By Alex Rose
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. 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.
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. There are well over 2,000 species of nudibranchs and they are all grouped into four distinctive suborders. Doridina, 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.
There are thousands of species of sea slugs in the world and they are all fascinating in their own way. A fantastic new book authored by Terry Gosliner, Angel Valdes, and David Behrens has recently been released, Nudibranch & Sea Slug Identification: Indo-Pacific, and it is absolutely the most comprehensive resource available for quick and reliable sea slug identification. I’m looking forward to breaking it in some more in the Philippines in 2016.