Nudibranchs are fantastically amazing animals. They come in just about every color imaginable, are shaped in the most peculiar of ways, eat some of the weirdest things, have gills growing out of their backs, smell and taste with stalks growing out of their heads, and can be toxic. A bit more than you might expect from an animal sometimes called a sea slug right?
Speaking of that name, the term sea slug can be a bit of a misnomer. There are five main groups (orders) that make up sea slugs, one of which is the nudibranchs (Nudibranchia), meaning that all nudibranchs are sea slugs, but all sea slugs are not nudibranchs. Let’s take a moment to get to know a bit more about these intriguing creatures and how to properly classify them.
Sea slugs are in the phylum Mollusca, along with snails, bivalves, and cephalopods. In Latin, Mollusca means “soft body,” an appropriate description because while many of the animals in this category have shells, they lack a true skeleton. Sea slugs as well as snails make up the class Gastropoda, and are further classified in the subclass Opisthobranchia to which all sea slugs belong. Most opisthobranchs lack an external shell, but sometimes have a small internal shell instead.
The Five Orders of Sea Slugs
This subclass is comprised of five orders: Cephalaspidea, the headshield slugs; Anaspidea, the sea hares; Notaspidea, sidegill slugs; Sacoglossa, sapsucking slugs; and Nudibranchia, the nudibranchs. Almost all the sea slugs that can be kept by hobbyists in captivity either fall into the sea hare or sapsucking slug category, while there’s really only one true nudibranch that can thrive in a tropical home reef aquarium. This statement may lead the reader to wonder why almost no nudibranchs are suitable for life in the tank, and the answer has to do with their strange and particular eating habits, an interesting topic that we will discuss after we cover some amazing facts about nudibranchs and the characteristics that make them unique in the already alien world of marine invertebrates.
The term nudibranch literally means naked gill from the Latin word “nudus” (naked) and the Greek word “brankhia” (gill). Their name refers to the structures that protrude from their backs, which in some cases are actual gills and in others are cerata, elongated respiratory organs that serve the same function as gills do. All nudibranchs have rhinophores, chemosensory structures on their heads that look like antennae, and allow them to smell and taste the water around them. Some nudis are solar powered. They are capable of storing the zooxanthellae from the cnidarians they eat in their tissues and using the sugars produced by these photosynthetic symbiotic dinoflagellates to power some of their daily activities.
Some species of nudibranchs can also retain the stinging cells or nematocysts from the animals they consume and use these for protection against predators. As diverse as the body form and coloration of nudis are, the places they inhabit are equally as varied. Nudibranchs have colonized the seas of the world and can be found in waters ranging from the Philippines to Antarctica. that is less than an inch long as an adult and feeds on the soft tissue of shrimpgoby fins, to the huge, pelagic Spanish Dancer (Hexabranchus sanguineus) that can swim through the water and reach an impressive two feet in length.
There are well over 2,000 species of nudibranchs and they are all grouped into four distinctive suborders. Dordina, or the dorid nudibranchs, are by far the most populous group and represent what is considered a “typical” nudi body plan with their branched gills and bright color patterns. Dendronotina, or the dendronotid nudibranchs, have paired, branching gills and long rhinophores. Arminina, or the arminid nudibranchs, typically have a smooth, often striped, mantle with hidden gills underneath it that run the length of the body. Aeolidina, or the aeolid nudibranchs, are covered in cerata and have a pair of long pointed tentacles extending from their heads. Some nudibranchs are clad in the most outrageous colors and patterns, while others perfectly mimic soft corals and sponges and are nearly invisible to all but the trained eye. Nudibranchs range in size from the tiny black Gymnodoris nigricolor that is less than an inch long as an adult and feeds on the soft tissue of shrimpgoby fins, to the huge, pelagic Spanish Dancer (Hexabranchus sanguineus) that can swim through the water and reach an impressive two feet in length.
It is often said that for every invertebrate in the ocean, there’s a nudibranch that eats it, and this generally seems to be true. Nudis tend to be predatory, and are extremely specialized when it comes to diet. They can eat things including sponges, hydroids, corals, anemones, tunicates, and will sometimes eat other nudis too. There’s even a pelagic one called the Blue Angel nudibranch (Glaucus atlanticus) that preys on the highly venomous Portuguese Man-of-War jellyfish (Physalia physalis) and then stores the jelly’s intact stinging cells, or nematocysts, in its own tissues as protection against predators.
There are thousands of species of sea slugs in the world and they are all fascinating in their own way. The largest species diversity can be found in the warm waters of the Indo-Pacific in places like Raja Ampat, Indonesia and Anilao, Philippines. These areas host the perfect conditions for explosive adaptive radiation because of the warm temperatures, high levels of nutrients from cold, deep water upwellings, and their protection from drastic environmental changes during the last ice age because of the deep underwater trenches that surround them.
Very few nudibranchs are conducive to captive aquarium life, and even most public aquariums avoid displaying the vast majority of them. In general, it is best to leave these animals in the wild and enjoy them in photos and videos instead of in tanks though there are a few notable exceptions such as the Aiptasia eating nudibranchs often known as Berghias, and the algae eaters Elysia and Aplysia. As scuba divers like to say: “Take only photos, leave only bubbles.”
Alex Rose holds a B.S. in biology and a M.S. in aquatic biology, and she has a wide variety of experience in the biological sciences, including bioacoustics research, exhibit construction, science writing, teaching, public presentation, and aquatic animal husbandry and breeding. Alex is a professional violinist, photographer, PADI divemaster and lover of all things aquatic. Her driving goal is to find ways to protect our world’s marine habits through diving, writing, education and research. Visit her website at alexroserenaissance.com. You can also read more on the Sustainable Reefkeeping page.