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What is a Nudibranch?

Maybe you’ve come across pictures flipping through a magazine, have a fun-loving diver friend who won’t shut up about how much they love them, or you’ve chatted with Hannah at her pop-up shop! No matter what brought you here, let’s take a look at this bright and colorful group of sea creatures we are lucky enough to share our planet with.

In their essence, nudibranchs are very pretty slugs that can breathe underwater. Although related, they are far from your average, drab garden slug! This incredibly diverse group has over 3,000 species varying in color, size, and shape. Arguably the most flamboyant and fashionable group residing in our oceans, some are so tiny you can barely see them whereas others are the size of a football. The largest on record is the beautiful Spanish dancer nudibranch measuring in at an impressive 52 cm (over 20 inches)!

Spanish Dancer Nudibranch
Spanish Dancer Nudibranch (Hexabranchus sanguineus)

 

Nudibranch (NEW-dih-bronk) comes from their scientific name Nudibranchia, derived from the Latin word nudus meaning “naked” and the Greek word brankhia meaning “gill.” This refers to their exposed gills which are located in a beautiful plume on their behind or sprout out across the back of their body. Nudibranchs are divided into two groups, dorids and aeolids, with the gill placement depending on which they belong to. Dorid nudibranchs have their gills gathered in a tuft at the back of their body like a little bouquet of lungs which they use to breathe. Aeolid nudibranchs have finger-like gills covering their entire back side which they use not only for breathing but also for digestion and defense.

Dorid vs Aeolid Nudibranch Graphic

 

Besides these unique gills, nudibranchs can be identified by their rhinophores. These look like a pair of bunny ears popping out of the top of their head. Nudibranchs have terrible eyesight due to their minuscule eyes, so they depend on their rhinophores to sense chemicals dissolved in the water. These nifty appendages are how nudibranchs find food and mates. But wait, there’s more! Rhinophores can also sense changes in water pressure and vibrations allowing them to evade predators. They may not be able to move very fast but many dorid nudibranchs are able to quickly retract and protect their rhinophores and gills if they sense any suspicious water movement.

Rhinophores in Nudibranchs

 

How are they able to be so glamorous, small, and slow without getting eaten? They generally taste bad and in some cases they’re even toxic! They have a unique ability to absorb toxins and stinging cells from their food and repurpose them in their own skin.

Glaucus atlanticus sea slugs eating a Portuguese man o' war
Glaucus atlanticus is a great example of a dangerous sea slug - they feed on other pelagic creatures, including the Portuguese man o' war as shown here. This sea slug stores stinging nematocysts from the siphonophores within its own tissues as defense against predators. Humans handling the slug may receive a very painful and potentially dangerous sting.

 

Lucky for us, nudibranchs are found in oceans worldwide. They are found on tropical coral reefs, in cold water kelp forests, floating in the water column, or cruising along on the sand. Due to their miniature size they’re not always the easiest to spot. However, with some patience and looking closely at their food sources (sponges, hydroids, corals, anemones, fish eggs), you might get lucky and find one! You don’t even need to be a diver to see them. Tidepools are an excellent and accessible place to observe them. If you’re not close to the ocean, your local aquarium might have some on display.

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