Featured Creature: Nudibranch

What psychedelic sea creature is known for its amazing array of colors, its creative survival tactics, and its wide dietary range that even includes its own kind?

The nudibranch! (Pronunciation: Nood – eh – brank)

(Photo by Eric Schlogl from ALAMY)

I first became acquainted with nudibranchs due to a fellow nature lover in my life. A few months ago, my girlfriend told me she was heading to New York to get two new tattoos, a turkey tail mushroom and a nudibranch (I know, what a nerd). While I was familiar with the fungus, a common sight here in the Northeast, I had no idea what a nudibranch was, and when she explained that it was some sort of sea slug, I was highly skeptical of how that might look on her arm. Despite my doubts, the tattoo turned out beautifully, and the more I learned about the creature behind it, the cooler it seemed to me. 

More than just a slug

The nudibranch may be known colloquially as a sea slug, but more formally it is a marine mollusk, like the much larger octopus. Nudibranchs, whose name literally means “naked gill”, are shell-less mollusks with gills exposed on the outside of their bodies. They grow anywhere from less than an inch to just under two feet long, and inhabit oceans around the world. While they are found at nearly all depths, there is the greatest diversity of nudibranchs in warm, shallow waters around reefs. 

This family of creatures (Nudibranchia) has tremendous variety and a penchant for clever survival tactics. Among various types of nudibranchs, different species sport incredible colors and patterns. They can derive their colors from the prey they eat, which includes a variety of sponges, portuguese man-o-wars, algae, anemones, corals, and barnacles. 

Poster from Etsy

This flashy appearance serves as a warning to any potential predators who may try and feast on such a beautiful dinner, because interesting pigmentation is not the only thing lurking beneath the nudibranch’s surface. It turns out that nudibranchs can actually ingest stinging cells (or nematocysts) from their prey, absorb them into their bodies and deploy them as their own defense mechanism. 

Watch and learn more about this process (and enjoy the amazing array of sea slug species):

Perhaps due to the success of their defenses, nudibranchs have few predators to contend with. They only have to deal with harm from certain crabs, humans, and other nudibranchs. While nudibranchs often prey on (or are preyed on by) other nudibranch species, certain types have been known to even cannibalize their own kind. 

These sea slugs spend much of their life hunting and eating, traversing the sea floor for food sources. One type of nudibranch has a special long term strategy to satisfy its appetite. The Pteraeolidia semperi actually stores algal cells within its outer appendages that continue to live and photosynthesize, providing sugars to both organisms and giving the nudibranch a bluish hue. 

(Clovelly, Sydney Diet: large solitary hydroid. Features: captures and farms microscopic plants (zooxanthellae) in its own body which pass a significant proportion of the produced sugars on to the nudibranch for its own use. The slug gains enough photosynthetically derived sugars to sustain it without feeding. They can store undigested nematocysts in the cerata on their backs and use it for their defense. Size: 70mm; Discovered: 1864, Angas. Photo by Sylke Rohrlach)

Variation built right in 

Nudibranchs are not only impressive and adaptable when it comes to sustaining life, but also when it comes to creating it. They are hermaphrodites whose sexual organs emerge when engaging in reproduction and can change encounter to encounter. Indeed, when two nudibranchs meet to reproduce, they engage in a dueling process to see who will inseminate the other. In some species (such as Nembrotha lineolata), both nudibranchs can experience fertilization in the process, and they both lay eggs afterward in a trademark spiral formation. 

(Nudibranch egg ribbon at Malahi – Red Sea, Egypt; Photo by Alexander Vasenin)

While eggs sometimes contain toxins that repel predators, nudibranchs in the larval stage are still susceptible, including to the dangers in the environment at large. Ocean acidification harms young larvae, and warming and pollution also present harsh challenges that negatively impact nudibranch population and distribution. 

On the other hand, nudibranchs are great ecosystem indicators, as the diversity of species present in an area broadcasts that ecosystem’s health. As restoration ecologists note when observing systems at work, the greater the diversity and complexity they contain, the more robust, resilient, and highly functioning they are. 

Biodiversity not only enhances our oceans and landscapes, but is also a source of tremendous beauty. I was tempted to fill today’s Featured Creature with dozens of images in place of any text at all, and it was difficult to choose just a handful. But I’m glad to offer a small glimpse into these creatures’ existence and I hope you’ll take some time to stoke your inspiration and keep exploring on your own.

(Also known as Sea Swallow, Blue Glaucus, Blue Sea Slug, Blue Ocean Slug and Lizard Nudibranch. Washed ashore at Bronte Beach, Sydney, NSW. Diet: Bluebottle (physalia physalis) Speciality: Stores stinging nematocysts from the bluebottle within its own tissues, which is additional protection from predation Discovered: 1777, Forster. Photo by Sylke Rohrlach)

With appreciation for the many miraculous ways of being,

Maya Dutta