northwest,” an incredible treasure trove and a book I highly recommend buying if you dive the temperate PNW waters. Prior to this book I was using Rick Harbo’s “Pacific Reef & Shore,” another great guide but one that’s definitely more pocket size.

Beyond my fascination with nudibranchs, I picked this one to start with because I’d seen so many of them in the Hood Canal, a first for me. While I’ve seen nudibranchs there before (the Sea Lemon and the Frosted Nudibranch), I’d never seen so many of this one, easily identified as a Red Flabellina. When I saw them I was at a depth of between 40 and 50 feet. Oddly, I found they were widely distributed in the more open, muddy, exposed areas, as not as much near the kelp and artificial reefs. This surprised me as I knew they fed on hydroids. Why would they be out here? I’ll get to that but first a little more information.

Briefly, the flabellina is a genus of sea slugs (specifically aeolid nudibranchs). It can be found in the sea of Japan, and from south Alaska to central Oregon. It is a subtidal species meaning it is found in areas that are always below low tide (AKA covered with water) but that are still fairly shallow. They have been identified to depths of 65m (215ft).

Because I’ve spent so much time looking at pictures of nudibranchs, I was careful to pay close attention to the ones I saw just off of Mike’s Beach Resort in the Hood Canal, looking for any unique identifiers that would allow me to match them exactly to the right picture later. In the case of the Red Flabellina it was fairly easy. They are translucent nudibranchs with gills that have a tiny white tip on the end, and a fine, burgundy-colored core running through each of the gills. The name for these “frilly fronds” or gills is cerata and that reddish core is actually an extension of the animal’s digestive tract, providing extra surface area for the process of digestion. Note, the color of this core (and therefore the color of the nudibranch in large parts) can change slightly depending on what they’re eating, which is why it’s also important to look for other features identifying these as the Red Flabellina.

They are also fairly easy to distinguish from the Red-Gilled Nudibranchs because their cerata are continuous (not clustered), and because they are a lighter color.

Red-Gilled nudibranchs. Notes the clustered cerata in comparison to the continuous cerata of the Red Flabellina

The other cool thing I noticed, thanks again to Andy Lamb’s book, were the many nudibranch eggs in the vicinity. Previously I would not have been able to identify them as eggs. Once you’ve seen a picture though, you can’t miss these swirly white ribbons. (and better yet, the spiral shape of them. More on that here).

If you know nothing about the anatomy of a nudibranch, I highly recommend studying this nudibranch diagram so that you can better talk about and identify them, even just understand them!

The little “snail-like” feelers are called the oral tentacles. The ones below them are the foot tentacles. The usually smaller tentacles that stick up are called rhinophores. These are basically the nudibranch’s scent or taste receptors. Behind those are the eyes. I know, right? I didn’t think they’d be there either, and they’re tiny! Those little colored bits at the tip of the cerata (the white bits in the case of the Red Flabellina) are called cnidosacs and wait until you find out what those are…

Stinging cells.

No joke. Sometimes they are called cnidoblasts or nematocysts. Interestingly, these stinging cells are not made by the nudibranch, but rather by the species they feed upon: cnidarians such as sea anemones, hydroids, jellyfish, corals and siphonophores. While the nudibranch does not make these, they are used for self defense.

And this discussion on what nudibranchs eat brings me full circle to the beginning of this post and my observations of them in the Hood Canal. According to observations from people on the Sea Slug forum, the Red Flabellina potentially eats tube worms, or some other type of worm. This would certainly explain why I saw so many of them out in more open, muddy areas. More observation is required now I know this is potentially a “thing.”

Other than that, I think that’s all of I’ve got on the Red Flabellina. I’m quite eager to write an article on the Barnacle-Eating nudibranch, but for purposes of diversity I may choose something else first and I’m thinking it’s going to be the Skeleton Shrimp!

If you want a nice, light but interesting overview of nudibranchs, check out this article on Thought Co.


Featured photo of Red Flabellina courtesy of Ratha Grimes

Photo of Red-Gilled Flabellina courety of Vlad Karpinskly

Written by Candice Landau
I'm a PADI Scuba Diving Instructor, a lover of marine life and all efforts related to marine conservation, a tech diver and a member of various scuba organizations in the Pacific Northwest. I write articles related to diving and spend my non-diving time writing and providing digital marketing services to nonprofits and businesses.